The transformation of an African Easterly Wave (AEW) into a tropical cyclone in north Atlantic is studied using mainly the UKMO Limited-Area Unified Model (LAM) (0.4425 degrees horizontal resolution) with initial conditions and sea-surface temperatures (SSTs) taken from the operational UKMO analysis. Initially, the AEW trough appeared to be influenced by the dry, low potential vorticity, lower-tropospheric Saharan Air Layer (SAL) and no intensification
occurred. The SAL is an inhibiting factor for the development of the AEWs because inflow of this dry air mass in the trough of the system may cause strong downdrafts lowering the boundary layer equivalent potential temperature, stabilising the low-level environment and therefore reducing the convective activity. When the simulated AEW moved away from the influence of the SAL it continued to contract in scale and transformation into a weak tropical cyclone (with 10m. wind speed equal to 16.37 m/sec) with a warm core structure occurred. Previous studies have shown that the reduction of the radius of maximum winds is necessary for intensification of a vortex. Strong convective heating leading to strengthening of the AEW trough and
therefore of the surface winds and the surface latent heat fluxes occurred during the first stage of the transformation. After this stage the system probably intensified through the WISHE mechanism. Hence, the transformation of the AEW trough (with stronger relative vorticity near 700mb) into a strong barotropic vortex together with the enhancement of the middle tropospheric relative humidity due to the continuing convective activity were the processes taking place initially. The strengthening of the absolute vorticity and increase of the middle tropospheric relative humidity are necessary for
reducing the Rossby radius of deformation (thus reducing the gravity wave activity) and for reducing the inhibiting effect of the downdrafts, respectively. After the WISHE mechanism is switched on, the system will attain a warm core structure in order to be in balance. In this simulation the system transformed into a tropical cyclone with maximum cyclonic winds near the surface and an anticyclone aloft (but at larger radii). The processes taking place in the intensifying period were explored further using the mesoscale version (0.15 degrees horizontal resolution) of the UKMO Unified
model. In this run the AEW trough transformed into a hurricane with a very realistic warm core structure and maximum 10m. wind speed equal to 42 m/sec. The role of the warm SSTs that prevailed in the region (during intensification) was explored; the initial conditions were taken from August 1995. An identical LAM simulation performed with climatological (that is, cooler) SSTs resulted in a much weaker system and no intensification occurred; pointing out the important role of the SSTs. The role of the ongoing convective activity in the scale contraction of the system was explored in a 'dry' run in which the surface fluxes of moisture were switched off