Comparison of model predicted cloud parameters and surface radiative fluxes with observations on the 100 km scale

Summary Cloud parameters and surface radiative fluxes predicted by regional atmospheric models are directly compared with observations for a 10-day period in late summer 1995 characterized by predominantly large-scale synoptic conditions. Observations of total cloud cover and vertical cloud structure are inferred from measurements with a ground-based network of Lidar ceilometers and IR-radiometers and from satellite observations on a 100 kilometer scale. Ground-based observations show that at altitudes below 3 km, implying liquid water clouds, there is a considerable portion of optically non-opaque clouds. Vertical distributions of cloud temperatures simultaneously inferred from the ground-based infrared radiometer network and from satellite can only be reconciled if the occurrence of optically thin cloud structures at mid- and high tropospheric levels is assumed to be frequent. Results of three regional atmospheric models, i.e. the GKSS-REMO, SMHI-HIRLAM, and KNMI-RACMO, are quantitatively compared with the observations. The main finding is that all models predict too much cloud amount at low altitude below 900 hPa, which is then compensated by an underestimation of cloud amount around 800 hPa. This is likely to be related with the finding that all models tend to underestimate the planetary boundary layer height. All models overpredict the high-level cloud amount albeit it is difficult to quantify to what extent due to the frequent presence of optically thin clouds. Whereas reasonably alike in cloud parameters, the models differ considerably in radiative fluxes. One model links a well matching incoming solar radiation to a radiatively transparent atmosphere over a too cool surface, another model underpredicts incoming solar radiation at the surface due to a too strong cloud feedback to radiation, the last model represents all surface radiative fluxes quite well on average, but underestimates the sensitivity of atmospheric transmissivity to cloud amount. Received August 31, 2000 Revised March 15, 2001     

Development of the water budget for three extra-tropicalcyclones with intense rainfall over Europe

Summary  Three cyclones developing between 28 August and 6 September 1995 were studied with respect to the temporal evolution of their water budget components. The cyclones were simulated with the regional model REMO. Water budget values were determined from hourly model output for circle areas with 500 km radius around the pressure minimum. The results show a maximum liquid water path of about 0.12 kg m⁻² and a maximum ice water path of 0.16 kg m⁻². In the vertical cloud structure the medium cloud layer disappears at the end of the life cycle for all three cyclones. The release of precipitation onto the Baltic Sea drainage basin is different for each cyclone. It lies between 13 and 22 · 10¹² kg. This is about 50% of the total precipitation in the whole area for the strongest cyclone and 65% for the others. The P — E (precipitation minus evaporation) is 15 · 10¹² kg for two of the cyclones and 10 · 10¹² kg for the third one. Received August 7, 2000 Revised March 19, 2001     

The Baltic Sea Experiment BALTEX: A brief overview and some selected results of the authors

The mechanisms responsible for the transfer of energy and water within the climate system are under worldwide investigation within the framework of the Global Energy and Water Cycle Experiment (GEWEX) to improve the predictability of natural and man-made climate changes at short and long ranges and their impact on water resources. Five continental-scale experiments have been established within GEWEX to enable a more complete coupling between atmospheric and hydrological models. One of them is the Baltic Sea Experiment (BALTEX).In this paper, the goals and structure of BALTEX are outlined. A short overview of measuring and modelling strategies is given. Atmospheric and hydrological model results of the authors only are presented. These include also the validation of precipitation using station measurements as well as validation of modelled cloud cover with cloud estimates from satellite data. Furthermore, results of a large-scale grid based hydrological model to be coupled to atmospheric models are presented.This research has never been possible without the contribution of research groups and operational institutions from all 10 member countries. We concentrate here on results obtained at the GKSS research center.