2002 Annual

Wednesday, 16 January 2002: 4:44 PM
Cloud Observing System derived characteristics of convection in the convective boundary layer
Richard S. Penc, Creighton University, Omaha, NE
Components of the Penn State Cloud Observing System (COS) were deployed near the southeast shore of Lake Ontario during the Lake Ontario Winter Storms Project (LOWS). COS is an integrated suite of ground based sensors including a 400 MHz wind profiler, radio acoustic sounding system, laser ceilometer, and standard surface instrumentation. These components were chosen in order to define the convective character of the convective boundary layer (CBL) during lake-effect events. Data were collected for two single band snowstorms, two multiple snowband events, and six submarginal events. The convective character for these events was compared.

Lifting condensation level was estimated from surface parameters and compared against ceilometer derived cloud base measurements in order to identify periods of drying within the mixed layer that may be associated with entrainment of free air into the CBL. CBL depths and growth were inferred from wind profiler signal to noise ratio data and measured parameters such as wind shear. These data were available from the integrated COS dataset. Profiler derived quantities were compared against rawinsonde data available nearby to ensure consistency.

COS measurements suggest that mixed layer growth was greatest during precipitating convection, typically about 4 cm s-1. Estimated CBL heating rates and CBL growth rates were highly correlated, and correspond closely to periods when the observed cloud base deviated from surface based LCL estimates, suggesting entrainment and drying of the mixed layer was enhanced during the convection. In this case, the cloud base "decouples" from the surface, which may sometimes be noted in sounding data as a small discontinuity in CBL profiles. Typical CBL heating rates were between 0.3 and 0.6 0C hr-1, and increasing to 0.6 and 1.20C hr-1 during the more intense convection. Mixed layer growth rates of 4 cm s-1 were typical during convection. Additionally, the cloud environment was warmed and dried by the convection through subsidence and warming as seen in the time series of observed cloud base that exhibit large excursions on short time scales. Entrainment estimates suggest that these mixed layers were controlled primarily by large scale forcing, but entrainment was important in modifying the convective environment as well. Integration of measurements available from COS were useful in identifying thermodynamic and convective characteristics of the convective boundary layer in these lake-effect snow situations.

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