Investigation of the influence of new active remote sensing instruments on numerical weather prediction models
Zusammenfassung der Projektergebnisse
Within this project data from the novel Water Vapour Lidar Experiment in Space (WALES) airborne demonstrator remote sensing instruments were used to investigate the humidity structure of Warm Conveyor Belts and their role for weather prediction. DLR’s institute for atmospheric physics developed and deployed Differential Absorption Lidars to observe range resolved water vapor profiles beneath the aircraft at high accuracy since 2002. Previous studies proved the ability of airborne lidar profiles to quantify representation of moisture fields in the lower troposphere. Warm Conveyor Belts are airstreams within mid-latitude cyclones carrying warm and moist air from the southern mid-latitude boundary layer to the northern mid-latitude upper troposphere in 1-2. During the ascent of 400-600 hPa in 48 h the air cools adiabatically until saturation is reached and cloud formation starts. Cloud condensation causes intense release of latent heat that may intensify the cyclone. It was hypothesized that the cross-adiabatic flow of the WCB airstreams impacts upper level dynamics by forming negative PV anomalies downstream of the cyclone. The insufficient representation of moist processes and moisture transport in simulated WCBs is believed to affect the short to medium-range forecast skill of numerical weather prediction (NWP) models. At first a large data set of DIAL observations from 2002 to 2008 was collected and potential intersections with WCB were analyzed based on a Lagrangian diagnostic. Two identified cases showed observations in lower tropospheric WCB inflow regions and formed the basis to investigate the origin of the moisture errors and their impact on the forecast skill. A research flight over the Iberian Peninsula intersected a WCB in the boundary layer inflow region just prior to the strong ascent. Comparison of lidar humidity measurements with ECMWF analyses revealed a significant overestimation of the simulated of about 1 g kg-1 on average. A Lagrangian moisture source diagnostic showed that these large deviations occur within airmasses that were transported from the western Mediterranean towards Spain and experienced intense local moisture uptake. Inaccuracies in surface evapotranspiration, in horizontal moisture advection and in turbulent vertical transport of moisture in the atmospheric boundary layer contributed to the erroneous humidity in the inflow region of this summertime WCB. A second WCB inflow airmass was observed east of Japan during a research flight. The comparison with the ECMWF analysis fields revealed a model moist bias in the WCB inflow of about 1.4 g kg-1 on average. The assimilation of the lidar water vapour data into the ECMWF IFS reduced the bias to 0.3 g kg-1. Two forecast experiments were performed to quantify the impact of the moisture analysis error on the forecast skill: One with the operational analysis containing the moisture bias and another one where the moisture bias was reduced through the assimilation of DIAL observations. Less latent heat was released at lower levels in the forecast with less moisture in the WCB inflow resulting in a weaker WCB. Also the increase of potential temperature is lower and the reduced vertical lifting weakened the surface cyclone. As a result of the WCB outflow at lower altitudes, a lower tropopause (10-20 hPa), a less pronounced ridge and a weaker jet stream (5-15 %) were found downstream. A comparison with operational analyses showed that the errors in the moisture analysis degraded the forecast skill, both of the midlatitude cyclone and the WCB outflow. This project demonstrated that novel airborne lidar observations are useful to study the moisture distribution in WCB inflow regions and to validate analysis fields of NWP models. It was confirmed that WCB inflow regions represent sensitive regions for the forecast of extratropical cyclones and the weather downstream.
Projektbezogene Publikationen (Auswahl)
- 2011. Airborne lidar observations in the inflow region of a warm conveyor belt. Quarterly Journal of the Royal Meteorological Society, Special Issue: Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave radiation (GERBILS), Vol. 137. 2011 , Issue 658, Part A, pp. 1257–1272.
Schäfler A., Dörnbrack A., Wernli H., Kiemle C., Rahm S.
(Siehe online unter https://dx.doi.org/10.1002/qj.827) - 2012. Investigation of the Warm Conveyor Belt Inflow - a combined approach using airborne lidar observations and ECMWF model simulations. 2012, Dissertation, LMU München, Fakultät für Physik.
Schäfler A.
- 2015. Impact of the inflow moisture on the evolution of a warm conveyor belt. Quarterly Journal of the Royal Meteorological Society, Vol. 141. 2015, Issue 686, Part A, pp. 299–310.
Schäfler, A., Harnisch, F.
(Siehe online unter https://doi.org/10.1002/qj.2360)
