The Detection and Significance of Diurnal Pressure and Potential Vorticity Anomalies East of the Rockies

Abstract

Harmonic analysis of pressure, temperature, and precipitation data from 1000 Automated Surface Observing System (ASOS) stations reveals a mix of stationary and east?west moving disturbances east of the Rockies. Optimization of the pressure data using a ?temperature-based tide assumption? separates a strong sun-following continentally enhanced tide from a smaller eastward-propagating wave (EPW). The latter signal moves at a similar speed to the previously discovered eastward-moving precipitation systems. Analysis of ASOS summer precipitation data confirms eastward propagation, but east of 90°W it shows nonpropagating diurnal convection at a fixed local time (i.e., 1800 LST). Analysis of winter days still finds the EPW, suggesting that it is the cause and not the result of the propagating precipitation. A possible mechanism for the EPW is developed from the linear Bousinesq equations with heating and wind shear. Solutions show eastward-moving diurnal pulses of potential vorticity (PV) generated by imposed heating over the Rockies. Because of the background shear, these pulses produce vertical motion in the lower troposphere. The PV hypothesis for precipitation propagation was tested with North American Regional Reanalysis (NARR) data. Diurnal drifting thermal and PV anomalies are clearly found near the 500- and 600-hPa levels in both winter and summer. In winter, the PV signal is weaker, moves faster, and does not influence precipitation. The existence of the winter PV signal again suggests that it is the cause, not the effect, of summer propagating precipitation.