Abstract
Abstract. Improvement of cloud modelling for global and regional climate and weather studies requires comprehensive information on many cloud parameters. This information is delivered by remote observations of clouds from ground-based and space-borne platforms using different methods and processing algorithms. Cloud liquid water path (LWP) is one of the main obtained quantities. Previously, measurements of LWP by the SEVIRI (Spinning Enhanced Visible and InfraRed Imager) and AVHRR (Advanced Very High Resolution Radiometer) satellite instruments provided evidence for the systematic differences between LWP values over land and water areas in northern Europe. An attempt is made to detect such differences by means of ground-based microwave observations performed near the coastline of the Gulf of Finland in the vicinity of St Petersburg, Russia. The microwave radiometer (RPG-HATPRO, Radiometer Physics GmbH – Humidity And Temperature PROfiler), located 2.5 km from the coastline, is functioning in the angular scanning mode and is probing the air portions over land (at an elevation angle of 90∘) and over water (at seven elevation angles in the range 4.8–30∘). The influence of the land–sea LWP difference on the brightness temperature values in the 31.4 GHz spectral channel has been demonstrated, and the following features have been detected: (1) an interfering systematic signal is present in the 31.4 GHz channel, which can be attributed to the humidity horizontal gradient, (2) clouds over the opposite shore of the Gulf of Finland mask the LWP gradient effect. Preliminary results of the retrieval of LWP over water by the statistical regression method applied to the microwave measurements by HATPRO in the 31.4 and 22.24 GHz channels are presented. The monthly averaged results are compared to the corresponding values derived from the satellite observations by the SEVIRI instrument and from the reanalysis data.
Highlights
Improvement of global and regional climate and weather forecasting models requires comprehensive information on atmospheric composition, physical and chemical processes, and in particular information on interactions between different components of the climate system: the atmosphere, water areas, land surfaces, snow and ice cover, and the biosphere. Boe and Terray (2014) analysed the role of soil– atmosphere interactions, cloud–temperature interactions, and land–sea warming contrast in European summer climate changes
In order to give an impression of the origin of the liquid water path (LWP) gradient signal, in Fig. 5a we present a simplified schematic picture of the MW radiation transfer from the atmosphere to an instrument which makes an observation at some elevation angle
In order to estimate the component in measured quantity, which is related to the LWP land–sea gradient effect, we analyse the difference between the mean values of Tb datasets which were calculated for situations without and with the gradient
Summary
Improvement of global and regional climate and weather forecasting models requires comprehensive information on atmospheric composition, physical and chemical processes, and in particular information on interactions between different components of the climate system: the atmosphere, water areas, land surfaces, snow and ice cover, and the biosphere. Boe and Terray (2014) analysed the role of soil– atmosphere interactions, cloud–temperature interactions, and land–sea warming contrast in European summer climate changes. In the present study an attempt was made to find a kind of supplement to satellite measurements in a coastal area in the form of detection of the land–sea LWP gradients by means of ground-based microwave observations The concept of these measurements is straightforward: a radiometer which is located close to a coastline can probe the air portions over land and water surface if it works in the angular scanning mode at an appropriate direction. Ground-based MW measurements characterise only the local-scale LWP distributions in the close vicinity of the observational point, and this is their disadvantage when compared to satellite measurements They can provide important information on the diurnal cycle of LWP over land and water surface with high temporal resolution, and they can be used for validation of satellite data on LWP obtained for the coastline area near the groundbased validation point. We managed to apply these measurements to the task under consideration and received promising results
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