Friday, July 06, 2007

Sounding Analyses When Data Are From Unreliable RRS Sippican Sondes

Prepared 4:00 pm MST on July 6, 2007

I think that the Tucson NWS upper-air soundings from yesterday afternoon and from this morning illustrate several problems that forecasters, and other users, will have with the data from the unreliable Sippican sondes being used operationally by the NWS RRS program.

First, let's examine the 5 pm MST (0000 UTC) Tucson sounding from yesterday - See Fig. 1. This sounding was taken on a very hot afternoon, about two hours before severe thunderstorms moved into the Tucson metro area. Things of importance:

  • The well-mixed adiabatic boundary layer (BL) reaches to 600 mb in the temperature profile. A layer of steep lapse rate is just above, reaching from 600 to 450 mb, i.e., an excellent thermal situation for high-based convection. Winds in the thermal BL are light and variable, meaning that advective changes were minimal, until the storms moved into Tucson.

  • The moisture profile from the 1st data point above the surface to 700 mb appears to be too dry in the extreme. There is no physically realistic BL captured in the Td profile. There should either be a BL well-mixed in moisture or, if shear at 600 mb was mixing dry air downward into the BL, decreasing mixing ratio with height would be expected.

  • Computed CAPE for this severe thunderstorm proximity sounding was zero.

The analysis and forecasting dilemma is obvious. Since the Sippican sondes tend to measure Td (RH) too low, we have no idea what the actual BL moisture structure was
prior to the thunderstorms and macrobursts.

Second, let's look at this morning's sounding - See Fig. 2. Things of importance:

  • The moisture profile bears no resemblance to that observed the evening before. Now we must ask whether we should accept this profile as accurate.

  • Note that if this profile becomes well-mixed (in T and q) during daytime heating to 550 mb, there would be at most a tiny sliver of CAPE. There is advection from the SE to consider however.

  • The GPS IPW at the time of the sounding was about 6 mm greater than that of the sounding. This calls into question, again, the accuracy of the Td profile.

Given the unreliable character of the Sippican moisture measurements, it is not clear how the forecaster, or other users, can actually diagnose what is the real potential for convective storms. The morning forecast discussion from the Tucson NWS Office, based to some degree on this sounding, concluded that:


"KTUS 12Z SOUNDING ANDGOES PW IMAGERY SHOWS MOST OF THE MOISTURE INCREASE SINCE YESTERDAYHAS BEEN AT MID LEVELS WITH SEVERAL DEGREES OF WARMING IN THE500-400MB LAYER. THAT SHOULD COOL A BIT AS AN INVERTED TROUGH OVER NTX TRIES TO EITHER MIGRATE W INTACT...OR SHEARS TOWARD US IN THERATHER FAST BUT DIFFLUENT FLOW ALOFT. WE ALSO HAD A 40KT NE SPEEDMAX AT 500MB THIS MORNING WHICH IS GOING TO HELP IN THE SHEARDEPARTMENT THIS AFTERNOON/EVENING. BUT THE UPPER 40/LOW 50 DEWPOINTSWE HAVE RIGHT NOW ARE NOT GOING TO YIELD MUCH CAPE. SO UNLESS WE SEEDEWPOINTS REVERSE COURSE OR AT LEAST HOLD WHERE THEY ARE...I DOUBT WE`LL SEE MORE COVERAGE THAN WHAT WE HAD YESTERDAY."

Interestingly, here at the house, there have been thunderstorms and occasional showers since about 2 pm. Updraft bases appear to be at about 600 mb and an intense downburst appears to be in progress a couple of miles to the east. I suspect that the morning sounding was dry and that there has also been some additional increase due to advection. This combines to make this afternoon much more convectively active than yesterday.

1 comment:

  1. I agree--sounding assessment is tricky with the known problems. Thank goodness for PWAT sounders across the region.

    The emphasis on the AFD and SPC forecasts, however, was whether coverage of the *severe* potential would be greater on Fri compared to Thu. Sure, it was more convectively active on Friday over Thursday. But, it is tough to say, at this point, on the coverage of severe thunderstorms based on no mention of such in media, print or LSRs from Friday. Have you seen any bonified severe reports from Friday?

    Jon

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