Ensemble Sensitivity Analysis in the Operational Met Office in the UK Ensemble System

The Met Office in the UK is exploring the use of ensemble sensitivity analysis (ESA) as an operational tool to support its upcoming focus on its global ensemble system. Ensemble sensitivity analysis is a technique that identifies atmospheric flow features throughout a forecast period that are relevant to high-impact forecast aspects such as high winds, heavy precipitation, and extreme temperatures (known as response functions). ESA typically highlights the importance of the position or magnitude of features like upper-level troughs, ridges, and wind maxima/minima in the jet stream, as well as structure in low-level pressure and moisture fields, to the response function. Since ESA also identifies specifically how features are associated with differences in high-impact response functions (e.g., an eastward shift of a 300hPa geopotential height trough off the U.S. east coast might be associated with heavier precipitation two days later in the UK), it can add substantial value to the forecasting process through forecaster awareness. This value can be realized through both improved dynamical understanding of high-impact flows and ensemble subsetting, a method that weights ensemble members more if they are more skillful in sensitive areas.

 

The Met Office in the UK has created a real-time ESA tool for initial evaluation to understand its value in the forecasting process. Wind, precipitation, temperature, and visibility response functions to seven-day forecast time over the UK, both coverage and maximum values, serve as the response functions. Sensitivities to geopotential height and wind speed aloft, surface pressure, and simulated water vapor imagery are produced every six hours from the response function backward to initial forecast time. This presentation involves what operational forecasters and research personnel have learned from day-to-day ensemble sensitivity fields, the use of ESA in the forecasting process, and the climatological nature of sensitivity. Future plans for the Met Office in the UK ESA tool will also be discussed.