Dynamical effects of ice sublimation in a frontal wave

Abstract

The dynamical role of ice sublimation in weather systems is briefly reviewed. Observations are presented from the Fronts and Atlantic Storm-Track Experiment (FASTEX) Intensive Observation Period 16 that show variations of static stability, humidity and mesoscale circulation corresponding to those associated theoretically with sublimation of ice precipitation. It is thus suggested that the observations display the mechanism proposed by Clough and Franks in which forward mesoscale flows 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 of The Mel. Office's Unified Model. Of these a mesoscale model integration with 11 km resolution and 45 levels clearly displays the symptoms, and is diagnosed 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, frontal troughs and mesoscale precipitation distribution. Il is also demonstrated that the static-stability transition, mesoscale circulation and mid-tropospheric 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-latitude weather systems, affecting stratification and the formation of features such as fronts and rain bands. These are substantially affected by the fall and evaporation of ice crystals, which are both important to temperature and moisture transports and the behaviour of NWP models on time-scales of hours to days. In our integrations dynamical feedback due to sublimation cooling coincided with extreme negative potential-vorticity values and potentially conditional symmetric instability, hence the anticyclonic motion occurring in the cloud head or deep cloud of the moist warm sector as in this case. In moist warm sectors a substantial role for sublimation may be anticipated more generally, particularly for air trajectories receiving most ice precipitation. We suggest that the described phenomenon be referred to as sublimation enhanced descent or SED. It is concluded that in view of tins demonstrated sensitivity substantial attention should be 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.