Wednesday, August 08, 2007

An Examination of Suspect Thermodynamic Structures in the Tucson Upper-Air Sounding AT 12 UTC 8 August 2007

Those who have read my previous posts are aware that I have been documenting data problems in upper-air soundings taken by the NWS using the new Sippican sonde. This morning's TWC sounding illustrates a suspect thermodynamic structure that I have observed several times here during the last few weeks.

First, a brief discourse on boundary layer (BL) structures. The afternoon BL is usually, but not always, well-mixed in both potential temperature (theta) and in moisture mixing ratio (q). Such BL layer structure is illustrated in the Phoenix 00 UTC sounding from last evening (Fig. 1) and in the TWC 00 UTC sounding (Fig. 2), ignoring the dry spike in contact layer. TWC uses the new Sippican sonde, whereas, Phoenix uses a Vaisala sonde with a different type hygristor. Both afternoon soundings depict a well-mixed BL extending from just above the surface to about 750 mb.

When winds are light and there is little advection or turbulent mixing during the night (the situation present over southeastern AZ during the past night), a cool and stable nocturnal BL develops near the ground, with the residual BL from the previous afternoon remaining in place above the nocturnal BL. The Phoenix 12 UTC sounding this morning (Fig. 3) indicates a residual BL present from about 850 to 700 mb.

However, the TWC 12 UTC sounding this morning (Fig. 4) indicates a much different structure aloft over Tucson this morning. Note that the theta profile indicates a residual BL aloft in about the same layer as found in the Phoenix sounding. However, the structure of q does not indicate a residual BL in the moisture field. Above the nocturnal BL q decreases steadily from about 12 g/kg to about 8.5 g/kg at 750 mb and then jumps up to 10 g/kg at 700 mb. This structure is physically very suspect, although I can not prove that the q values are bad, I feel strongly that they are.

Based upon the TWC afternoon sounding at 00 UTC, one would expect to find a residual BL well-mixed in moisture, as in the Phoenix sounding, with nearly constant q values of about 10 g/kg. The Sippican sonde seems to have responded slowly to the abrupt change in q above the nocturnal BL decreasing steadily from 900 to 750 mb and then jumping back up to the expected value of ~ 10 g/kg at 700 mb.

Interestingly, although I am personally convinced that the TWC morning moisture profile data points are erroneous from 900 to 700 mb, the sounding's total IPW agrees very well with the GPS IPW. Thus, at times the GPS-derived data may indicate that a Sippican sounding is accurate wrt IPW, even though there may be serious problems within the details of the Sippican data.

Errors such as the ones likely present in this morning's TWC sounding can have serious implications if one is trying to forecast the likelihood of strong convective storms.

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