Joint Centre for Mesoscale Meteorology (JCMM)

Observations and model simulations of the FASTEX IOP frontal wave - effects of sublimation
by Sid A Clough, Humphrey W Lean, Nigel M Roberts and Richard M Forbes
Submitted to the Quarterly Journal of the Royal Meteorological Society, December 1998

The dynamical role of sublimation in weather systems is briefly reviewed.  Observations are presented from FASTEX IOP 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 rainbands.

A set of 3 model simulations of the event has been made with versions of the Meteorological 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 with omission of 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.  It is also demonstrated that the stability transition and mesoscale circulation are substantially weakened in this integration, thus confirming that the hypothesised mechanism is also operating in the 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 rainbands.  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 was associated with the extreme negative potential vorticity values predicted by the model, and hence the anticyclonic motion occurring in the cloud head or deep cloud of the moist warm sector as in this case.  In this latter situation a substantial role for sublimation may be anticipated more generally.

We suggest that the described phenomenon be referred to as sublimation enhanced descent (SED).

It is concluded that in view of this demonstrated sensitivity substantial attention should be given to refining microphysical parametrisations in NWP models, and that observations from the FASTEX experiment provide a suitable basis for validating these schemes.

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