Dynamical effects of ice sublimation in a frontal wave

The dynamical role of ice sublimation in weather systems is briefly reviewe d. observations are presented from the Fronts and Atlantic Storm-Track EXpe riment (FASTEX) Intensive Observation Period 16 that show variations of sta tic stability, humidity and mesoscale circulation corresponding to those as sociated theoretically with sublimation of ice precipitation. It is thus su ggested that the observations display the mechanism proposed by Clough and Franks in which forward mesoscale Rows are associated with moist adiabatic descent supported by the sublimation cooling. This mechanism was suggested as an important stage in the evolution of many mesoscale rain bands. A set of three model simulations of the event has been made with versions o f The Met. Office's Unified Model. Of these a mesoscale model integration w ith II km resolution and 45 levels clearly displays the symptoms, and is di agnosed to demonstrate its consistency with the Clough-Franks mechanism. An integration omitting the cooling due to sublimation differs significantly from the full model experiment in the structure of low-level wind fields, f rontal troughs and mesoscale precipitation distribution. It is also demonst rated that the static-stability transition, mesoscale circulation and mid-t ropospheric potential-vorticity perturbations are substantially weakened in this integration, thus confirming that the Clough-Franks mechanism is also operating in the numerical weather prediction (NWP) model. We deduce from these studies that ice precipitation and its sublimation has a major role in determining mesoscale circulation and structure in mid-lat itude weather systems, affecting stratification and the formation of featur es such as fronts and rain bands. These are substantially affected by the f all and evaporation of ice crystals, which are both important to temperatur e and moisture transports and the behaviour of NWP models on timescales of hours to days. In our integrations dynamical feedback due to sublimation co oling coincided with extreme negative potential-vorticity values and potent ially conditional symmetric instability, hence the anticyclonic motion occu rring in the cloud head or deep cloud of the moist warm sector as in this c ase. In moist warm sectors a substantial role for sublimation may he antici pated more generally, particularly for air trajectories receiving most ice precipitation. We suggest that the described phenomenon be referred to as sublimation enha nced descent or SED. It is concluded that in view of this demonstrated sensitivity substantial a ttention should he given to refining microphysical parametrizations in NWP models, and that radar and sounding observations from the FASTEX experiment provide a suitable basis for validating these schemes.