Fluctuations of High Clouds and 500-mb Geopotential Heights in Baroclinic Waveguides

Abstract

A time filter that passes waves with periods in the 2.5-6.0 day band is applied to a six-winter record of the Nimbus-7 THIR/TOMS high cloudiness and the NMC 500-mb geopotential height in the northern extratropics. The strongest correlations between fluctuations in geopotential and high cloudiness are found in the baroclinic waveguides, where the fields of both geopotential and high cloudiness exhibit 1arge variabilities. Over many grid points in the waveguides, positive anomalies in high cloud areas are found to be approximately one-third of a wavelength to the east of negative anomalies in 500-mb heights (band-pass troughs), and negative anomalies in high cloud arms are found to be approximately one-sixth of a wavelength to the west. A map of the standardized anomalies in the cloud area associated with height fluctuations above the mean forms a simple negative of the map of the cloud anomalies associated with height fluctuations below the mean. The analysis presented here suggests that the high cloud structures of baroclinic waves are less spatially coherent than the internal geopotential height structures. Over the North Pacific, small-scale (latitudinal wavenumber 13-18) fluctuations in geopotential appear to play a greater role in forcing high cloudiness than do medium-wale (latitudinal wavenumber 7-12) fluctuations in geopotential.