Tuesday, May 15, 2012

Time To Look At GPS Precipitable Water

With the onset of very hot and dry conditions over the Southwest, it seems time for my annual reminder that the NWS RRS soundings continue to have problems accurately measuring RH, leading to unreliable calculations of precipitable water (PW). The two time series above show GPS PW (blue) versus RRS sounding PW (green dots) for the past 5 days at El Paso (top) and Tucson (just above). The PW values are usually too low for the RRS sounding data, as per the typical hot conditions situation with sounding RH too low near the surface.

The 5-day time series for Flagstaff is shown below. The sounding data for 00z UTC on 13 May were much drier (about 0.5 cm) than were the GPS data. The Flagstaff upper-air sounding for this time is shown in the skewT plot at the bottom. The sounding has a huge dry spike from the surface to 550 mb, where the RRS RH is too low.

So, the bottom line is that users should beware during the next few months.

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