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ABSTRACT
Over the past few years the way in which central forecast guidance is disseminated by forecasters in the Met Office headquarters has been changing, with an increasing reliance on modification of variables' output from NWP models. The editing of grids of forecast data at the Met Office Operations Centre in Exeter is described, and two case studies are presented. Results of verification of modified versus raw fields are shown, presenting the concept of "lead time gain" as a unifying measure of relative forecast accuracy. At all lead times net lead time gain outweighs time spent considering and effecting modifications.
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1. Overview
Since 1997, grids of numerical weather prediction (NWP) output have been modified by forecasters at the National Meteorological Centre, Bracknell, United Kingdom, and now the Operations Centre at the Met Office's new headquarters in Exeter. An application that allows modification of fields via quasigeostrophic potential vorticity (Carroll 1997) has been in use, allowing forecasters to reposition, intensify, or weaken features such as depressions and fronts. Working on a large-scale domain that corresponds with the standard analysis and forecast guidance products, ASXX, FSXX forecasters modify, when necessary, output from the glpbal model at a resolution of about 50 km out to 132 h. Synoptic features are manipulated via their associated potential vorticity (PV) along with the boundary temperature; the PV is then inverted to give a new balanced and realistic set of dynamic fields. Wind is retrieved by remapping the ageostrophic component and adding it to the recalculated geostrophic part of the wind, while a simple boundary layer model helps recalculate low-level winds. Humidity and precipitation have the same remapping function applied as the PV, ensuring that these water parameters retain a realistic distribution in relation to the dynamic forcing features. The whole process is quick and ensures that the new fields are realistic, the PV approach imposing a dynamical consistency.
More recently, since October 2002, a new application has been in use that extends the functionality to allow modification of more parameters on a finer grid; the focus here being mainly sensible weather on the mesoscale up to 36 h ahead. The two applications are referred to as field modification I and field modification...