Diurnal Variability of Low-level Clouds in the Southeast Pacific Simulated with an Upgraded Multiscale Modeling Framework

Diurnal variability is one of the basic modes of atmospheric variability. This variability is obviously driven by the diurnal cycle of solar radiation. The amplitude and phase of this daily variability are different for different types of cloud systems. Due to its shallowness, low-level clouds are sensitive to the solar heating. In this study, we analyze this diurnal variability in the southeast (SE) Pacific low-cloud region that is simulated from an upgraded multiscale modeling framework (MMF). An MMF is a promising approach to climate modeling. It replaces traditional cloud parameterizations with a 2D cloud-resolving model (CRM) in each atmospheric column. The CRM component in the upgraded MMF contains an advanced third-order turbulence closure, helping it to better simulate low-level clouds. The October hourly data, which correspond to the peak stratocumulus activity in the SE Pacific, are analyzed in this study. The low-level cloud cover reaches its minimum in the early afternoon while reaches its maximum in the early morning, with amplitudes exceeding 20% locally. The cloud liquid water path shows a similar diurnal variation, with amplitude of nearly 75% of the peaks. These results suggest that the changes of cloud thickness play an important role in the diurnal variations. Diagnosed cloud base and cloud top heights reveal larger changes in cloud base height than in cloud top height, implying that solar heating penetrates the cloud layer during daytime and entrainment of dry air dilutes the cloud water without totally destroying the stratocumulus clouds. It seems that surface latent heat does not play an important role in the diurnal variation.lt;img border=0 src="images/A43K-04_B.jpg">