The Hampstead storm: A numerical simulation of a quasi‐stationary cumulonimbus system

Abstract

Abstract A numerical simulation of the storm which occurred over north London on 14 August 1975 is presented. This storm was notable in giving record rainfall totals for the London area while confining the rainfall to an area of about 100km 2 despite lasting three hours. The results of the simulation define a distinctive model of the storm dynamics, featuring an impulsive yet quasi‐stationary system of cumulonimbus which contributed successively to the rainfall over a limited area. Furthermore the simulation emphasizes the dominant role of the downdraught outflow which establishes and sustains the storm through its interaction with the low‐level flow. A mechanism by which the prevailing vector shear maintains the storm is presented. Trajectories computed for the simulated storm show an interlocking updraught/downdraught structure similar to that proposed by Browning (1964). In order to simulate the observed high surface rainfall it was found necessary to model the high fallspeeds associated with hail by a simple modification to the ‘warm’ rain parameterization. The simulated and observed rainfall patterns compare remarkably well. The localization of the storm is considered but the local orography is thought to have no significant influence. Dynamical points arising from the model identify aspects of storm modelling that require further investigation.