Latent heating and cloud processes in warm fronts

Abstract

The results of two studies are presented in this thesis. In the first, an extratropical cyclone that crossed the United States on April 9-11 2009 was successfully simulated at high resolution (3km horizontal grid spacing) using the Colorado State University Regional Atmospheric Modeling System. The sensitivity of the associated warm front to increasing pollution levels was then explored by conducting the same experiment with three different background profiles of cloud-nucleating aerosol concentration. To our knowledge, no study has examined the indirect effects of aerosols on warm fronts. First the budgets of ice, cloud water, and rain in the simulation with the lowest aerosol concentrations were examined. The ice mass was found to be produced in equal amounts through vapor deposition and riming and the melting of ice produced ~75% of the total rain. Conversion of cloud water to rain accounted for the other 25%. When cloud-nucleating aerosol concentrations were increased, significant changes were seen in the budget terms, but total precipitation was relatively constant. Vapor deposition onto ice increased, but riming of cloud water decreased such that there was only a small change in the total ice production and hence there was no significant change in melting. These responses can be understood in terms of a buffering effect in which smaller cloud droplets in the mixed phase region lead to both an enhanced Bergeron process and decreased riming efficiencies with increasing aerosol concentrations. Overall, while large changes were seen in the microphysical structure of the frontal cloud, cloud-nucleating aerosols had little impact on the precipitation production of the warm front. The second study addresses the role of latent heating associated with the warm front by assessing the relative contributions of individual cloud processes to latent heating and frontogenesis in both the horizontal and vertical directions. Condensation and cloud droplet nucleation are the largest sources of latent heat along the frontal surface and together produce rates of horizontal frontogenesis that are of the same order of magnitude as the deformation and tilting terms at midlevels; however near the surface latent heating does not cause strong frontogenesis. In the vertical, frontogenesis caused by these two processes is nearly everywhere higher than frontogenesis caused by dry dynamics, and are the primary mechanisms through which high static stability is found along the frontal surface. The horizontal and vertical components of frontogenesis are combined in a new way to form an expression for the frontal slope tendency. While dynamic processes lead to increases in frontal tilt, latent heating often counteracts this tendency. This indicates that the direct effect of latent heating on the thermal structure of the front is to decrease the slope and in that sense weaken the warm front.