A Case Study of Radiative Forcing upon a Tropical Cloud Cluster System

Abstract

Tropospheric radiative convergence profiles from Cox and Griffith are used to assess the radiative forcing upon a tropical cloud cluster located in the vicinity of the GATE A/B-scale array during 4?6 September 1974. A background discussion summarizes some of the previous investigations that served as motivation for the present study. The atmospheric response to differential radiative cooling between the cluster and its surrounding environment is examined by means of ?slab? and cross section analyses over the Cox-Griffith array. A radiatively derived vertical motion model is constructed to investigate the role of radiation with respect to larger-scale dynamics during a daytime (0600?1200 LST 5 September) and nighttime (1800?2400 LST 5 September) period of the cluster life cycle. Radiative forcing is found to be strongest during the initial stages of cluster development. Throughout the cluster life cycle, the radiative forcing is consistently strongest in the middle troposphere (400?700 mb). As the cluster system intensifies, daytime shortwave warming superimposed upon the longwave cooling lessens the total radiative cooling in the surrounding cloud-free region, resulting in a lessening of the differential radiative cooling. Increased amounts of middle and high cloud remnants also contribute to the observed weakening of radiative forcing during the mature and dissipating disturbance stages. Cross section analyses reveal that E-W gradients of radiative convergence between the cluster and its surroundings are comparable in magnitude to the N-S gradients. The radiatively derived vertical motion model yields a qualitatively realistic total area of cluster influence for a nighttime case, 1800?2400 GMT on 5 September. The model assumption of a closed mass system breaks down during the daytime (0600?1200 LST, 5 September) period, yielding an unrealistically 1arge total area of cluster influence. This suggests the occurrence of significant cluster-scale interactions with large-scale circulations during the daytime period. Radiative forcing appears to play a more significant role in dynamical interactions during the nighttime period, when circulations seem to be somewhat more localized. The maximum in-cluster precipitation intensity lags the incidence of strong radiative forcing by 6?8 h, in general agreement with GATE composite observations. Continental oceanic differential beating must also play a significant role in modulating cluster- and large-scale dynamical interactions, accounting for the anomalously long precipitation lag observable in the GATE cluster. The interpretations presented herein are based solely upon this single case study and may not necessarily be representative of cluster disturbances as a whole.