Dynamics and organisation of precipitation bands in the midlatitudes.

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Abstract

Thesis title: Dynamics and organisation of precipitation bands in the midlatitudes.Submitted by Jesse Norris to The University of Manchester for the degree of Doctorof Philosophy (PhD). 24 March 2014.This thesis was funded by the Natural Environmental Research Council (NERC)as part of the Diabatic Influences on Mesoscale Structures in Extratropical Storms(DIAMET) project. The thesis is presented in alternative format, meaning that theresults of the thesis take the form of three journal articles, each telling a distinctstory within the subject matter, but collectively highlighting the sensitivity of bandsto frictional and diabatic processes.Paper 1 is an idealised-modelling study with the Weather Research and Fore-casting (WRF) model, in which moist baroclinic waves are simulated from an initialzonally uniform midlatitude jet on an f -plane at 20-km grid spacing, and the sen-sitivity of the resulting precipitation bands is explored. Paper 2 employs furtherWRF idealised-baroclinic-wave simulations and takes a simulation from Paper 1,after the cold front has formed, as the initial condition. A nested domain at 4-kmgrid spacing is inserted when this simulation is re-initialised to invesigate the sensi-tivity of finer-scale precipitation cores along the surface cold front. In both Papers 1and 2, friction and latent-heat release enhance multiple banding at the two distincthorizontal scales, while surface fluxes hinder multiple banding.Paper 3 studies postfrontal snowbands over the English Channel and Irish Seaduring extreme cold-air outbreaks in the winters of 2009–10 and 2010–11, via a cli-matology of precipitation-radar, sounding, and SST data, and real-data WRF sensi-tivity simulations of one such band over the English Channel. The observational andmodelling results show that strong winds and large differential heat fluxes betweenland and sea were necessary to generate banded precipitation. Coastal orographyand the land–sea frictional contrast aided the morphology of bands, but bandedprecipitation did still form in the absence of these influences in the sensitivity sim-ulations.These three studies and the thesis as a whole highlight the role of frictional anddiabatic processes in modifying various types of precipitation bands within baroclinicwaves, and in generating bands that would otherwise not exist.

Keyword(s)

rainbands, snowbands, mesoscale, extratropical cyclones, cold-air outbreaks, baroclinic waves, diabatic, WRF, radar

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