Impact of airborne Doppler lidar observations on ECMWF forecasts

Abstract

The wind field over oceans is still poorly observed. Single-level wind measurements are provided at the surface by buoys, ships and satellite scatterometers, while aircraft observe wind mainly at the cruise level along the air traffic corridors. Wind profiles are only provided by a small number of radiosondes launched from ships. Satellite cloud-drift winds are numerous, but they have fairly large errors due to inaccurate height assignment. In such a framework, Doppler wind lidar offers a great opportunity to measure the wind field either globally with a polar-orbiting satellite or regionally when mounted on aircraft. During the Atlantic THORPEX Regional Campaign (A-TReC) in autumn 2003, the airborne Doppler lidar of the Deutsches Zentrum für Luft- und Raumfahrt (DLR) was used to observe wind profiles over the Atlantic Ocean. In eight flights, the system measured a total of 1,600 profiles that were used experimentally in the global assimilation system at ECMWF. These lidar observations had a significant impact on the analyses as well as on forecasts due to their high accuracy and spatial resolution. On average, the measurements reduced the forecast error of geopotential height for days 2–4 over Europe by 3%. Furthermore, forecast errors of wind, temperature and humidity fields over Europe were reduced. This is a promising result, considering that observations have been gathered from only 28.5 flight hours in a two-week period. Dropsondes released in the same area where the Doppler lidar was operating showed good agreement in terms ofmeasured winds, but smaller analysis impact and less reduction of the forecast error. A detailed assessment of the experiments can be found in Weissmann and Cardinali (2007).