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Adjusted Radar Comparisons at USGS Rain Gages
near Irwin Creek, Charlotte, NC

(includes Level2 vs N0Q comparison)
Apr 1, 2013 to  Sep 30, 2013 COMPLETED

NEXRAD Radar rainfall adjustment

A do it yourself real-time radar rainfall adjustment methods using free data and software.  5-minute radar over a 30.5 sqmi catchment is adjusted using 1, 2a, 2b, 3 and 4 Wundergages and evaluated using 6 USGS tipping buckets and runoff at a USGS streamflow gage.  If I can, anybody can do it.


As a radar beginner wrestling with NEXRAD radar for 3 years, I finally figured out what wiser men have said all along... and that is that radar rainfall must be adjusted.  It is time to put emphasis on ADJUSTING THE RADAR instead of looking at ADJUSTING THE RUNOFF.  The runoff did not need help but has been honed sharp, it just needed better rainfall.  Anyone wishing to do so, can download my free Windows programs (see links at top) to do the same thing shown here in real-time or for historical analysis. Any questions are most welcomed. (yes, that's The King, pre STP colors working at a Charlotte gas station.... so you too can be somebody someday with the right radar colors.)



Radar N0Q 0.6 mile pixel versus USGS 8" tipping bucket


There is an old saw (a NWS rationalizaion) that you cannot expect to get an 0.6 x 0.6 x 0.6 mile volume of rainfall to fit into an 8 inch rain gage with the wind blowing from 10,000 feet and therefore the obvious collary would be you cannot adjust a radar value written from on high with an 8 inch rain gage.... poppycock!  Only a radar rube like me would have fallen for that line of reasoning.  Dang them, they should stop misleading us poor rubes.  You most certainly can and if you don't you are a bigger rube than I!


Although I have found that the radar seasonal error can be corrected quite well by a single constant correction factor for rainfall/snowfall accumulations for climate studies like snowpack and melt, rainfall-runoff at the catchment level requires a basin correction factor on at least an hourly basis (if not more?) applied to the 5-minute radar rainfall from a standard z-r relation (cool or convective).  This is equivalent to changing the 'a' in Z=a*R^b each hour, so let's get to it.


Study Setup:

I did a web search for professional rainfall gage networks that are available on line in real-time.  I found that the USGS has had a network available for the Charlotte, NC area that has been collecting 5-minute rainfall since 1992.  Their network location link is:  http://nc.water.usgs.gov/char/rainfall.html


The rainfall data site ID's are 15 digits instead of 8 and so I had to update GetRealtime to recoginize these ID's.  And as long as we are at it, I have choosen a runoff basin and selected the 6 rain gages in it for comparisons.  The basin selected was for the 30.5 sqmi USGS gaged basin at Irwin Creek near Charlotte, NC.  The basin centroid is 81 miles east by northeast of the GSP radar and mid-beam elevation is 8,600 ft.  The N0Q 1000x1000 image pixels are 0.57 miles x 0.69 miles.  Each gage has a decimal pixel location where a 0.5 would be the ideal center of a pixel.


The 6 USGS raingages are:














So now we have known points that we can stuff 0.6 mile pixel rainfall in.  My way to adjust radar is to use free Wunderground Weather Stations.  They usually have 5-minute time steps so that the hourly total will be near hour hh:00 and are available online for free in REALTIME.  I weeded out some of the closer Wundergages and found these three that look very reliable, but I have fallen for bad ones before so we shall see (and I did... WG South has been replaced 4/19/2013 and again 5/6/2013.  Wundegrage East had began having incomplete days and was replaced 5/21/2013 and then that one was replaced yet again on 6/2/2013.. and again 6/13/2013.)


In a real world application, I would imagine one could talk to the Wundergage owners and slip them a fin to inspect there setup and do some QA or have your employees or relatives install and operate Wundergages at their homes at strategic locations.


The 3 Wundergages located on the location screen below are (ok, 2 have changed slightly but lat/long and distances below are correct for current use):

Wundegrage East is KNCCHARL42 at 10.2 miles from basin centroid


Wundegrage South is KNCCHARL76 at 6.3 miles from basin centroid


Wundegrage West is KNCGASTO8 at 12.2 miles from basin centroid



A 4th Wundergage was added to the north to see if that would hlep, it didn't:

Wundegrage North is KNCCHARL12 at 8.3 miles from basin centroid



So far I have found using the hourly ratio (not daily, thats a last resort) of the Gage/Radar to determine an hourly adjust factor to apply to each 5-minute radar value in that hour and that is what we will do here.  The adjustment to the 5-minute radar at each USGS rain gage will be the hourly 3WgageAvg / 3RadarAvg.  You could use the sums instead of averages but the sums do not compare graphically as nice and looses some connotation.  And should a Wundergage fail, the averages could still give a good ratio but the totals would not.  GetRealtime Help web page describes the needed setups and its Setup Wizard will help you figure it out.  Remember we are using 5-minute hourly totals and that hopefully the only ratios that will be computed are viable ones above 0.05 inches for the hour at the Wundergage.  But on the other hand setting the 0.05 too high allows radar over estimation to go uncorrected or even zeroed out.  There are other refinements to this method that the user can make, like the 0.05 test before ratioing, then what upper and lower limits of computed ratio to use.  These factors can make a big difference as we shall see.... for the worst at extreme times... so 20 times radar is not encouraged.  My steups are:



COMPUTE-Hour; 10654; Rainfall; Irwin Cr, NC Avg of 3 Gages; 0; 0; (P1+P2+P3)/N

COMPUTE-Hour; -10654; Rainfall; Irwin Cr, NC Avg of 3 Radars; 0; 0; (P1+P2+P3)/N

COMPUTE-hour; 31654; Ratio; Irwin Cr, NC Avg of 3 Gages; 0; 0.05,1,5; P1/P2

.... at a gage adjustment example...

COMPUTE-unit; 11655; Rainfall; CHARLOTTE RAINGAGE Adjusted Radar; 0; -1, 1, 0.4, 0.5; P1*P2

Try to use the 11 datatype for adjusted rainfall... it really gets confusing so remember to use 11 or -11 instead of 10.  Ha, you won't believe this but it's the DataType stupid that determines the computation methods in GetRealtime.  I changed the code so 11 would work same as 10 for rainfall so my DatatypeSiteID' like 11654 would work like 10654 but in doing so I forgot to actually change the database datatypes from 10 to 11 and so doy!  It worked the same without the change.  Now I have the 11 datatype with no real purpose.  Dang!  So 10654, 11654, or even 32654 would work just as well  if the datatype is 10.  Live and learn... but I still like to look at 11654 and know it's adjusted even if I forgot to change the datatype from 10 to 11.


For the database GetAccess table 'rsite' you can use GetRealtime with the Setup Wizard with the above setup values and it will suggest the 'rsite' table setup.  All gage times are converted by GetRealtime to my computer's time which is Pacific Time (rsite -3) as well as the radar time which is done automatically.  And yes, keeping track of this setup stuff can cause a brain clot but I do not know how to make it simpler safely.  So just pace yourself and use Notepad and some junk mail envolope notes like I do.  And dont blame me, the NWS should adjust every pixel for us.  Ha!  The NWS radar rainfall products like N1P are complete duds as I have noted in several of my earlier learning studies and should be avoided.  Admittedly, I seem to pick sites 80 miles or more from the radar so this may have something to do with it.  On the other hand, I bet even N1P could be adjusted using my adjustment methods, but why would you?  Its for an hour, has 1.2 mile squares and has real bad rainfall precision steps so just stick with 5-minute 0.6mile x 0.5 dbz precision of N0Q for realtime or 0.15 mile x 0.5 dbz Level2 for historical studies.  Perhaps my next radar adjustment study will be to redo these rainfall adjustsments but with Level 2's 0.15 mile resolution to see if that is way over kill  or not.  For now let's just see how things work in real-time with N0Q.  A guy could make a career out of this stuff if it paid something and anyone really cared. (Update:  Level2 versus N0Q has been added at the bottom.)


W=Wundergage location, Gn=USG gage locations, and the Irwin Creek basin radar pixel boundary is shown:  


Boy, 20.8 miles is sure a long ways but these can be typical distances in areas of sparce Wundergages.  If we can't get these distances to work out then I will rethink a Plan B.  Maybe some full radar screen adjustement based on distance and sector (Hey, dont we pay these NWS guys to just that???).


Something to note about adjusting radar is that when the radar says near zero, not much way to adjust it. (perhaps using the dbz below 20dbz=0.02 may help here???) Happens again and again but dont fall for setting your adjust limit to 20... it will bite you bad... I would stick to 5 tops... and a lower limit of 0.3 seems to be where things are headed.  I quess this is why radar rainfall is usually obtained from a commercial radar provider so as to let them figure this all out but you will soon see why you may be in a better position and motivated to look into how to adjust your sites best.  But already having suffered through a brain clot or 2, you're nearly home.  Here is adjusted radar rainfall in all it's magnificent glory!!!


APRIL 4, 2013--N0Q Radar and 3Gage Adjusted Radar at USGS Rain Gage Comparison:

I hate to jinx this study so early but boy, I had hoped these adjustments would be alot better.  I can see I may be refinining GetRealtimes ratioing methods as experience gets experienced.

% Difference is the error in cumulative rainfall, a measure of bias, like runoff cares.

MAE % is the mean of the individual hourly unsigned differences, like runoff does care.


APRIL 11, 2013--N0Q Radar and 3G Adjusted Radar at USGS Rain Gage Comparison in GetGraph:  Note that the 2nd graph, Chraroltte SOUTH Wgnd shows zero rainfall while the violet radar shows 0.5".  This screwed up the whole adjustment but thats par for the radar course. ** I feel another brain clot **  The last 6 graphs at the USGS gages show the adjusted radar in violoet.  These screw ups tend to fade in time as bad Wundergages get weeded out.


APRIL 19, 2013--N0Q Radar and 3G Adjusted Radar at USGS Rain Gage Comparison (BAD RESULTS):  I thought that the Wundergages were just too far away so I tried using just the one closest gage but that gave even worse results.  For this storm, setting the 'Gage Minimum for Hourly Ratios' from 0.05 down to 0.02 would fix the trailing bad radar values near hour 18:00 but lets see what the next storm does before changing.  Using 0.02" would reduce the %Diff from +28% to -6% at Gage #1 and would drop from +78% to +35% at Gage #6.


APRIL 28, 2013--N0Q Radar and 3G Adjusted Radar at USGS Rain Gage Comparison:

The 'Gage Minimum for Hourly Ratios' has been lowered from 0.05 down to 0.02.  MAE's are still pretty poor.


May 5, 2013--N0Q Radar and 3G Adjusted Radar at USGS Rain Gage Comparison:

The 'Gage Minimum for Hourly Ratios' is 0.02".  Results are still pretty poor.  South Wunder Gage was dead so radar adjustment graphs use the 2Gages West and East Wundergages.


May 19, 2013--N0Q Radar and 3G Adjusted Radar at USGS Rain Gage Comparison:

The 'Gage Minimum for Hourly Ratios' is 0.02".  Results are the worst yet for 2 gage ratio.  The East Wundergage was down during the morning when it was not raining and may be off???.  (update: The East Wundergage failed again and was replaced May 21)  The storms were scattered thunderstorms where it was only raining at each Wundergage at different times so that the peak adjustments were really more like a single gage adjustment.  For thunderstorms it would be easy to think the 'Maximum Usable Ratio' should be reduced from winter's 5 to maybe 2 at this basin but it would have not been anywhere near enough so is not something reasonalble to do.  The closest USGS gage had 1.77 inches while the East WG had only 0.56 inches so may be the whole problem so no reason to panic yet.  Using the 3 WG ratio worked much better than the 2 WG ratio at 5 or 2 Max Usable ratio for this day... safety in numbers.

June 2, 2013--N0Q Radar and 3G Adjusted Radar at USGS Rain Gage Comparison:

The East Wundergage was down AGAIN and replaced on June 2.  Although the adjusted rainfall was below the USGS gages, the runoff came in a litte on the high side.

Ok, now that you have seen enough of the blow by blow cringing graphics, lets see the results over time as they pile up...



Study Results... as they come in:

As of 4/19/2013 these 3 storm results are not what I had expected and are about the worst I have seen.  I have not shown the closest 1 W gage ratio and 2 W gage ratio results but a closest 1 gage ratio seems to be a very bad choice at Charolette.  The best results so far are obtained by using 2 W gages on opposite ends of the area of interest .  This opposite ends theory seems to also hold true to a similar sized basin I have been looking at for urban Birmingham, AL but have not published.  My Las Vegas, NV study shows the closest W gage is better and has had a wee bit more testing... but not always, sometimes distance does not matter there either meaning an improved adjustment with distance.  I even tried adjusting based on the #1 USGS gage at the center of the basin which was actually worse than the 1 Wundergage adjust (see peak flow table below).  This radar stuff is just plain diabolical.  I have seen alot of adjustments but nothing as bad as April 19 above so I hope this is just a one off case.  But as noted above, setting the 'Gage Minimum for Hourly Ratios' from 0.05" down to 0.02" would reduce the storm total bias by half and may be the next step and doing so reduced peak runoff a good deal but nowhere enough.


May 19 was even worse than April 19 for the 2 WG ratio.  The 3 WG ratio was not so bad.  The 'Maximum Usable Ratio' value of 5 being lowered to 2 for thunderstorms at this basin would not be sufficient so that for these distances the more gages probably the better.  The offending East Wundergage failed again May 21 and was replaced with one 3 miles closer to the basin and may have been the real cause for the problem.


After mid-May the storm cell areas have become smaller and tend to affect only one Wundergage at a time at these far distances between Wundergages.  Daily ratioing as well as a 4th Wundergage has been added to see if this would help overcome the distances with these smaller storm cells.  You can see the results below on the peak flow comparisons.


Summary of the 2 East-West Wundergages adjustment at the 6 USGS gages and for the Basin:


Comparison of unadjusted and 2 gage adjusted N0Q basin average rainfall to 6 Gage Average as Hourly Mean Absolute Error by event and their average.  The adjusted green average line is supposed to be much below the pink unadjusted average line.


The adjusted basin radar vs 6 USGS gage average is a little less than hoped for.  I think the reason for this perception is that the UNADJUSTED radar is surprisingly not too bad at this location, but also 20.8 miles between Wundergages is not helping.  My other studies unadjusted radars were just not usable without adjusting (see 2nd graph below example).


As an example of adjusting a radar also 90 miles out using 3 Wundergages INSIDE of a 91 sqmi basin average radar comparison (note how bad the unadjusted radar is here and how nice it seems above, and yes comparing the basin adjusted with the Wundergages used to adjust it may be cheating):


Keep an eye on the all important Basin vs 6 Gage Average because that is what we are really using for runoff and remember even the 6 Gage Average aint sacred and could be less reliable and could even be interpreted as the 6 Gage averaging error as it has at times been shown to be based on peak flows.


You can see the 3gage ratio adjustments for each storm event on the Level 2 comparison below.


Gage/Radar Ratios:

The 3 gage ratio is the hourly ratio of the average of the 3 Wundergages divided by the average of the 3 radars at the gages.  The 2 gage ratio uses the West Wundergage and East Wundergage.  The 1 gage ratio uses the ratio at the South Wundergage only and being closest to the USGS gages I would have thought would provide the best results... so far that is not so.  The MAE (mean absolute error) in the graph titles below are computed when the 6 USGS gage hourly average is 0.1 inches or more.  The MAE should not imply that the 6 rain gages give better basin average rainfall because they actually don't seem to.  The April 28 peak and volume runoff from the 6 gages was too high and the 2Gage ratio was actually the best.




Daily Accumulations:  Don't let the linear plots of unadjusted daily totals fool you into thinking unadjusted radar is great like I did.  Some how the NWS has the bias down pat, but their 5-minute values and hourly sums can be COMPLETE DISASTERS so don't fall for adusted radar bias as any great feat.  It's those 5-minutes that count.  The daily accumulations do show that the USGS gages are working and that there is no large differences from gage to gage between the comparisons to radar and so all are working properly.  Actually this radar location has about the best UNADJUSTED radar rainfall I think I have seen for any radar site so far, cumulative wise.


Yeah, whatever....


Also it's a good idea to monitor the Wundergages to be sure they are working properly:

Wunder gages East, South, and North look nice and linear.  West is a bit sloppy and I will keep my eye on it.  Also note how the radar is above the East and South  gages but is always below the 6 USGS gages above.  Perhaps West slop is helping here.  Daily accumulations will not show if leaves get in the tipping bucket and cause slow tipping so an eye must be kept on the 5-minute plots vs radar values for sharp steps indicating that it's leaf free and that is done in my GetGraphs that is used for viewing all the hourly or unit data in real-time or the next day.




Typical Wunderground site and hard to maintain weather station:


Professional USGS rain gage setup ... ok so the only difference is a bigger roof top:


I'm a little concerned about all these USGS gages on flat rooftops getting roof splash up and into their gages so I will check them againtst this one #G6= 351954080493445; CHARLOTTE RAINGAGE FIRE STATION 28, NC which should be more splash proof.  For winter storms Jan 1, 2013 to Apr 4, 2013 the nearly same 12" has been measured at the splash proof and 2 nearby rooftop gages, so far so good:


A web search indicates that heavy rainfall can splash vertically over 4 FEET! but column (d) is more typical:





My runoff loss method here is GetRealtime's SCS curve number method adapted for continuous simulation and is honed sharp to cover all the bases if need be.  The 30.5 sqmi basin lumped radar average 5-minute rainfall was adjusted using the same hourly ratio of the 3Wunder/3Radar used to adjust at the USGS rain gages above.  I prefer to use the 3 Triangular Unit Graph method because it allows mimicking a bit of routing for a better calibration fit than a regional dimensionaless unit graph (you could create your own unit graph but why bother).  The Linear Reserovoir method allows for a bit of routing adjustment too and is almost as good as the triangles and you might even prefer it for its ease of adjustment.  I always check my initial unit graph method with the SCS dimensionaless graph just to be sure the peak flows are reasonably close to what it says and then adjust the recession to the flow gage.  The % impervious can be adjusted by fitting the smaller peaks where SCS CN losses are greater than rainfall.


I was surprised how easy it was to guess at the inital runoff coefficent setups.  I just grabbed a setup for Birmingham, AL, set the minumum base flow, lag time, and forced most of the runoff into the peak Triangle runoff, and fiddled with the recession slope, all in about 1 hours time and was surprised how good it worked.  But these runoff coefficients I have used here don't have a snowball's chance in hell of working for the next storm forecast in about 2 days.... watch and learn... oops...ok the next one then.  You need a few storms to zero in on the longer term dry CN and such as the continous system storage parameters gets initialized (see below).


Initial Calibration:  (actually I don't think I ever changed this much)


This April 19 runoff had terribly bad adjustments to the radar.  Boy this is one big black eye for adjusting.  I should add UNadjusted but I know that's just crazy talk.  Using a 0.02" instead of the 0.05" limit before ratioing would help alot but not enough and is not shown but I suspect we will soon be using 0.02" if this keeps up.  (Actually I have been using a 0.02" as the wundergage trigger for ratioing for my other studies so I guess I must have already figured this out.  But wiser men suggest 0.1" so what can you do.)  If you are wondering why not 0.01" is that is special case and will zero out any radar rainfall over the basin if there is no rainfall at the Wundergages.  Not a deal killer for larger storms and I did try it here but did not help.  The Apr 4 storm of over 1" produced about the same peak runoff as this 0.05" adjusted storm and thats a black eye... uh maybe you need to adjust the runoff recovery factor like you suggested after one storm duh!


April 28 Runoff (includes just using the 6 USGS raingage average which I am shocked was not better):


May 5 Runoff:  Something just does not look right about the lag times on all thes hydrographs.  I wonder if some sort of regulation is occurring like fast storm drains , interbasin flows, or detentions.  With the continuous soil moisture pretty well initialized prior to this May 5 storm, I will try to recalibrate this storm some more because something is fishy...  Ah Ha!  After an online search of the Irwin Creek Basin it turns out there is no significant regulation but the basin consists of two parallel branches of similar size with their confluence 2.8 miles above the Irwin Creek stream gage.  I think the 30.5 sq mile basin needs subdividing at the 11.1 sq mile Steward Creek branch and luckily it too is gaged.  I think I will continue with this lumped 30.5 sqmi example to show as a reason to subdivide and just add another hydrograph line for the subdivided case.  After subdividing the basin for the May 5 storm I didnt get consistant lag times as I hoped but I think I discovered the real problem.  The recent storms are training along a southeast to northwest track while slowly moving eastward.  The lower portion of the basin is getting heavy rainfall while the upper basin portion not so much.  This causes a faster runoff near the gage until the whole basin is being covered.  I know you are thinking why I didnt I add a 3rd sub below the confluence and well I should have but I'm hoping eventually the storm tracks will be west to east and even more subdividing will not be needed but we shall see.  Ok, I went ahead and subdivided the basin into 3 subs but that still did not improve things much.  So much for my storm tracking theory.  I guess that just leaves poorly adjusted rainfall although using the 6 USGS gage basin aveage was no improvement.  I could use the subdivided basins with closest USGS rain gages but I won't.


I have since revised GetRealtime 3.1.2 to include a variable Lag time and that made an improvement in peak flow comparisons for both bias and %MAE. My variable lag is computed as Lag = Lag0 - SoilSpace * Lag0 / 2. This reduces the Lag0 entered for dry conditions to 1/2 Lag0 when saturated. The lag time and all unit graphs are updated on startup soilspace and when the computed Lag changes by 5 minutes. So as rainfall increases, soil space is reduced, and the Lag time is reduced. Under dry contitions soilspace increase and Lag time increases. Pretty clever eh!


Looking at some of the rainfall accumulation graphs you can see where radar and adjusted radar can preceed actual rainfall accumulatins at the 6 USGS gages by several hours.  I quess this is the real problem with lag times, although the 6 gage runoff only improves the lag somewhat better.  Hmm... 10% impervious could be causing the early rise on the smaller peaks and then gets lost in the large peaks.  It's probably a trade off between getting the peak values correct or getting the timing correct over the full range of peak flows so I won't do nothing, but you could try.

Routing File:

Three sub basins Irwin Creek, NC

GET 4 30670 Upper Irwin Cr Subbasin #1

GET 4 30671 Stewart Cr Subbasin #2


ROUTE Modpul ModpulLowerIrwin.Rat   (Tatum 0.75 hrs travel worked as well)

GET 4 30672 Lower Irwin Cr Subbasin #3




Modpul Rating File:

Lower Irwin Creek, Steward branch to USGS gage

bw=30', bs=3, cs=6.8 ft/m, cl=2.8 m, n=0.035 using my free stuff ChannelStorage.exe and Google Earth

 5 0 0

5.2 3.144004 2.077091

5.4 10.04043 4.235637

5.6 19.85955 6.475636

5.8 32.29116 8.797091

6 47.16438 11.2  ... etc out to...

22.2 9999.145 476.3462



May 19 Runoff, Wundergage East produced a very high ratio and screwed it all up but the 3 WG ratio did a heroic job of compensating for the East WG screw up:

June 2 Runoff, Wundergage East replaced again.

June 28 Runoff, Wundergage East had 3.45 inches of rain in 4 hours, WG South 0.88 inches, and WG West 1.30 inches.  Although the adjusted %MAE was very very bad, the % Diff was right on and so helped bring the peak runoff right in on target.  You can see the 3gage ratio adjustments for each storm on the Level 2 comparison below.


July 11, 2013  All the rain was building over the basin and not so much at the adjusting rain gages.  Basin reflectivity gradients were so pronounced that there was even a difference in the Level2 higher spatial resolution boundary vs N0Q rainfall.  A 4th Wundergage was added to see if that would help which sometimes does and sometimes doesn't but did help on this event.



Current Setup (updated 5/8/13) for continuous simulation for graphs below April 1, 2013 to...:



Continuous runoff comparison graphs each month:


April 2013


May 2013 Wundergage East replaced and 2Gage adjusted soil conditions reset May 21:


June 2013 Wundergage East died and was replaced AGAIN on June 2:


July 2013 :


August 2013 :


September 2013 :



Ongoing 2Gage adjusted Soil Space converted to CN:

Runoff wise, this is the most important thing of all.  See my GetRealtime Help.  2 gage adjust soil conditions reset May 21 after bad Wundergage replaced.  Divergence from the 6 USGS gages is probably not good and will lead to over/under runoff which is probably not good.  I need to add a table soon of observed lag times vs peak flows to verify that the use of a variable lag is warranted.


Observed lag time at the USGS gage and computed lags based on soil storage:


The 'observed' USGS lag time was computed as the modeled peak time versus USGS gage peak time difference added to the 6 USGS average tipping bucket model's constant lag time.  This let the model figure out the excess rainfall centroid for us since lag=centroid of rainfall to peak flow times.  I tried using different soil lag time formulas and adjusting the triangular unit graphs and did not get an over all improvement over the yellow lag formula above Lag=2.25-Soil*2.25*0.5.  This allows reducing the lag in half from dry conditions to wet.  And just because the USGS knows the peak and lag time, it apparently does not mean that the radar and model will like it.  The violet formula above may look better but had slightly greater %MAE.  This soil lag technique did improve over the non-variable lag time of 2.25 hours significantly.  I think the radar sees the rain before it actually falls and so it could just be floating up there!?.  And my evaluation of the lag time is based on best peak flow, not best lag.  This method needs checked out on a basin with sharper peaks than the round topped ones here where differing the 0.5 value in the lag formula above may show improvements but 0.5 seems safer than trying greater values that may look better as the violet graph data above.


Summary of event hourly peaks at USGS flow gage vs 6 USGS tipping bucket average and Wundergage radar rainfall adjustments:




My Conclusions:  I am hoping 2 Wundergages on opposite sides of the basin aligned with storms general directions will be the way to go.  The leading gage outside the basin will start providing adjustments before the storm arrives over the basin... or so the theory goes.  There is not alot I can find on the Web about practical applied studies for idiots.  Kriging 50 gages is just not my cup of tea... even if I knew how to Krig or had 50 gages then why would I want to use radar anyway.  I have tried using just the 2 Wundergages without adjusted radar and found that's not a smart idea so its either a 1 gage adjustment or 2 adjustment... or 3 tops.  And 1 gage nearest the basin is the worst case so far because I have had to replace it twice.  But also I tried using USGS #1 gage at the center of the basin for 1 gage adjustment and very very interestingly that did not  improve on the Wudergage south 1 gage adjustment for peak flows.  So it seems 2 gage adjustment at a minimum and probably 3 gages for more safety.  The Apr 28 jump in USGS 6-gage runoff as well as the peak flow table above seems to say adjusted basin RADAR IS BETER than a basin GAGE average from 1 rainfall gage per 5 square miles.


The May 19 ratios indicate that 3 Wundergages at these distances are much safer than 2 and certainly 1. Using a lower 'Maximum Usuable Ratio' of 2 instead of 5 which was better suited for winter stratiforms than for tall and scattered convective would have helped some but 3WG was still much better.  Once the bad Wundergages get weeded out, hopefully better conclusions can be made.


And remember that storms are moving about 1 gage an hour so the closest gage will hopefully have the most influence for better or worse.  The Wwest gage may have little infuence at it's distance but sure can't hurt... or can it???  I added a 4th Wundergage North to see if it helps.  This is getting interesting.  I think a smarter mind is needed here on just how to evaluate all this.  On the other hand, who lives in Charlotte anyway. ;-)  If using the USGS gage at basin dead center for adjustment is bad, then this radar stuff is just cat's sleeping with dogs.  Boy, is this ever a good plug for figuring out your own adjusted radar.


Because of the Wundergage distances and storm cell travel time giving short point duration less than an hour I added a table of hourly peak flow based on DAILY RATIO adjustment to see if that would help.  Sometimes it does and sometimes it don't.  For larger storms Hourly adjustments seem safer.  If you only have one Wundergage, then Daily ratio is better.  If you subdivide the basin then Hourly ratio is subtantially better.  Why?  Who knows why.  Maybe more time will tell.


On June 6 I had a huge spike in runoff with the 2Gage adjusted radar that triggered a snafu flag.  I tracked it down to the Wizard setup for 'Gage Minimum for Hourly Ratios'.  Although setting this value at 0.02, it did not preclude radar values coming in at 0.001 and creating a 5 limit ratio factor.  I have changed the Getrealtime code so that both the Gage AND THE RADAR must exceed this 0.02 limit before issuing a new ratio factor.  In this case the Gage had 0.22 and the radar at the gage had 0.015 and the next hour at the gage had below 0.02" causing the continuing 5 limit ratio to be applied across the basin that had significant radar rainfall.


 I can see why engineers would think this radar stuff is pretty flakey, cause at first glance it is.  We just need to show them you can adjust it!!!... for free... most of the time... usually... ok you might have to use your engineering degree now and then.  I guess they aren't as gullible as I was.  Hey, I just sell tools, not the rules, so dog gonnit innovate... and buy some rain gages.



Level2 0.15 mile versus N0Q 0.6 mile (and 'NCQ')


Is the extra effort to use super high resolution Level2 reflectivity worth it rain wise or will it actually hinder because of trying to hit a 8 inch rain gage with only 0.15 miles verus the 0.6 miles of N0Q?  I sure hope N0Q wins out because downloading and processing 4 megabyte zip files for a 5-minute image animation KMZ file via the WTC Toolkit is a real aggrevation (it takes me 1 minute for each radar image so 1 day would take 5 hours).  Why can't the NWS break out the 0.5 degree base reflectivity from all the other angles or better just give us an image like N0Q???  Who uses all those high angles anyway!  Ok, maybe Dr Forbes but nobody else.  Or maybe the NWS has seen this play out before and knows the outcome here and why there is N0Q.  Those ol' goats at headquarters may be sleepy but they didn't get old without learning a thing or two.


Iowa Mesonet's N0Q composite cutting service has been added to GetNexrad.  Iowa's cutting service works for any lat/long box of any size but GetNexrad cuts each radar site as 'NCQ' 750x674 pixels out to only 150 miles which provides smaller downloads and faster loops.  The drawback to using these composite N0Q's is the highest dBz from nearby radars gets used which causes much higher rainfall.  Virga and melting layer can over ride the nearest radar by these higher dBZ's from farther away radar sites scanning higher elevations.  Also, the higher elevations (23,000 vs 8, 000 ft) lets the wind blow it farther when gage adjusting.  If this 'NCQ' shows promise then it will be added to GetRealtime.  'NCQ' is not a NWS product code.  It's my code for 'C'omposite N0Q = NCQ.  At first look here, you definately do not want to use this composite with out rain gage adjustment but it may prove useful in certain locations and when there is no melting layer in winter. 


Let's look at the 30.5 sqmi Basin Average Radar versus the 6 USGS Gage Average. 

% Difference is the error in cumulative rainfall (r-g)/g.

% MAE is the mean of the individual hourly unsigned differences for pairs greater than 0.1".

AVERAGE unsigned is like a %MAE of the table events.

AVERAGE bias is your normal average that indicates bias of the table values.

Wind is storm cell radar speed and direction of movement. Wind at the rain gages is usually calm.


So far the unadjusted basin area radar is essentially the same for Level2 vs N0Q as would be expected.  The big difference shows up in the adjustments based on point values at the 3 rain gages.  Come on N0Q!  You're pulling ahead.  It would be an ironic paradox if the old saw of not fitting a super high resolution 0.15 mile radar in an 8" rain gage proves true, but you CAN fit a 0.6 mile N0Q radar.


A rain drop falling from 10,000 ft at 19 mph in a 10 mph cross wind will be blown 1.0 miles.  But after more thought the clouds are moving at 40 mph and surface winds are calm so wind is not the problem presumably because the 40 mph extends down to about 500 feet up.  On the other hand this sure looks like something is up with rainfall directions not being straight down the last few thousand feet:


I had a problem with the WCT toolkit (big difference between level2 and N0Q basin values for 6/10/13) where I somehow confused the WCT's generation of the  KML location file, but while tracking the problem I also tried Level2 gage locations as 4 nearest pixels.  Nice idea but because I had zoomed the WCT in on Charlotte, the 4 nearest image pixels were in the same level2 pixel.  I then tried Unzoomed and got 0.6 mile pixels and so gave up.  I have tried the 4 nearest N0Q pixels at a gage location for my Las Vegas Valley study and that was shown to be a very bad idea.  So it would appear that the ideal radar pixel size is 0.6 miles, not 1.2 miles at Vegas, and not 0.15 miles as will be shown here.  Expecting to adjust a 4km (2.4miles) NWS multi-sensor MPE rainfall product pixel with a single rain gage would simply be ludicrous (that would take 16 rain gages for the pixel).  I guess you could use 16 N0Q adjusted radar pixels based on a single gage to adjust a MPE product LOL!!!  What in the world where they thinking with 4km pixels.  This is getting interesting... if you want the best radar estimate of rainfall at a point, then the N0Q is better than Level2 and it is its equal for areas.  Prior to 2007 Level2 was equivalent to N0Q resolution, then super high resolution was introduced for tornado velocity detection.  N0Q was introduced in 2011.  It would seem the NWS has indeed seen this play out before.  For 2007-2011 you can use Level2 but do not zoom and check your pixel size is 0.6 miles if you are going to rain gage adjust it.


Another thing to think about is that based on the irratic hourly adjustments at the 6 USGS rain gages is that 1-km radar point rainfall, contrary to the current popular vouge, may be better suited for lumped runoff modeling than the effort needed for gridded point distributed models. At the 30.5 sqmi basin size and sub-divided 10 sqmi basin sizes here, these point rainfall errors seem to get corrected much better basin wise than at the 6 tipping bucket points and their average.  See summary table of the Basin vs 6 gages above; %Diff 19% vs 30% and %MAE 32% vs 40%. So I would do some experimenting on point rainfalls with your area's available rainfall adjustments before thinking you can model a shopping mall parking lot with radar.


Executive Summary:  Aside from a few minor brain clots getting started, I think engineers can follow all this and could actually do better at this radar stuff than I with some motivation based on their actual site and actually getting paid.  I give you the tools so have at it and questions are most welcome.  And no, I have never met The King... yet.


If anyone would like a zip file of all the Microsoft Excel 2003 xls files that automate generating all the above Gif graphs from the GetRealtime Access database with just a click, send me an email.


Some additional page links about ET and Nexrad Radar help and comparisons:

List of How To Videos on Youtube
Help Page for GetNexrad.exe
Nexrad Rainfall to Tipping Bucket Comparison
ET and Radar Rainfall along the Lower Colorado River, AZ-CA
Nexrad Rainfall-Runoff Comparison Las Vegas Valley, NV
Nexrad Rainfall-Runoff Comparison San Jouqin Valley, CA
Nexrad Rainfall-Runoff Comparisons in northwestern Arizona
Nexrad Snowfall Comparisons in western central Sierras, CA

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