Radar discrepancy question

[Joe Zemek]

So it's 3:40 MST and I am seeing a massive variance between composite and base reflectivity for Phoenix area precip. Base is accurate; composite looks like a nice late evening in late July!

NWS radar image link:
http://radar.weather.gov/radar.php?rid=iwa&product=NCR&overlay=11101111&loop=no

Flagstaff and Tucson aren't having this problem. Anyone know the particular error at work here?

 

[Scott Olson]

Doesn't look like an error, the base reflectivity is sampling it at less then 1800ft. The strongest echoes reside between 2-4kft. Not all of that precipitation is making it to the lowest tilt of the radar thus the base looks rather unimpressive.

 

[Jeff Snyder]

Also, looking at surface obs, dewpoints depressions look to be in the 15F range, so some of the precipitation is likely evaporating before reaching the altitude of the base reflectivity scan (I assume it's the 0.5 degree scan). Perhaps more importantly, I think you'd find that the altitude of maximum reflectivity (remember, composite reflectivity shows the maximum reflectivity over all elevation scans) is near the melting level. Given the relatively deep mid-level troughing over the southwestern US at this time, temperatures aloft are quite cool over Arizona (~-6C at 700mb over central Arizona per SPC/RUC Mesoanalysis). At some altitude above the surface, the temperature increases to above freezing, and the snow melts (though it begins the process around -10C). This results in the "bright-band" radar feature, which is a good indicator of the freezing/melting level. In this particular case, RUC/SPC mesoanalysis shows the freezing level about 6kft ASL, which is 4.5-5kft above the ground. Looking at several elevation scans from the Phoenix radar, it appears a probable bright-band (best seen on the higher elevation scans) is focused around 3-4kft AGL (or 4-5kf ASL). Add in some wetbulb cooling in the saturated precipitation areas aloft, and it matches up rather well. If you "add" the affect of bright-banding over all elevation scans, you'll have a much more quasi-horizontally-expansive area of higher reflectivity than you'd see at any elevation angle alone (such as the 0.5 degree scan you mention).

 

[Joe Zemek]

(Stan or Kyle voice) Hey, I learned something today!

I guess I was working on a base assumption that the composite scan was off because it did not moderate the effect. Because of the cool, minimally dynamic system character and the precip reaching the ground--accurately depicted by the base scan as steady light rainfall--it seemed very unlikely that there were significant convective echoes anywhere in this entire system, so the concept of a bright return from sampling the melting level makes much more sense. I suppose if I had more experience with LP storms I'd have noticed these larger reflectivity variances by now

Cool! And thanks!