The effects of diabatic heating on storm track dynamics

The dynamical effects of diabatic heating in Northern Hemisphere winter storm tracks are studied in the context of a nonlinear primitive equation model with vertical and horizontal resolution adequate to capture shallow, short wavelength baroclinic waves. The model is forced by realistic orography and diabatic processes including radiative relaxation, surface heating and condensational heating. Time-mean analyses of the statistically equilibrated eddy ensemble includes time-mean statistics, global spectral energetics, local energetics and wavepacket maps. Preliminary investigations of linear and nonlinear zonal baroclinic instability and nonlinear baroclinic wavepacket evolution are presented to verify and extend the results of Mak (1998) regarding surface heating effects on modal baroclinic instability. To address the fully nonlinear storm track dynamics four numerical experiments are analyzed: (1) CONTROL with orography, drag and radiative relaxation, (2) SURF with additional surface heating and (3) FULL with additional surface and condensational heating which yields eddy variance and time-mean flow structure in best agreement with observations and (4) FLAT without orography but including full diabatic heating. Orography forces stationary waves which control the storm track localization, while radiative relaxation serves to maintain baroclinicity. Surface heating suppresses storm track amplitude in contrast to condensational heating which enhances the storm track amplitudes. Both heating processes localize storm tracks over oceans by attenuating eddy variance over continents. Generation of correct Atlantic-Pacific storm track asymmetry is associated with preferential amplification of the Atlantic storm track by heating. This is associated with a midlatitude baroclinic waveguide seeded upstream of the Pacific storm track and attenuated downstream of the Atlantic storm track. The time-mean jet modifications follow eddy flux perturbations induced by zonally inhomogeneous heating while the surface temperature field is strongly controlled by surface heat flux regulation.