Andre C.A.P. van Lammeren

Ground-Based and Satellite Observations of Cloud Fields in the Netherlands

Abstract A study is performed on the combination of ground-based and satellite observations for the derivation of cloud properties. Ground-based measurements from a lidar ceilometer and an infrared radiometer were combined with measurements of the NOAA Advanced Very High Resolution Radiometer and Meteosat satellite instruments. Two case studies are presented: a case with streets of fair weather cumuli and a case with a weak cold front involving cumulus, stratus, and cirrus clouds. From the combination of ground-based and satellite observations, a much better description of the cloud field geometry, cloud base, and cloud top can be obtained than with satellite or ground-based observations alone. The combination of satellite retrievals and lidar-ceilometer measurements is promising. This concept is widely applicable because lidar ceilometers are available on airports all over the world and the used infrared sensors are relatively cheap and can easily be installed. This opens the way for a much improved auto...

High time resolution cloud measurements with lidar during CLARA

Abstract The objective of the Clouds and Radiation (CLARA) project is to improve observation techniques in order to obtain a better understanding of the role of clouds in the Earths radiation budget. In 1996 three measurement campaigns took place to collect an extensive set of remote and in-situ measurements of clouds. Beside methods for the accurate retrieval of cloud base height, vertical extent and optical properties by synergetic use of CLARA instrumentation, variability is one of the key points sought after. The RIVM High Temporal Resolution Lidar (HTRL) recorded single-shot lidar returns. These measurements are used to study the influence of averaging on estimations of cloud geometry and optical properties. It is shown that unless information on the variability is retained, biased estimates of cloud parameters are made.

Cloud boundary height measurements using lidar and radar

Abstract Using only lidar or radar an accurate cloud boundary height estimate is often not possible. The combination of lidar and radar can give a reliable cloud boundary estimate in a much broader range of cases. However, also this combination with standard methods still can not measure the cloud boundaries in all cases. This will be illustrated with data from the Clouds and Radiation measurement campaigns, CLARA. Rain is a problem: the radar has problems to measure the small cloud droplets in the presence of raindrops. Similarly, few large particles below cloud base can obscure the cloud base in radar measurements. And the radar reflectivity can be very low at the cloud base of water clouds or in large regions of ice clouds, due to small particles. Multiple cloud layers and clouds with specular reflections can pose problems for lidar. More advanced measurement techniques are suggested to solve these problems. An angle scanning lidar can, for example, detect specular reflections, while using information from the radars Doppler velocity spectrum may help to detect clouds during rain.

RIVM's automated lidar systems for climate research

At the Dutch National Institute of Public Health and Environmental Protection (RIVM), an automated elastic backscatter lidar at 1064 nm monitors aerosol and low clouds on a routine basis. The lidar data are presently used for several climate-related research issues: (i) the attenuation of solar UV-B irradiance by mixed layer aerosol is assessed by combining the results of a radiative transfer model, measured irradiances from the UV monitoring station of RIVM, and mixed layer heights obtained by lidar. (ii) The lidar forms part of the KNMI Cloud Detection System, which monitors cloud parameters in an area of 120 X 120 km2 in a central part of the Netherlands. (iii) Aerosol vertical profile information has been used in a case study at ECN to quanitfy the total radiative forcing by anthropogenic aerosol. At present, a second lidar system, especially designed for cloud detection, is being built in the framework of a new national climate research program in the Netherlands.

Cloud detection system in the Netherlands

A cloud observational system, which combines ground and satellite measurements, is used to characterize cloud fields over a central area in the Netherlands. The results will be used for the improvement of parameterizations in climate models. In the Netherlands a ground-based cloud detection network of 11 stations is installed. Each station consists of a lidar-ceilometer and an upward looking narrow beam infrared radiometer. The observations of the network are combined with cloud parameters derived from NOAA/AVHRR and Meteosat. Information on the actual atmospheric conditions like standard rawinsonde data, and numerical weather forecast analysis data are also used. The system aims at a detailed quantitative description of cloud cover, cloud structure and radiative properties in an area of 120 by 120 km2, the typical size of a general circulation model (GCM) grid box. The cloud detection system (CDS) will be operated for a period of two years. A first analysis of the measurements from the CDS for September 11, 1994 is presented. The ground based measurements are combined with the satellite measurements. It is concluded that this combination of measurements gives a good description of the temporal and spatial variability of the clouds over the TEBEX area. The concept, to use the lidar-ceilometer measurements to derive cloud characteristics for meteorological purposes, has much potential. Thousands of lidar ceilometers are already installed all over the world. The investiments needed to use these measurements for meterological applications would be limited.

Sensor synergy for cloud observation: theory and observation (for liquid water clouds)

Several different instruments are necessary to extract a full set of cloud properties, such as cloud boundaries, cloud optical thickness, particle size and particle phase. The sensor synergy used in this study is a combination of active and passive instruments. Both radiative and intrinsic cloud properties are extracted, and the relation between the two is studied for liquid water clouds. The experimental method for extracting IR emissivity (epsilon) form IR radiometer and lidar measurements will be explained. Cloud Liquid Water Path (LWP) can be extracted from the microwave radiometer. Comparing the two leads to a clear relation between (epsilon) and LWP in the case of liquid water clouds. This can be understood if one considers a simple approximation to the extinction efficiency of liquid water droplets in the IR, leading to an RI emissivity that solely depends on LWP, and not on the particle size distribution. Similar methods can be applied to space-based observations, such as combining LITE and Meteosat measurements to derive IR emissivity. First results are shown.

Long-term cloud observations from a satellite and ground-based remote sensing network

Spaceborne remote sensing observations ar used to obtain better understanding of cloud properties and cloud physics. To validate and supplement the satellite observations, continuous cloud measurements have been made by a groundbased network (CDS) located in the Netherlands. The spatial and temporal variability of cloud cover has been compared extensively with collocated synoptic observations. Frequency analysis of groundbased and spaceborne radiometer signals identify cloud layer temperatures within the area. CDS provides a powerful database that comprise tow years of different atmospheric conditions, from which a.o. cloud cover, cloud top, cloud base, optical thickness, IR emissivity and LWP can be obtained.

The Emissivity of Cirrus Clouds Derived from LITE and Meteosat Measurements

By combining the LITE meassurements and the colocated (in time and space) Meteosat measurements, the emissivity of cirrus clouds in the Infra-Red (IR) is derived. The IR-emissivity can not be obtained from the data of each single instrument alone. This shows the advantage of a complementary set of instruments on (possible future) space platforms.

Data integration of ground based and satellite observations of clouds and radiation in the Netherlands

A cloud observational network is used to characterize cloud fields over a 120*120km^2 area in the Netherlands. The system consists of a network of stations for ground based remote sensing and a processing environment for AVHRR and Meteosat measurements. The aim is to obtain a detailed quantitative description of cloud properties within a gridbox of a climate model. Results are to be used for the improvement of parameterizations of clouds and cloud- radiation interaction in weather and climate models. Research will focus on the characterization of the variability in time and space of cloud properties. Currently the system is fully operational. Case studies are performed to investigate the individual instrument response to clouds and their correlation to other measurements. In this paper some results for 4 August, 1993 are presented.