RRS problems

OVERVIEW OF PROBLEMS WITH DATA FROM THE RRS SIPPICAN SONDES
by Bob Maddox
19 Aug 2007

This commentary also can be found on the MadWeather Blog

I have seen a copy of an internal NWS issue paper on the RRS sonde, including "talking points" for the media. After reading this document, I have a number of concerns:

The sonde problem is apparently being approached by higher levels of the NWS as if it is essentially a problem related to the performance of the Sippican hygrister in dry, desert environments.

In my opinion, nothing could be further from the truth.

After looking at many RRS soundings during the past month and a half, I conclude that there are serious problems with both the hygrister and the thermistor and, probably, with the sonde's internal electronic components. This last statement is just a hypothesis, since I don't know much about the technical details.

I am collecting bad RRS soundings from all over the country so that I can provide an extensive documentation of the problems. I provide here a quick overview of the
problems I have seen to date:

1 - Extreme dry layers above the surface with a slow recovery, leading to soundings that are too dry. (Note that this "too dry in low levels" problem was first detected by NWS personnel in the Southwest. However, the NWS instruction manual for operators of the RRS system indicates that other of the sonde's problems were detected during evaluation; however, implementation proceeded regardless.) See Fig. 1.

2 - Slow response to dry layers aloft when sonde is launched in a relatively humid environment, leading to occasional soundings that are too wet. See Fig. 2 and my MadWeather Blog post of August 8th.

3 - Both the thermistor and hygrister are apparently poorly vented and one or both are easily wetted, leading to a plethora of problems caused by the latent heat effects of condensation, evaporation, freezing, and sublimation. (The wetting problems apparently relate to the compact size of the new overall instrument package that was evidently developed initially for military field use.) See Fig. 3 and Fig. 4.

4 - The RRS soundings are sometimes missing winds through significant layers (the ultimate example of which I distributed the other day - for a MN sounding, hardly a desert environment!) - See a previous post made today. The problems the missing winds can cause are many, some of which have serious implications for forecasting. This problem may be related to the sonde's internal electronic components. See both Fig.1 and Fig. 5.

5 - Other problems, again probably related to internal electronics, cause high to very
high frequency noise leading sometimes to many many "special" points - in both
T and Td. See Fig. 6.

Fig. 1 - This sounding from Salt Lake City, UT, illustrates three of the problems listed. A layer that is too dry (# 1 above) exists from the surface to 700 mb and produces a boundary layer structure in moisture that is physically unrealistic. The superadiabatic layer and rapid drying from about 500 to 480 mb is probably due to evaporation when the wetted instruments (# 3 above) emerged from a cloud layer - note that both thermistor and hygrister seem to have been affected, since T and Td don't seem to recover until about 420 mb. This sounding has no observed winds other than the surface (#4 above).

Fig. 2 - This sounding from Tucson, AZ, illustrates problem number 2 listed above. The layer from 900 to 700 mb is an elevated residual layer in temperature; however, the moisture structure is not physically realistic. The Td decreases slowly through the layer, rather than exhibiting a nearly well-mixed structure, as would be expected in the old boundary layer. See post of August 8th which discusses this situation in more detail.

Fig. 3 - This sounding from Flagstaff, AZ, illustrates two of the problems listed above. The sounding appears too dry from the surface to 700 mb, resulting in an unrealistic moisture structure in the boundary layer (# 1 above). The layer from 630 to 600 mb exhibits cooling due to a wetted thermistor (# 3 above). It is not clear whether the hygrister was affected, nor is it clear how long it took the instruments to recover - the data may have been inaccurate up to about 520 mb.

Fig. 4 - This sounding from Gray, ME, illustrates two of the problems listed above. The sounding exhibits a superadiabtic layer (#3 above) from 580 to 570 mb. The data then exhibit high frequency noise from 570 to 400 mb (# 5 above). There are several other suspect points in the sounding and there seem to be few data points above 400 mb.

Fig. 5 - This sounding from Chanhassen, MN, exhibits problem # 4 above. The sounding has only 5 observed winds and led to the earlier post today.

Fig. 6 - This sounding from Tampa Bay, FL, exhibits extreme high frequency noise below 500 mb. Interestingly, as in Fig. 4, once the high frequency noise ends, there are few data points recorded for the rest of the flight.

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