Thunderstorm Cloud Top Dynamics as Inferred from Satellite Observations and a Cloud Top Parcel Model

Abstract

A Lagrangian model applicable to the overshooting region of thunderstorm tops is used to describe the temperature-height path taken by updraft core parcels as they penetrate above the tropopause, reach their maximum height and descend in the periphery of the convective tower. The model is run under a variety of ambient and in-cloud conditions in order to simulate certain temperature-height relationships observed in satellite observations. Observations indicate that in the majority of observed storm tops, the satellite-observed cold point in the IR brightness temperature (TB) field is collocated with the highest point in the convective overshooting region and the TB-height relations are near adiabatic. The parcel model quantitatively reproduces this type of relationship for model runs where the mixing parameter is relatively small. Another type of storm has a close-in, cold-warm TB couplet with a dimension of approximately 20?40 km and a V-shaped cold TB pattern. In some cases of these V-shaped storms, the cold point is clearly located upwind of the high point. Model runs have been made to reproduce a number of these salient features for these types of storms. With larger values of the mixing parameters (presumably related to larger shear), the model produces temperature-height relationships that are, of course, much closer to ambient than to adiabatic, as is observed in these cases. With the larger mixing parameter, the cold-high offset is also produced, for model runs having a relatively large initial vertical velocity and under conditions of a strong inversion. The amount of the cold-high offset is shown to be a direct function of the strength of the inversion. The cause of the close-in warm point is also explored with the simple model. As has been shown in three-dimensional cloud model results, the warm point in the cold-warm couplet can be related to internal cloud subsidence on the downwind side in association with mixing with the environment. This effect is also reproduced in the parcel model with the occurrence of a warm point being related to conditions of an intense updraft and strong mixing. The model also points to parcels subsiding from their maximum height and crossing the ambient lapse rate from negative to positive buoyancy on the downwind side and then coming into equilibrium at a relatively high level above the tropopause on the downwind side. This effect may be related to the top of the downwind anvil cloud being elevated significantly above the equilibrium point or tropopause. Another interpretation of this model result may be related to the above-anvil cirrus noted by a few investigators. The temperature-height distributions produced by the model in a Lagrangian framework are converted to the spatial domain by the assumption of steady state conditions and are compared to temperature-height cross sections determined from GOES IR and stereoscopic height fields. The locations of cold points, high points, warm points, and the magnitude of cold-high offsets compare favorably between the model and the satellite observations.