Development of a New Numerical Model for Studying Microscale Atmospheric Dynamics

Studies of the small-scale atmosphere improve our understanding of dynamics and interactions that occur over distances of a few metres to a few hundred metres. \Vith increased understanding of the science and advances .in computer technology, the ultimate aim is to improve the skill of high resolution numerical weather prediction (NWP) models, thereby helping to protect against the effects of severe localised weather events such as flash floods. Increasing the resolution of N\VP models results in more of the variations in the underlying orography being captured by the model grid. Consequently, high resolution models must be able to simulate flows over steeper and more sharply varying terrain than is captured in more coarsely resolved models. Most current NWP models resolve flows over hills using terrain-following grids, which have been seen to suffer from numerical instabilities and erroneous flo\v-fields above steep hills. This thesis discusses the development of a three-dimensional, nonhydrostatic, fully ___~ ~~p~ess~~le_ model designed for studies_of_''-C!l ~~!S~_~e~()l~~ion~yn~mics~ _T~~ ~. model implements a terrain-intersecting grid, such that the introduction of uneven orography results in cut-cells. A finite-volume discretisation approach is used to solve flows through the irregularly-shaped cut-cells. The model applies a fully explicit time-splitting integration method to help efficiently resolve acoustic waves, combined with leapfrog time-differencing and centred spatial-differencing schemes. Results are presented for a series of idealised cases of flows over hills with increasingly steep gradients, and horizontal resolutions ranging from 2km to 40m. \Vhere possible, comparisons are made with analytical solutions and results from other established models. The terrain-intersecting model is seen to show go.od agreement with both analytical solutions and traditional terrain-following models; and compares favourably with results from a step-orography approach. When applied to relatively steep gradients, the terrain-intersecting method shows no evidence of the instabilities associated with terrain-following grids.EThOS - Electronic Theses Online ServiceGBUnited Kingdo