Trajectory mapping of water vapour measured by the Microwave Limb Sounder

• Main Author: Black, Robert
• Published: University of Edinburgh 1997
• Subjects: 551.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.641675

This thesis is concerned with the development, and subsequent use of a new trajectory mapping technique to produce synoptic analyses of water vapour in the middle atmosphere. The technique has been applied to water vapour measurements in the middle atmosphere made by the Microwave Limb Sounder instrument (MLS) aboard the Upper Atmosphere Research Satellite (UARS). These measurements cover the time period early October 1991 until mid-April 1993. The technique assumes that 1) the water vapour data used to initialise the trajectories are conserved and 2) the trajectories are adiabatic for the length of the advection. The technique uses a simple trajectory model and numerical wind fields interpolated to isentropic surfaces to produce equal-area gridded analyses. Trajectories are initialised using MLS water vapour data and have their value altered as MLS measurements are assimilated into the trajectory scheme at their time and position of measurement. In this manner the evolution of the water vapour field is constrained as much by the MLS water vapour measurements as by the wind fields, in contrast to pure trajectory techniques which are heavily sensitive to the quality of the meteorological analyses. The synoptic analyses produced by the technique are shown to greatly enhance the measurements of the MLS by increasing the spatial resolution and are more accurate than analyses produced by gridding the daily data and approximating this to be a synoptic map. Analyses have been used in two case studies of atmospheric phenomena. The first has been to quantify, over an annual timescale, the irreversible transport of tropical air into northern and southern hemisphere mid-latitudes. Version 3 data were used. It is found that during 1992 roughly fifty per cent more tropical air is exchanged into northern mid-latitudes than into southern mid-latitudes. The exchange is associated with breaking planetary waves episodically disturbing the poleward edge of the tropical reservoir. The second case study involved looking at the temporal evolution of Arctic water vapour measured during the northern hemisphere winters of 1991-92 and 1992-93.