Diurnal Variations in Cloud Frequency over the Gulf Stream Locale

Abstract

This paper documents evidence of a diurnal variability in cloudiness over the Gulf Stream locale. The Gulf Stream locale (GSL) is defined as the region covering 31°?38°N, 82°?71°W. The Gulf Stream, which occupies a portion of the GSL, is a warm current of water that flows south to north along the east coast of the United States and provides conditions conducive for the development of cloudiness. Cloud heights derived from the GOES VISSR (Visible-infrared Spin Scan Radiometer) Atmospheric Sounder (VAS) are obtained and used to produce a 7-yr climatology of the diurnal variation in the frequency of low-, middle-,and high-level cloudiness. The climatology is segregated into summer and winter seasons. Diurnal variations are found during the summer and winter. Satellite observations over land indicate a maximum in the frequency of low cloudiness during daytime and a minimum at night. In addition, high cloudiness is found to increase significantly late in the afternoon and evening. Over the Gulf Stream region, high cloudiness is found most frequently in the mid- to late morning hours. A midafternoon maximum in low cloudiness is found along the coastline of Georgia and South Carolina and north of the Gulf Stream east of Virginia. Nocturnal minimums in low cloudiness are reported in these regions. Results suggest that summertime low and high cloudiness over the GSL are related to prevalent convective activity. An analysis of the diurnally oscillating pattern of boundary layer convergence, derived from analyses from the National Meteorological Center's step coordinate model, indicates a strong relationship to the presence of high cloudiness. The strong correspondence between the timing of these two parameters suggests that atmosphere dynamics play a significant role in the diurnal cycle in high cloudiness. In winter, when convective activity is suppressed there is less detectable response of the atmosphere to the 24-h solar cycle manifest in the diurnal variations of clouds. Nevertheless low- and midlevel cloudiness are found most frequently in the predawn hours, except over the Gulf Stream where low clouds exhibit an afternoon maximum and a nocturnal minimum. Surface observations of cloudiness support the diurnal variations reported by VAS.