Monday, 7 July 2014
Forecasting the lifetime of stratocumulus clouds over coastal areas is notoriously difficult. Prior studies have shown that numerical weather prediction (NWP) models consistently forecast shorter dissipation times over land when compared to observational data. The purpose of this study is to determine the sensitivity of the stratocumulus dissipation time to a comprehensive set of parameters such as initial cloud thickness and land surface properties to help explain the errors arising in NWP forecasts. In order to study the factors controlling the stratocumulus cloud dissipation over land, we employ a mixed layer model (MLM) and large eddy simulations (LES). The LES is coupled with a land surface model, and a diurnal cycle is simulated for the well-mixed CGILS stratocumulus case. The well-mixed assumption holds throughout the diurnal cycle as, initially, nocturnal longwave cooling at the cloud top drives the turbulence. Increasing solar insolation at the surface increases the surface fluxes and as the stratocumulus cloud dissipates, the main source of turbulence shifts from the cloud top to the surface. The MLM includes a temperature-dependent longwave radiation scheme, a solar-zenith-angle dependent shortwave radiation scheme, entrainment closure, and a land surface model that computes the surface energy balance. In order to study the sensitivity of stratocumulus dissipation times, perturbations are applied in the MLM and the model response is validated through the LES for a limited number of cases. Utilizing the MLM framework and the sensitivity study, we are able to isolate the different feedback mechanisms affecting the dissipation time. For example, we can map out how surface moisture content controls the surface latent heat flux and, in turn the boundary-layer total water mixing ratio and the cloud dissipation time. The advantage of using the MLM is that different mechanisms and feedbacks controlling stratocumulus cloud thickness can be examined rapidly through sensitivity studies.
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