Synoptic Disturbances Found in Precipitable Water Fields North of Equatorial Africa.
TEXAS A AND M UNIV COLLEGE STATION DEPT OF METEOROLOGY
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The origin and structure of tropical synoptic scale precipitable water PW anomalies estimated from TOVS satellite observations are analyzed as they propagate eastward across northern Africa during MAM 1988. Previous studies, accomplished by Mackey 1996, determined the location and season for the strongest and most coherent 3 to 8 day filtered PW signal. NCEPNCAR reanalysis 2.50 resolution provides data for a case study analysis of seven prominent anomalous PW events during the period. Six case wind field composite analysis is accomplished with both actual winds and reduced shear winds zonal mean removed at each latitude band. Case study analysis revealed a doppler shifted propagating Rossby Wave k 6, c -5 msec at 500hPa and 250hPa that moved onto the west African coast imbedded within a mean zonal flow of approximately 17 msec. The wave then moved east across the continent at approximately 12 msec near 20 deg N. The waves associated cusp-like feature advects deep tropical moisture northward, resulting in Mackeys PW anomaly. Analysis of potential vorticity, specific humidity, temperature, and wind field composites at 850hPa, 500hPa and 250hPa fit well within the range of Rossby wave structure. Descriptive statistical analysis established confidence intervals for the composites at two resolutions within the localized grid 20 deg W to 40 deg E, 0 deg to 30 deg N. An empirical model is provided showing a four stage development of the PW anomalies during the five primary days of their life cycle and days prior.