Low-Frequency Variations of Circulation and High Cloudiness in the Northern Extratropics: Spatial and Temporal Relationships

Abstract

The spatial and temporal relationships between fluctuations in geopotential height and high-cloud fractional area in low-pass (periods greater than 10 days) and intermediate-pass (10?30 days) time scales are investigated and compared with relationships in the fast-pass (2.5?6 days) time scale. NMC 500-hPa height and Nimbus-7 THIR/TOMS high-cloud data are used for extended 6-month (October-March) winters (1979?85). Summary correlation maps describe the spatial phase relationships between the heights and the clouds over the full domain of the northern extratropics. As we move from the slower low-pass and intermediate-pass regimes to the fast-pass regime, the temporal variance of the height field decreases, but the temporal variance of the high cloudiness increases. Surprisingly, the 500-hPa height and high-cloud fields are more strongly correlated in the slower time scales. The spatial phase relationships between the height and cloud fields are generally different in the low-pass and fast-pass regimes. Over portions of the jet exit regions, one-half of the variance in the low-pass cloudiness can be explained by a correlation with the height fluctuations at nearby points. Over these areas, the low-pass height fluctuations are approximately equivalent barotropic, and the heights are correlated with clouds quite strongly downstream (to the east) but only weakly upstream (to the west). This contrasts with the spatial phase relationships found in the more baroclinic fast-pass regime, where fluctuations in height are correlated with clouds by about the same absolute magnitude either downstream or upstream. An analysis of the temporal evolution of intermediate-pass height and cloud fields reveals the cloud signal of two-dimensional Rossby wave dispersion over a small portion of the northern extratropics.