Abstract
AbstractThe potential vorticity (PV) in warm conveyor belts (WCBs) is strongly influenced by the latent heating associated with the various microphysical processes occurring during the formation of clouds. The first‐order effect is that PV increases below the level of maximum diabatic heating and decreases above. Thus, the WCB reaches the upper troposphere with low PV values and has the potential to influence the large‐scale dynamics. In order to quantify the influence of different microphysical processes on the diabatic heating rates (DHRs) and associated PV development during the ascent, a Lagrangian analysis is used and applied to a regional model simulation of a selected WCB event. First, the individual DHRs caused by the various microphysical processes are calculated with the COSMO model. Then, the DHRs and the associated changes in PV are evaluated along the WCB trajectories. The relative role of the different microphysical processes is quantified for the latent heating and the diabatic PV modification, for which the gradient of the latent heating and the absolute vorticity are crucially important. It is shown that condensation of vapour and depositional growth of snow each contribute ∼10 K to the total latent heating. However, the diabatic PV modification due to condensation is stronger since it occurs close to the cold frontal low‐level maximum of the z‐component of the absolute vorticity, ηz, whereas a similar heating rate gradient caused by depositional growth of snow modifies the PV much less since it occurs in a region with much weaker ηz. This highlights the importance of ηz for determining the PV modification due to a certain diabatic heating rate. Furthermore, cooling processes like the evaporation of rain, also co‐occurring with high ηz near the surface cold front, have the potential to strongly modify the PV below the WCB. Copyright © 2011 Royal Meteorological Society
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