Airborne lidar observations in the inflow region of a warm conveyor belt

Kurzfassung

Warm conveyor belts (WCBs) are key flow structures associated with extratropical cyclones. They transport moist air from the cyclone�s warm sector poleward and upward close to the tropopause level, leading to the formation of elongated cloud bands, intense latentheating and surface precipitation. In this study a comprehensive dataset of airborne lidar observations of moisture andwind fromdifferent campaigns has been investigated with a trajectory-based approach to identify �lucky encounters� with WCBs. On 19 July 2007, an upstream flight over the Iberian Peninsula during the European THORPEX Regional Campaign (ETReC 2007) in Central Europe intersected two WCBs: one in the upper tropospheric outflow region about 3 days after starting the ascent, and the other one in the boundary layer inflow region over Spain justprior to the strong ascent.Comparisonof the lidarhumiditymeasurements with analysis fields from the European Centre forMedium-RangeWeather Forecasts (ECMWF) reveals significant positive deviations, equivalent to an overestimation of the modelled humidity, in this low-tropospheric WCB inflow region (of about 1 g kg�1 (14%) on average and with peak deviations up to 7 g kg�1). It is noteworthy that this substantial bias occurs in a potentially dynamically highly relevant airmass that will be subsequently lifted within aWCBto the upper troposphere.ALagrangian moisture source diagnostic reveals that these largemoisture deviations occur within airmasses that, according to the ECMWF analyses, are coherently transported from the western Mediterranean towards Spain and experience intense moisture uptake over the Ebro valley. It is suggested that inaccuracies in surface evapotranspiration, horizontal moisture advection, and turbulent vertical transport of moisture in the atmospheric boundary layer potentially contribute to the erroneous lowtropospheric humidity in the inflow region of this particular summertime WCB over Spain in theECMWFanalyses.