Recorded presentation12.4
The detection and significance of diurnal pressure and Potential Vorticity anomalies east of the Rockies
Yanping Li, Yale Univ., New Haven, CT; and R. B. Smith
Harmonic analysis of data from 1000 automated surface stations finds the known eastward moving diurnal summer precipitation anomaly and a large sun-following continentally enhanced tide. Optimization using the “temperature based tide assumption” suppresses the tide and reveals a smaller pressure signature moving east along with the precipitation. This pressure “wave” is present on dry days also, and in winter, indicating that it is the cause of the precipitation anomaly, not its result.
A possible mechanism for the pressure wave is developed from the linear Bousinesq equations with heating and wind shear. In addition to the inhomogeneous continental tide, it shows eastward moving diurnal pulses of potential vorticity (PV) generated by imposed heating over the Rockies. Because of the background shear, they produce vertical motion in the lower troposphere.
The PV hypothesis is tested with the North American Regional Reanalysis (NARR) data. Diurnal drifting PV anomalies are found around 500 to 600 hPa level in both winter and summer. In winter the PV anomalies are weaker and seem to form further west, and they do not trigger convection, the eastward propagating signal only shows up in the surface pressure observation.
Session 12, Orographic, coastal and other thermally driven mesoscale circulation systems II
Wednesday, 19 August 2009, 10:30 AM-12:00 PM, The Canyons
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