Modeling of Cold Fronts

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Since the inception of the polar front theory, the nature of fronts and their relationship to cyclogenesis and precipitation has been an active area of meteorological research. The study of fronts is important because they are often the locus of significant weather occurrences (strong temperature changes, strong winds, formation of clouds and precipitation) that present hazards to human activities in midlatitudes. While much has been learned about fronts through observational, theoretical, and numerical modeling work in the past century, the development of certain types of frontal structures are still not clearly understood. In this research, the Principal Investigators will study two aspects of mid-latitude cold fronts: the forward-tilting structure that some lower-tropospheric cold fronts adopt under certain meteorological conditions and in certain geographical areas; and the corrugated narrow rainband structure that many oceanic cold fronts adopt at their leading edge. Specifically, they will test the effects of horizontal contrasts in static stability (and potential vorticity) on the formation of forward-tilting cold fronts; and the relative importance of horizontal shear instability, precipitation microphysics, and trapped gravity waves in organizing the corrugated cellular structure of narrow cold-frontal rainbands.A natural approach to this problem is to carry out simulations with a numerical model, because it can be used to isolate the essential physical and dynamical processes involved, and outputs can be compared to the many observed cases of the phenomena in question. A combination of both "idealized" and "real-weather" simulations offers the greatest chance of success, and that is the approach that will be taken in this study. With idealized simulations, initial and boundary conditions are specified and physics are simplified in order to isolate and understand particular mechanisms and controlling parameters. With real-weather simulations, initial and boundary conditions based on 3 D observational analyses are used to drive simulations with full model physics, to test if the proposed controlling parameters and mechanisms behave as expected in the full complexity of the atmosphere. The Weather Research and Forecast Model is ideally suited to carry out the research. This mesoscale model is specifically designed to resolve processes on the scale of 1-10 km, and has the unique capability of being configured to perform both idealized and real weather experiments. A variety of simulations will be designed to test the hypothesized processes mentioned above. These will include control simulations of fronts, and tests of sensitivity to parameters that are considered important in controlling the hypothesized mechanisms, such as vertical profiles of shear and stability, horizontal wind shear patterns, microphysical processes and diabatic heating, etc. Intellectual Merit: Both the forward-tilting structure and the corrugated narrow rainband structure of some lower-tropospheric cold fronts are well documented, but not well understood, phenomena. Several viable hypotheses involving dynamical and cloud microphysical processes have been proposed to explain these phenomena, but the task remains to test them, in order to understand the parameters that will help predict where upward motion, clouds, and precipitation occur and how they are organized. This study will investigate these controlling mechanisms and thereby advance our understanding of these important frontal phenomena. Broader Impacts: The phenomena to be studied are of direct importance to weather prediction because they determine where precipitation and hazardous weather occur relative to the surface cold frontal boundary, a feature that tends to draw much attention from weather forecasters. Working toward better understanding and prediction of where and when disruptive weather occurs clearly has direct potential benefits to society.