Numerical Simulation of an Explosively Deepening Cyclone over the North Atlantic that was Unaffected by Concurrent Surface Energy Fluxes

Abstract

The effect of surface sensible and latent heat fluxes on explosive cyclogenesis has been found to be quite variable. Sensitivity tests reveal little or no influence in some cases and dramatic effects in others. However, in nearly all cases where little impact was found, the predicted deepening rate was significantly less than the observed rate, opening the possibility that the unexplained deepening might be a consequence of a shortcoming of the flux formulation. We present here the results of a test of the impact of the fluxes in a case of extreme deepening that was accurately predicted by the ECMWF T106 operational model. In the test the fluxes were switched off during the 24-h period of most rapid deepening. The result was to change the predicted deepening rate of 48 mb by only 1 mb. This confirms that in certain circumstances the concurrent fluxes can have negligible effect on the deepening despite their great importance in other situations. It is shown that downward heat and moisture fluxes (cooling and drying) occurred in the warm sector close to the low center, whereas upward heat and moisture fluxes (warming and moistening) occurred in the cold air well to the rear of the low. It is known from theory that this pattern is not conducive to cyclonic development. The cause of the vast difference in the effect of the fluxes on cyclone development is discussed in terms of the stage of development, the geographical location, and the degree of atmospheric preconditioning.