Boundary Layer Control of Nocturnal Convection Associated with a Synoptic Scale System

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  • 1 Department of Meteorology, University of Utah, Salt Lake City, UT 84112
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Abstract

A case is presented with pronounced diurnal variations in precipitation and low-level circulations over the Great Plains during 5–10 May 1979. Well-defined nocturnal convection, along with distinct maximum precipitation events in Nebraska, was associated with strong diurnal wind oscillations at 850 mb in Oklahoma, Kansas and Missouri. The LFM-II operational model did not predict these phenomena even though hourly precipitation frequencies at midnight were five times as numerous as in the afternoon in Nebraska, while the 850 mb winds for 1200 GMT at four rawinsonde stations in the Great Plains were approximately double the speeds at 0000 GMT for three consecutive days.

A numerical prediction model was used to relate boundary layer flow modulations to synoptic scale slow for this case. The model was driven by a diurnal cycle and employed terrain following coordinates with input data for 1200 GMT 8 May. Experimental forecasts displayed diurnal wind modulations at 500 m with rising motions on the order of 4 cm s−1 at the 2200 m level along the leading edge of the nocturnal jet. The timing and locations of these features compared favorably with the Nebraska thunderstorms. Sensitivity studies suggest that influences by topographically bound low-level circulations upon the short-term evolution of convection may be more predictable than larger scale influences.

Abstract

A case is presented with pronounced diurnal variations in precipitation and low-level circulations over the Great Plains during 5–10 May 1979. Well-defined nocturnal convection, along with distinct maximum precipitation events in Nebraska, was associated with strong diurnal wind oscillations at 850 mb in Oklahoma, Kansas and Missouri. The LFM-II operational model did not predict these phenomena even though hourly precipitation frequencies at midnight were five times as numerous as in the afternoon in Nebraska, while the 850 mb winds for 1200 GMT at four rawinsonde stations in the Great Plains were approximately double the speeds at 0000 GMT for three consecutive days.

A numerical prediction model was used to relate boundary layer flow modulations to synoptic scale slow for this case. The model was driven by a diurnal cycle and employed terrain following coordinates with input data for 1200 GMT 8 May. Experimental forecasts displayed diurnal wind modulations at 500 m with rising motions on the order of 4 cm s−1 at the 2200 m level along the leading edge of the nocturnal jet. The timing and locations of these features compared favorably with the Nebraska thunderstorms. Sensitivity studies suggest that influences by topographically bound low-level circulations upon the short-term evolution of convection may be more predictable than larger scale influences.

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