Impact of the inflow moisture on the evolution of a warm conveyor belt

Abstract

This case study of a warm conveyor belt (WCB) event that was probed on a research flight during the THORPEX (The Observing-System Research and Predictability Experiment) Pacific Asian Regional Campaign (T-PARC) field experiment in 2008, investigates the sensitivity of the forecast of the WCB, the associated cyclone and the downstream waveguide to the moisture content in the inflow region of the WCB. By assimilating water vapour profiles of a differential absorption lidar (DIAL) into the European Centre for Medium Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS), the inflow moisture in the analysis fields is adjusted and humidity is reduced in a broad region around the flight track. The initial reduction of moisture in the WCB inflow affects the latent heat release along the WCB, as well as the potential vorticity (PV) production at lower levels. This change led to a substantially lower outflow height of the forecasted WCB. Further, the height of the tropopause was reduced up to 20 hPa, which caused a change in the jet stream wind speeds of up to 15% downstream. Although the impact on the developing surface cyclone was small, improvements of the PV structure as well as of the kinetic energy could be identified.