The Effect of Latent Heat Release on the Evolution of a Warm Occluded Thermal Structure

Abstract

The effect of latent heat release on the development of the occluded thermal structure in a major winter storm is examined through comparison of full physics (FP) and no-latent-heat-release (NLHR) simulations of the event performed using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Though both simulations possess a well-developed occluded thermal ridge near the surface, the 3D structure of their respective occluded quadrants is quite different. In particular, the FP simulation depicts the canonical, troposphere-deep warm occluded thermal structure, whereas the NLHR simulation produces only a shallow, poorly developed one. Consistent with these differences in tropospheric thermal structure, the FP cyclone displays a robust treble clef potential vorticity (PV) distribution in the upper troposphere in its postmature phase, while a considerably less robust version characterizes the NLHR simulation. The PV minimum of the treble clef overlies a poleward sloping column of warm, weakly stratified air that extends through the depth of the troposphere and is a signature of the trowal, the essential structural feature of warm occluded cyclones. Consequently, examination of the role played by latent heat release in production of the occluded thermal structure in this case is made through consideration of its influence on the evolution of the upper-tropospheric PV morphology. It is found that direct dilution of upper-tropospheric PV by midtropospheric latent heat release initiates formation of a local, upper-tropospheric PV minimum, or low PV tongue, to the northwest of the surface cyclone center. The production of this PV minimum initiates a cutting off of the upper-tropospheric PV anomaly associated with the surface development. The upper-tropospheric circulation associated with this cutoff anomaly, in turn, forces the advection of low (<1 PVU) values of PV into the developing PV trough. This combination of kinematic and diabatic processes acts to produce both the tropopause PV treble clef as well as the underlying warm occluded thermal structure in the FP simulation. In contrast, though an adiabatic kinematic tendency for production of a treble clef PV morphology operates in the NLHR simulation, the resulting PV and thermal structures are weaker and slower to evolve than those produced in the FP simulation. Thus, it is suggested that latent heat release plays an indispensable role in the production of the characteristic occluded thermal structures observed in nature.