Evolution of Tropical Plumes in VAS Water Vapor Imagery

Abstract

Tropical plumes are common tropical synoptic disturbances marked by continuous upper tropospheric cloud bands extending out of the ITCZ into midlatitudes. Thirty-five tropical plumes over the northeast Pacific are composited in GOES VAS water vapor imagery at four time periods throughout their evolution, from a precursor stage 48 h before they fulfil an objective plume definition until they mature 24 h after this definition stage. A ?quiescent? composite is constructed for 35 days in which no synoptic activity is occurring. Composites of outgoing longwave radiation data are constructed using the same days, for comparison. Precursor signals are identified in the vapor imagery before clouds develop: a moist midlatitude trough, a synoptic scale wave in the moisture/cloud field along the ITCZ, and an anomalously dry intervening subtropical high, all appearing in an anomalously dry tropical environment. This pattern is contrasted with the synoptically quiescent composite, a nearly zonally symmetric pattern in vapor imagery, with a convectively active ITCZ, flanked by a linearly shaped subtropical high; the quiescent tropics are generally moist. The plume evolves as a stationary, tropical, dry/moist dipole, separated by an exceptionally strong cloud/moisture gradient All features within individual composites and most variations from stage to stage are statistically robust. Tropical plume evolution is accompanied by a systematic drying of the tropical eastern Pacific atmosphere before development, and moistening and increased cloudiness with development. The precursor pattern is used as a forecasting tool applied to an independent set of vapor imagery; 65% of 29 plumes were forecast correctly by position and time of evolution. Forecasting errors were systematic.