Two years of winter season data were considered; the first was comprised of observations from December-February of 1997-1998; the second from January-March of 2001. In total, 953 hours of overcast low stratiform clouds were analyzed. All clouds were divided into two distinct categories: 531 hours of boundary layer stratocumulus (with tops below 1.5 km and a typical depth of several hundreds meters), and 422 hours of low altitude stratiform clouds (with bases below 2 km and a typical depth of up to a couple of kilometers). To study the role of precipitation each category was further partitioned into sub-sets of precipitating and non- precipitating cloud segments using as a threshold radar reflectivity value of -17 dBZ.
The results show that for boundary layer stratocumulus the total duration of non-precipitating segments is about 3 times greater and they are 2.4 times more frequent compared to their precipitating cloud segments. For low altitude stratiform clouds, on the contrary, the total duration of precipitating segments is about 1.3 times greater with about the same frequency of occurrence compared to their non-precipitating cloud segments. In boundary layer stratocumulus about 95% of non- and precipitating cloud segments have duration less than 6.2 hrs and 4.0 hrs, respectively. In low altitude stratus 95% of non- and precipitating cloud segments have duration less than 4.25 hrs and 5.5 hrs, respectively.
The analysis also shows that for boundary layer stratocumulus the PDFs of reflectivity, averaged over all non- or precipitating segments, are quite symmetrical and are very well approximated by a two parameter Gamma function. For low altitude stratiform clouds those average PDFs are highly skewed due to its greater reflectivity and demonstrate markedly different character (negative skewness) compared to boundary layer stratocumulus, related to difference in cloud driving forcing mechanisms.
The average PDFs of LWC for non-precipitating clouds are obtained using radar data and Frisch et al technique. Boundary layer and low altitude clouds have obviously markedly different PDFs of LWC suggesting presence of significant differences between their variability properties. The average PDFs of precipitation flux were retrieved from the radar data using relationships derived from LES simulations. The average precipitation flux is ~ 3 times greater in low altitude stratiform clouds than in boundary layer stratocumulus.
Our analysis suggests that precipitating clouds exhibit much stronger variability than non-precipitating clouds. This effect of precipitation is especially pronounced in low altitude stratiform clouds. Separate parameterizations of sub-grid scale variability for non-precipitating and precipitating clouds are needed. Variability in boundary layer stratocumulus and low altitude stratiform clouds differs substantially and also needed to be parameterized separately.
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