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Slide Text Space SUBMIT ALL submit all SUBMIT submit Submit Unmute volume radar sampling issues principles of doppler weather radar instructor: andy wood technical problems? if you encounter any technical problems with this lesson, please contact the rac team directly by e-mail (nws.wdtd.rachelp@noaa.gov) or you can use the wdtd feedback forum web page (https://training.weather.gov/wdtd/contact/feedback.php) to send us a message. technical problems? complete the quiz at the end of this lesson, there is an embedded quiz. complete this quiz by selecting the best answer for each question. you need to correctly answer 70% of the quiz questions to receive completion credit in the lms. complete the quiz review lesson take your time and review the lesson content provided in this presentation. review lesson course completion information introduction in order for nws forecasters to receive credit for this course in the nws learning center, you will need to take the following steps review lesson complete the quiz technical problems? roadmap learning objectives identify the key effect radar sampling issues have on radar-identified features identify a negative cue identify the primary factors in radar beam height estimation errors and uncertainty identify how aspect ratio affects radar signatures identify how radar horizon affects the parts of the storm radar can see learning objectives identify why buffers should be placed around radar signatures for public warnings identify how a user can overcome beam blockage issues identify how viewing angle primarily affects velocity interpretation identify the most likely scenario to experience noticeable side lobe contamination identify the products that are directly affected by non-uniform beam filling (nbf) and differential attenuation motivation for this lesson nearly 43% of rcas (28 of 65) listed radar sampling issues as a contributing factor sampling issues: e.g., small scale, brief, viewing angle, range rf, vcp contributing factors listed in root cause analyses (rcas) for missed tornado events quiz! quiz! quiz! quiz! quiz! quiz! negative cues negative cues negative cues negative cues negative cues negative cues beam heights beam heights beam heights beam heights beam heights beam heights aspect ratio aspect ratio aspect ratio aspect ratio aspect ratio aspect ratio radar horizon radar horizon radar horizon radar horizon radar horizon radar horizon feature location feature location feature location feature location feature location feature location beam blockage beam blockage beam blockage beam blockage beam blockage beam blockage viewing angle viewing angle viewing angle viewing angle viewing angle viewing angle side lobes side lobes side lobes side lobes side lobes side lobes nbf & diff atten nbf & diff atten nbf & diff atten nbf & diff atten nbf & diff atten nbf & diff atten radar sampling issues negative cues inferring something significant from the absence of data aren’t the kids kinda quiet in there? mom and dad are gonna love this!!! negative cues in meteorology major storm moves into major metropolitan area, but no reports are received no reports??? negative cues in meteorology warnings are not being transmitted by nwr or tv negative cues in meteorology time stamp for radar data stops updating surface observations go missing where there should be information forecaster misses or does not alert office to major radar signature and therefore no warning issued reflectivity suggests a supercell structure, but there is no apparent mesocyclone in velocity lack of sampling by radar due to overshooting low-level features radar height estimation & uncertainty actual height beam center height radar height estimation & uncertainty howard et. al. 1997 radar height estimation & uncertainty howard et. al. 1997 aspect ratio size of the feature compared to size of the beam determines how it is resolved aspect ratio radar horizon low-level, more detail midlevel & overshooting radar horizon ha ha! you missed! occurs when the radar beam overshoots a feature because of the curvature of the earth (ignoring elevation angle) mitigation step: switch to a closer radar, if possible feature location upper level features are projected directly below onto a map where feature is projected by radar where feature is on ground use caution when inferring precision of radar-based locations feature location speheger and smith, 2006 feature location speheger and smith, 2006 ktlx 14.0 ktlx 10.0 ktlx 4.3 ktlx 0.5 actual path beam blockage occurs when objects fully (or partially) obstruct the radar beam. complete beam blockage partial beam blockage side lobe contamination pieces of energy outside the main lobe intercept a target and return to the radar main lobe side lobe side lobe piltz and burgess (2009) ~54 dbz z gradient side lobe impactscumulative within 60 nm of radar main lobe side lobe side lobe main lobe beam blockage does missing data in oval mean no precip there? nope! beam blockage storms will appear weaker here then they actually are elevation side lobes example rda viewing angle different radars will sample different portions of the storm from different sides viewing angle switch radars if a better viewing angle is available viewing angle horizontal side lobes example rda nbf & differential attenuation heavy precip/ hail core nbf/differential attenuation possible non-uniform beam filling (nbf) gradient of φdp usually uniform across beam gradient can develop in hail storms at mid ranges or line of storms along radial results in low cc down rest of radial non-uniform beam filling (nbf) differential attenuation you’re getting weak mr. horizontal! differential attenuation lower zdr values along radial behind strong cores result of differential attenuation don’t confuse nbf and da… nbf results from φdp gradient in beam and lowers cc down radial differential attenuation results from attenuation differences between horizontal and vertical affecting zdr nbf stronger da stronger incorrect incorrect. side lobes will cause weak reflectivity returns clockwise from heavy cores or velocity shadows in supercell thunderstorms. continue continue continue continue continue continue incorrect incorrect. nbf will lead to radials of lower cc due to gradients of phidp continue continue continue continue continue continue incorrect incorrect. radar horizon explains why low-level features may be missed at long ranges continue continue continue continue continue continue incorrect correct! continue continue continue continue continue continue which limitation explains why features might have less detail at longer ranges? aspect ratio aspect ratio aspect ratio aspect ratio aspect ratio aspect ratio radar horizon radar horizon radar horizon radar horizon radar horizon radar horizon non-uniform beam filling (nbf) non-uniform beam filling (nbf) non-uniform beam filling (nbf) non-uniform beam filling (nbf) non-uniform beam filling (nbf) non-uniform beam filling (nbf) side lobes side lobes side lobes side lobes side lobes side lobes question #1 of 9 incorrect incorrect. because of sampling limitations, many times features on radar appear less impressive than in reality. continue continue continue continue continue continue correct correct! continue continue continue continue continue continue the majority of sampling limitations with the wsr-88d result in features appearing less impressive than the actual feature. true true true true true true false false false false false false question #2 of 9 incorrect incorrect. remember that negative cues are the “absence of data suggests something significant is occurring”. continue continue continue continue continue continue correct correct! continue continue continue continue continue continue identify all of the following scenarios that are negative cues. softball size hail is reported in a rural area softball size hail is reported in a rural area softball size hail is reported in a rural area softball size hail is reported in a rural area softball size hail is reported in a rural area softball size hail is reported in a rural area a surface observation site stops reporting after a storm moves over it a surface observation site stops reporting after a storm moves over it a surface observation site stops reporting after a storm moves over it a surface observation site stops reporting after a storm moves over it a surface observation site stops reporting after a storm moves over it a surface observation site stops reporting after a storm moves over it large hail is reported in a mountain town but the radar data does not suggest it large hail is reported in a mountain town but the radar data does not suggest it large hail is reported in a mountain town but the radar data does not suggest it large hail is reported in a mountain town but the radar data does not suggest it large hail is reported in a mountain town but the radar data does not suggest it large hail is reported in a mountain town but the radar data does not suggest it no warning recevied by media on a classic supercell moving into a city no warning recevied by media on a classic supercell moving into a city no warning recevied by media on a classic supercell moving into a city no warning recevied by media on a classic supercell moving into a city no warning recevied by media on a classic supercell moving into a city no warning recevied by media on a classic supercell moving into a city question #3 of 9 incorrect incorrect. lower portions of storms will actually appear stronger as the range decreases to the feature. continue continue continue continue continue continue incorrect incorrect. as range increases, the curvature of the earth causes the beam to get higher and higher above the ground. continue continue continue continue continue continue incorrect incorrect. as range decreases, the beam can exist lower to the ground. continue continue continue continue continue continue incorrect correct! continue continue continue continue continue continue the lowest portion of a storm detectable by radar becomes ______ as range _______. higher; increases higher; increases higher; increases higher; increases higher; increases higher; increases higher; decreases higher; decreases higher; decreases higher; decreases higher; decreases higher; decreases lower; increases lower; increases lower; increases lower; increases lower; increases lower; increases weaker; decreases weaker; decreases weaker; decreases weaker; decreases weaker; decreases weaker; decreases question #4 of 9 incorrect incorrect. aspect ratio usually causes things to appear weaker and slight larger than reality. continue continue continue continue continue continue incorrect incorrect. beam blockage will not affect feature location, but rather just feature strength or visibility. continue continue continue continue continue continue incorrect incorrect. typically radar circulations will be larger in size than the actual circulation. continue continue continue continue continue continue incorrect correct! continue continue continue continue continue continue why should locations of radar-identified circulations without ground truth include some form of uncertainty when conveyed to the public? the circulation on the ground may be displaced horizontally from the radar-identified circulation the circulation on the ground may be displaced horizontally from the radar-identified circulation the circulation on the ground may be displaced horizontally from the radar-identified circulation the circulation on the ground may be displaced horizontally from the radar-identified circulation the circulation on the ground may be displaced horizontally from the radar-identified circulation the circulation on the ground may be displaced horizontally from the radar-identified circulation the circulation on the ground may be larger than the radar-identified circulation. the circulation on the ground may be larger than the radar-identified circulation. the circulation on the ground may be larger than the radar-identified circulation. the circulation on the ground may be larger than the radar-identified circulation. the circulation on the ground may be larger than the radar-identified circulation. the circulation on the ground may be larger than the radar-identified circulation. beam blockage may be causing the circulation to be identified in the wrong location. beam blockage may be causing the circulation to be identified in the wrong location. beam blockage may be causing the circulation to be identified in the wrong location. beam blockage may be causing the circulation to be identified in the wrong location. beam blockage may be causing the circulation to be identified in the wrong location. beam blockage may be causing the circulation to be identified in the wrong location. the aspect ratio of the circulation to the size of the radar beam may make the circulation appear smaller than it is in reality. the aspect ratio of the circulation to the size of the radar beam may make the circulation appear smaller than it is in reality. the aspect ratio of the circulation to the size of the radar beam may make the circulation appear smaller than it is in reality. the aspect ratio of the circulation to the size of the radar beam may make the circulation appear smaller than it is in reality. the aspect ratio of the circulation to the size of the radar beam may make the circulation appear smaller than it is in reality. the aspect ratio of the circulation to the size of the radar beam may make the circulation appear smaller than it is in reality. question #5 of 9 incorrect incorrect. switching radars or change elevation are the only options short of cutting down the blockage (which would be tough for a mountain :)) continue continue continue continue continue continue correct correct! continue continue continue continue continue continue how can you overcome missing data due to beam blockage? switch to a different radar that does not have beam blockage in that area switch to a different radar that does not have beam blockage in that area switch to a different radar that does not have beam blockage in that area switch to a different radar that does not have beam blockage in that area switch to a different radar that does not have beam blockage in that area switch to a different radar that does not have beam blockage in that area go up in elevation where the beam blockage is not an issue go up in elevation where the beam blockage is not an issue go up in elevation where the beam blockage is not an issue go up in elevation where the beam blockage is not an issue go up in elevation where the beam blockage is not an issue go up in elevation where the beam blockage is not an issue activate attenuation correction at the rda to help recover lost signal activate attenuation correction at the rda to help recover lost signal activate attenuation correction at the rda to help recover lost signal activate attenuation correction at the rda to help recover lost signal activate attenuation correction at the rda to help recover lost signal activate attenuation correction at the rda to help recover lost signal raise the height of the radome using the adjustable-height pedestal raise the height of the radome using the adjustable-height pedestal raise the height of the radome using the adjustable-height pedestal raise the height of the radome using the adjustable-height pedestal raise the height of the radome using the adjustable-height pedestal raise the height of the radome using the adjustable-height pedestal question #6 of 9 incorrect incorrect. weaker signals could cause faulty velocity estimates, but viewing angle is not related to this. continue continue continue continue continue continue incorrect incorrect. this is possible but is not the primary reason. continue continue continue continue continue continue incorrect incorrect. non-uniform beam filling only affects cc. continue continue continue continue continue continue incorrect correct! continue continue continue continue continue continue viewing angle primarily affects velocity interpretation because: radar only measures the radial component of velocity. radar only measures the radial component of velocity. radar only measures the radial component of velocity. radar only measures the radial component of velocity. radar only measures the radial component of velocity. radar only measures the radial component of velocity. non-uniform beam filling (nbf) may bias velocity estimates. non-uniform beam filling (nbf) may bias velocity estimates. non-uniform beam filling (nbf) may bias velocity estimates. non-uniform beam filling (nbf) may bias velocity estimates. non-uniform beam filling (nbf) may bias velocity estimates. non-uniform beam filling (nbf) may bias velocity estimates. beam height estimates from one radar may make the circulation appear closer to the ground than it is in reality. beam height estimates from one radar may make the circulation appear closer to the ground than it is in reality. beam height estimates from one radar may make the circulation appear closer to the ground than it is in reality. beam height estimates from one radar may make the circulation appear closer to the ground than it is in reality. beam height estimates from one radar may make the circulation appear closer to the ground than it is in reality. beam height estimates from one radar may make the circulation appear closer to the ground than it is in reality. differential attenuation may weaken the signal too much to get accurate velocity estimates. differential attenuation may weaken the signal too much to get accurate velocity estimates. differential attenuation may weaken the signal too much to get accurate velocity estimates. differential attenuation may weaken the signal too much to get accurate velocity estimates. differential attenuation may weaken the signal too much to get accurate velocity estimates. differential attenuation may weaken the signal too much to get accurate velocity estimates. question #7 of 9 incorrect incorrect. while it’s possible elevation side lobes can penetrate the cone of silence, this is not a big deal continue continue continue continue continue continue incorrect incorrect. this is related to nbf, not elevation side lobes. continue continue continue continue continue continue incorrect incorrect. it is very unlikely there will be stronger echoes above very weak echoes in this region leading to vertical side lobe issues. continue continue continue continue continue continue incorrect correct! continue continue continue continue continue continue elevation side lobe contamination is often most noticeable in ___________________. the inflow region of supercell thunderstorms the inflow region of supercell thunderstorms the inflow region of supercell thunderstorms the inflow region of supercell thunderstorms the inflow region of supercell thunderstorms the inflow region of supercell thunderstorms the stratiform region of squall line thunderstorms the stratiform region of squall line thunderstorms the stratiform region of squall line thunderstorms the stratiform region of squall line thunderstorms the stratiform region of squall line thunderstorms the stratiform region of squall line thunderstorms down radial of intense hail cores down radial of intense hail cores down radial of intense hail cores down radial of intense hail cores down radial of intense hail cores down radial of intense hail cores within the cone of silence within the cone of silence within the cone of silence within the cone of silence within the cone of silence within the cone of silence question #8 of 9 incorrect nbf and differential attenuation primarily affect only the dual pol variables. continue continue continue continue continue continue correct correct! continue continue continue continue continue continue non-uniform beam filling (nbf) primarily affects ____ whereas differential attenuation affects ____. correlation coefficient (cc); differential reflectivity (zdr) correlation coefficient (cc); differential reflectivity (zdr) correlation coefficient (cc); differential reflectivity (zdr) correlation coefficient (cc); differential reflectivity (zdr) correlation coefficient (cc); differential reflectivity (zdr) correlation coefficient (cc); differential reflectivity (zdr) radial velocity (v); reflectivity (z) radial velocity (v); reflectivity (z) radial velocity (v); reflectivity (z) radial velocity (v); reflectivity (z) radial velocity (v); reflectivity (z) radial velocity (v); reflectivity (z) correlation coefficient (cc); reflectivity (z) correlation coefficient (cc); reflectivity (z) correlation coefficient (cc); reflectivity (z) correlation coefficient (cc); reflectivity (z) correlation coefficient (cc); reflectivity (z) correlation coefficient (cc); reflectivity (z) radial velocity (v); spectrum width (sw) radial velocity (v); spectrum width (sw) radial velocity (v); spectrum width (sw) radial velocity (v); spectrum width (sw) radial velocity (v); spectrum width (sw) radial velocity (v); spectrum width (sw) question #9 of 9 lesson complete! <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style LeadingMargin="0" FirstLineMargin="0" Justification="Left" ListLevel="1" LineSpacingRule="Multiple" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Plain" Start="0" Color="#000000,00" BulletChar="•" BulletFont="Arial"> <BulletPicture Size="0x0" /> </ListStyle> </Style> <Span Text=" Welcome to the Radar & Applications Course (RAC) Principles of Doppler Weather Radar. This lesson covers common radar sampling issues with the WSR-88D. Let’s get started!"> <Style FontFamily="+minor" FontSize="11" FontIsBold="False" FontIsItalic="False" FontIsUnderline="False" FontIsStrikeout="False" ForegroundColor="dk1,00" Elevation="Normal" DisplayCase="AsIs" LinkColor="dk1,00" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style LeadingMargin="0" FirstLineMargin="0" Justification="Left" ListLevel="1" LineSpacingRule="Multiple" LineSpacing="20" SpacingBefore="4.31999969" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Plain" Start="0" Color="#000000,00" BulletChar="•" BulletFont="Arial"> <BulletPicture Size="0x0" /> </ListStyle> </Style> <Span Text=" Here is the complete roadmap for the entire “Principles of Doppler Weather Radar” section of RAC. You are currently in the Radar Sampling Issues portion of this topic. Let’s keep going!"> <Style FontFamily="+minor" FontSize="11" FontIsBold="False" FontIsItalic="False" FontIsUnderline="False" FontIsStrikeout="False" ForegroundColor="dk1,00" Elevation="Normal" DisplayCase="AsIs" LinkColor="dk1,00" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" There are ten learning objectives for this lesson. Here are the first five. Please take a moment to review these objectives, as the quiz at the end of this lesson is based on these objectives."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Here are the last five learning objectives. Feel free to go back and forth between this slide and the previous one as needed."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" A root cause analysis study revealed that in 65 missed tornado events, 28 of those events (or nearly 43%) listed radar sampling issues as a contributing factor. As a result of this research, understanding common radar sampling issues helps forecasters mitigate these issues and avoid future missed events. Let’s take a look at these common issues."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Radar sampling issues come in all shapes and sizes. Some can make interpretation almost impossible, while others are just annoyances. However, when we understand how these situations can occur, then we can take steps to mitigate their impacts (if possible). This lesson introduces 9 common issues that are shown on the left. Click on the button for each issue to learn more. When you are done with each section, you will be directed back to this page. After completing all the sections, a button shall appear to take you to the quiz."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Before we tackle actual sampling limitations, we need to discuss the concept of negative cues. Many of the sampling limitations we’ll discuss in this lesson impact forecasters by masking radar signatures and making them appear LESS IMPRESSIVE than they are in actuality. In some situations, the sampling issue masks the feature altogether. In these situations, we must infer that something significant is present by the absence of data. This task requires the observation of negative cues. 
"> <Style FontSize="11" /> </Span> </Block> <Block> <Style /> <Span Text=" To better understand concept of negative cues, let’s look at an everyday situation. Let’s say a group of families gather at a house. The parents talk in one room while the kids play in another room. After some time, the parents notice the kids’ room is quiet. I mean too quiet. The parents notice there is NO noise coming from the kids’ room which gets their attention. The absence of noise when noise is expected provides a negative cue to the parents that something significant could be occurring in the kids’ room."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Let’s take a look at some examples of negative cues in the warning world. Our first example shows a classic supercell with a hook echo moving into a major metropolitan area. With such a large population being affected by this storm, you would certainly expect reports coming into the office, right? What if your office received no reports at all? Would that seem suspicious? The lack of reports from what appears to be a very severe storm should signal that something isn’t right. The lack of calls may be due to the storm’s damage, either because people aren’t thinking of calling the office. Maybe you’ve lost your phone lines and you can’t receive incoming calls. Whatever it is, the lack of reports coming into the office is a negative cue that something significant could be happening."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Here’s another example from real life. Your office sees this storm shown on the left and decides to issue a tornado warning (which is a good decision). However, you keep getting calls and tweets from the public and media partners asking, “Are you going to issue a Tornado Warning?” When you tell them that you have issued one, they proceed to tell you that they are getting no indication of a warning…even from weather radio. Oh, and by the way, they are also getting reports of a tornado.
"> <Style FontSize="11" /> </Span> </Block> <Block> <Style /> <Span Text=" In this situation, your partner is observing the negative cue with this storm that should have a Tornado Warning. What does this situation tell you? Maybe you issued a warning for the wrong storm. Maybe you forgot to click that last button to submit your product in WarnGen. Maybe the NWS network is down and warnings aren’t being disseminated to the public. You can learn a lot from the negative cues that others observe. "> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Numerous examples of negative cues exist in warning operations. However, how do these apply to radar sampling issues? Well, this slide lists some negative cues that are related to radar interpretation. One of the main points of this lesson is radar sampling issues often lead to negative cues in a warning environment. Therefore, have a good grasp on radar sampling limitations and you’ll be a better warning forecaster because you will be better equipped to observe and respond to negative cues."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Recall the radar beam spreads out as the pulse travels away from the radar, making the sampling area wider and wider. For locations as close as 60-70 miles from the radar, the beam’s width extends over 1000s of feet. Therefore, an object detected at the edge of the beam, like this cloud here, might have an actual height of 22,000 feet, but the beam center is located at 18,000 feet. So, the radar will think the cloud echo is at 18,000 feet even though it is much higher. Just keep this fact in mind when interpreting radar echo heights in AWIPS."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" A study done by Howard et. al. (1997) actually plotted the echo top heights as a function of range from the radar. Notice how all those radar echo top heights fall nicely along straight lines. Do you think this plot shows reality? Absolutely not! This graphic illustrates how using the beam center height to compute echo top heights is not the most accurate. In reality, storms exist in a continuous space, but the radar can only measure echo heights at discrete levels. So, what uncertainty exists in these height estimates? We’ll look at that next."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" This first graphic shows the uncertainty (in meters) of the target height estimate for each elevation angle in one of the original VCPs intended for general weather surveillance. This color scheme is a little unusual in that red shades are good, and green and blue tones are bad! Notice how accurate the estimates are within 50 km range and below 4 km height. Almost all estimates are within 1000 m. As height or range increases, the uncertainty increases to as much as 3 to 7 km due to the gaps in the scanning strategy. 
"> <Style FontSize="11" /> </Span> </Block> <Block> <Style /> <Span Text=" Now, look at a similar graphic for a different VCP that was intended for severe convection that has more tilts in the volume scan. The uncertainty increase happens at further ranges (past 75 km) and higher heights (above 8 km). So, choosing a scanning strategy with more elevation scans reduces the likelihood of uncertainty in your echo top height estimates. "> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" A target’s size compared to the size of the radar beam is called the aspect ratio. Apsect ratio matters because the radar beam spreads out with increasing range. So, a target of constant size will look different in the radar base data at different ranges. Take, for example, the idealized circulation shown on this slide. Close to the radar, the radar needs multiple beams to sample the circulation, providing a more detailed view of the feature. At medium ranges, the circulation may be roughly the size of the beam. So, you still get some detail, but not as much as you would at close ranges. At far ranges, only a portion of the beam is sampling the feature. In these cases, you’ll either get very little detail or the feature may not be resolvable at all. In other words, the greater the targets range, the less impressive a feature will appear because less detail is apparent."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" To further illustrate the concept of aspect ratio, we show an example of the same storm that is viewed to the south of a radar at three different ranges. I should note this example is from an old application that predates both the dual-pol and super-res upgrades to WSR-88D, so we will focus on the reflectivity and velocity displays. The displays show what the same storm looks like with range from the radar increasing from left to right among the examples. Notice the details you can see on the left at the closest ranges, especially the reflectivity gradient on the southeast side of the storm. At further ranges, the reflectivity appears less detailed, almost blocky. Remember that the beam will be observing the storm at a higher altitude at these farther ranges. Still, you can see how the storm might appear less intense when looking at the farther ranges as compared to the closer distances.
"> <Style FontSize="11" /> </Span> </Block> <Block> <Style /> <Span Text=" Switching over to velocity shows a similar pattern. On the closest look, you can see more details of the circulation. At the further ranges, the circulation appears weaker."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" This example shows a supercell thunderstorm producing a tornado moving through a major metro area. The reflectivity and velocity images on the left show the storm from a radar located to the east at very close range. The comparable images on the right display data from a radar located much further away to the northwest of the storm. Notice how the images on the left show more details in both products. The images on the right show the storm’s structure at midlevels, where the radar beam is overshooting the low-level hook echo in reflectivity, among other features. Therefore, if low-level features are your primary concern, then make sure you keep this limitation in mind and use the closest radar to observe these features. "> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Recall, as the beam propagates away from the radar, the earth’s curvature causes the radar pulse to sample areas of increasing height as its range from the radar increases. In practical terms, this relationship of increasing beam height with increasing range means low level features will not be seen by the radar at far ranges. To mitigate this problem, forecasters can switch to a closer radar when one is available. When you can’t switch to a closer radar, forecasters are forced to draw conclusions about unseen storm features based on the data you do have. Let’s look at an example on the next slide. "> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" As mentioned in another area of this lesson, the radar beam samples higher elevations of the atmosphere as its range from the radar increases. However, radar imagery projects these data onto the ground surface directly below that point. Many meteorological targets tilt vertically. In other words, features observed in the radar beam are not vertically stacked. For example, tornadic circulations will likely not be located directly underneath the midlevel mesocyclone. You can see from the illustration on the slide that the surface projection based on radar is here, but the actual surface feature is located here. Another example would be hail cores, which travel horizontally as they fall and not be located directly underneath the hail core aloft. So, what’s the lesson here? Be careful about how much precision you infer from a circulation’s location based on what you see on radar. "> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" The scatter plot shown here comes from a study by Speheger and Smith (2006). The diagram identifies the distance between where a radar identified circulation exists relative to the actual circulation location at the surface. Within 50 miles of the radar, errors remain small, averaging less than 3/4 mile and usually less than 2 miles. Once you get past a range of 50 miles, the distribution becomes more noticable. In this study, several circulations had errors as large as 4-8 miles! Therefore, be careful with how precisely you portray feature locations on radar compared with where they occur on the ground."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Let’s look at a specific example illustrating this issue, also from Speheger and Smith (2006). Notice how, as the radar elevation angle increases, the distance between the radar circulation location and the actual tornado path increases. So, again, be careful assuming that a radar-determined location precisely identifies the surface circulation location, especially on the higher tilts."> <Style FontSize="11" /> </Span> <Span Text="
" /> </Block> <Block> <Style /> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" When tall objects reside near the radar, these objects can obstruct the beam and cause a power loss significant enough where either no targets down radial from the object will be visible at all, or the targets will appear much weaker than if the obstruction was not present. Objects that can obstruct the beam include nearby trees, buildings, or even a mountain range. In the example shown, the trees block the entire beam where as the higher beam experiences only partial beam blockage."> <Style FontSize="11" /> </Span> <Span Text="
" /> </Block> <Block> <Style /> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" Recall from earlier lessons that some of the transmitted energy propagates outside the main lobe in areas called side lobes. During some situations, targets reflect this energy back to the radar and corrupt the returns collected in the main lobe (Piltz and Burgess, 2009). Side lobes can occur when a strong storm has a significant azimuthal reflectivity gradient…usually where low-level inflow enters the storm’s updraft. The gradient should be at least 54 dBZ over 3 degrees of azimuth of the main beam for the primary side lobe to contribute. Secondary and tertiary side lobes exist between 5 and 8 degrees, as well. Sidelobe contamination is cumulative in nature. In other words, the returned power from all side lobes all around the main beam need to be 54 dBZ stronger than the main beam. That contribution can come from multiple areas of the storm impacted by the side lobe. Likewise the storm core in question needs to be relatively close to the RDA, say 60 nm or less, for side lobe contamination to be possible."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" Let’s look at an actual example of beam blockage near a radar. The lowest tilt (top left panel of both images) shows a data gap to the northeast of the radar. Does this gap mean no precipitation is present here? Nope. An obstacles blocks the radar from seeing down radial targets in this area. In fact, the blockage looks complete. So, what might we have missed due to this beam blockage? In this case, there was a tornado with the storm located on the left edge of the beam blockage. Some cyclonic shear is visible aloft. However, we don’t know if the radar would have shown the circulation better near the surface because the data are unavailable."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" This example shows a situation where there is partial beam blockage to the west-southwest of the radar. In instances like this one, remember that any storms down radial of the blockage will appear weaker than if no blockage was present. Therefore, you will need to make some mental adjustments to your expectations."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" This next example results from side lobe contamination in mostly the vertical (or elevated) direction. The radar (located to the ENE) samples the storm at a range of 40 nm. As in the previous example, the reflectivity gradient on the inflow side of the storm lines up nearly parallel to the radar beam at 0.5 degrees. Unlike the previous example, the horizontal reflectivity gradient here is only around 40 dBZ over 3 degrees. Yet, the velocity and storm-relative motion look suspicious compared to most mesocyclones. If we look aloft, at 3.4 degrees, we see stronger Reflectivity values in that same area than we did in the storm inflow at 0.5 degrees. Notice, also, how the velocity values in these areas match the core aloft better than the core at the surface. The Spectrum Width is very high, also, in the same area where the velocity values are anomalously high.
"> <Style FontSize="11" /> </Span> </Block> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" A later lesson in RAC will discuss in more detail these situations and what steps to follow in order to identify them. "> <Style FontSize="11" /> </Span> <Span Text="
" /> </Block> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=""> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style /> <Span Text=" In several places in the US, especially in the Central & Eastern US, storms are often sampled by multiple radars. Storms lack symmetrical structure both in the horizontal and in the vertical, so getting multiple views of a storm improves storm sampling and generally helps you see what you want to see. Forecasters need to know when a better viewing angle for a storm is available. Let’s look at a few examples."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" Recall that your radial velocity measurements depend on your viewing angle. When the radar’s beam aligns with the wind direction, wind speed measurement accuracy maximizes. When the radial looks perpendicular to the wind direction, the radar can’t measure the wind speed at all. Usually, the radar observes an individual storm at some angle in between. 
"> <Style FontSize="11" FontIsBold="False" FontIsItalic="False" FontIsUnderline="False" FontIsStrikeout="False" /> </Span> </Block> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" This image shows the same radial velocity data when viewed from 3 radars: one located north of the echo on the left, one located east of the echo in the center, and one located south of the echo on the right. Notice how the radial velocity changes depending on the radar’s viewing angle, even though the actual wind field remains the same. Therefore, know your radar’s location relative to your area of interest, and switch radars when another site provides a better look…especially when looking at velocity signatures. "> <Style FontSize="11" FontIsBold="False" FontIsItalic="False" FontIsUnderline="False" FontIsStrikeout="False" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" Viewing angle impacts Radial Velocity the most frequently of all the base data, but other products, such as Reflectivity, are not immune. Three-body scatter spikes (TBSSs) often appear down radial of significant hail cores. The example on the slide show an example of a TBSS to the northwest of the storm’s core. If another storm was located in this area, this signature might not be visible from this radar. To see the TBSS, you would likely need to switch to a radar with a different viewing angle. Likewise, data from other products in a TBSS region can be corrupted. If the radar beam passes through a hail core and then through a mesocyclone, then the velocity data for the mesocyclone could be negatively impacted by the TBSS. The solution to this problem: Pick a different viewing angle."> <Style FontSize="11" FontIsBold="False" FontIsItalic="False" FontIsUnderline="False" FontIsStrikeout="False" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" Here’s a good example of primarily horizontal side lobe contamination. The radar (located to the NNE) samples the storm at a range of ~50 nm. A fairly tight gradient of reflectivity exists on the east side of the storm. Directly to the east of, and at the same range as, the storm core, a weak reflectivity feature protrudes out to the east of the storm. In this area, side lobe contamination has occurred. Notice the ZDR and CC values in this area aren’t indicative of precipitation. If we switch over to the other products, you see the HC algorithm has tagged the data as biological. Notice how the radial velocity values in this area better match those from the storm core then those in the adjacent radials.
"> <Style FontSize="11" /> </Span> </Block> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" The storm motion (218 degrees at 59 kts) takes this storm on a path roughly parallel to the radial. This orientation allowed the side lobe contamination to occur for several volume scans. Fortunately, forecasters can quickly identify the corrupted data and see that it doesn’t impact their interpretation of the key feature in the velocity data: the mesocyclone located directly to the west of the bad data. However, data corruption can be more difficult to spot and result in poor warning decisions if forecasters are not careful. Let’s look at another example to see how that can happen."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" The last two sampling issues on our list impact the dual-pol variables of Correlation Coefficient (CC) and Differential Reflectivity (ZDR), albeit in different ways. The first artifact, non-uniform beam filling, impacts CC while the second artifact, differential attenuation, impacts ZDR. As you’ll see on the next few slides, these two sampling issues often occur in tandem, but they are caused by completely different processes. The most common situation occurs when strong hail cores exist, especially when the cores are aligned down a radial from the radar. Let’s look at both of these issues in more detail."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" In most circumstances, differential phase remains uniform across the radar beam. One instance where this condition isn’t true occurs when hail storms are observed at medium ranges from the radar. Under these conditions, the radar beam can sample significant hail melt across the beam. When this happens, differential phase experiences little phase shift at the top of the beam, but a large phase shift toward the bottom of the beam where more liquid hydrometeors exist. This gradient in phase shift across the beam causes Correlation Coefficient to be reduced from that point and the rest of the gates located down radial. This condition, called non-uniform beam filling (or NBF), occurs down radial from the hail core. Now let’s look a similar situation where NBF can occur."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" This image shows storms that are causing non-uniform beam filling issues. Can you determine which storms are causing problems? Does it help when we switch over to CC? Hopefully, you said yes. The storm to the northwest caused significant NBF issues, as is visible in the reduced CC values down radial from the storm’s core. Just north of the radar, another storm appears to have some NBF issues, too. The CC values drop less prominently, but they are still noticeable."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" Now let’s move on to the second, related sampling issue that impacts ZDR. The radar transmits its pulse with two polarizations. As the pulse propagates through the atmosphere, each polarization usually attenuates (or slows down and weakens) at the same rate because most hydrometeors don’t attenuate S-band radiation significantly. In some cases, however, the horizontal pulse will attenuate significantly, but the vertical pulse will not. These situations occur when the pulse travels through storms with very heavy rain and hail cores, or when several storms are aligned along a radial. In these cases, the precipitation impacts the vertical pulse far less than the horizontal pulse. As a result, the differential reflectivity skews toward lower values down radial of differential attenuation regions than they would otherwise be. In fact, the values are often negative. Let’s look at an example."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" This example should look familiar. It’s the same case we just showed for the non-uniform beam filling example. Remember how we said that NBF and differential attenuation often occur in the same areas. Well, let’s look downstream of the two storms we noted before. We don’t see a noticeable change in Reflectivity even though there is probably some attenuation happening in that channel. Switching over to ZDR, we can see the attenuation more clearly down radial of both of these heavy rain and hail cores. These areas of negative ZDR oriented along the radial result from differential attenuation."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Style FlowDirection="LeftToRight" LeadingMargin="0" TrailingMargin="0" FirstLineMargin="0" Justification="Left" LineSpacingRule="Single" LineSpacing="20" SpacingBefore="0" SpacingAfter="0"> <ListStyle ListType="None" ListTypeFormat="Parentheses" BulletFont="Arial" /> </Style> <Span Text=" As I showed previously, NBF and differential attenuation often occur in similar situations. The causes include the radar beam passing through a heavy rain and hail core or strong storms aligned along a radial. However, the physical reasons they occur are very different. NBF results from a gradient in Differential Phase, or PhiDP, within the radar beam that reduces correlation coefficient values for all the down radial range bins. Differential attenuation results from the horizontal channel being attenuated more in these situations than the vertical channel, causing ZDR to be lower than expected. It may not seem like a big deal that these are different processes, but it helps to know the differences when you only observe one phenomenon, but not the other."> <Style FontSize="11" /> </Span> </Block> </Content> </Document> <Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Content> <Block> <Span Text=" Thanks for your attention! You are now complete for this lesson." /> </Block> </Content> </Document>