Role of the Convective Scheme in Modeling Initiation and Intensification of Tropical Depressions over the North Atlantic

Abstract:

The tropical cyclone mesoscale structure is not represented in global climate models (GCM) with horizontal resolution of 0.5° to 1°. It is however possible to simulate initiation and intensification of tropical depressions vortices (TDV) that have characteristics similar to observed tropical cyclones. This is especially interesting to analyze modification of the cyclonic activity related, for example, to global warming. In this framework, it is necessary to understand how TDV characteristics are related to the GCM formulation. We use the LMD-Z GCM to study the sensitivity of TDV characteristics to different formulations of the convective scheme. Using the “zoom” capability of LMD-Z, the horizontal resolution is set to 0.75° over a large region covering West Africa and the North Atlantic. The GCM is free to run in this region and is tied to ERA-Interim reanalysis outside that region. We use the Tiedtke convective scheme with entrainment and closure based on the moisture convergence, or with an entrainment based on the relative humidity of the environment, and additionally with a closure based on CAPE. Each configuration is run for 10 years between 2000 and 2009 with prescribed SST.

An original approach is used to track TDV and the results are compared to ERA-Interim. Another tracking algorithm based on vorticity, wind speed and warm core, is also used to estimate the associated cyclonic activity and to compare it to observations (i.e. the IBTrACS dataset). Results show a large sensitivity of the TDV characteristics (such as initiation, duration, strength) to the formulation of the convective scheme. For example, the entrainment based on the relative humidity tends to increase the convective precipitation in the moist environment of the vortices above the ocean, leading to stronger and more numerous TDVs with longer duration. On the opposite, most TDV initiated over West Africa dissipate too soon over the continent with the entrainment based on the relative humidity and do not reach the North Atlantic. The origin of the TDV in the GCM is thus dependent on characteristics of the convective scheme. Implications for the study of the sensitivity of cyclonic activity to large-scale forcing, related for example to a warmer climate, are discussed.