Koninklijk Nederlands Meteorologisch Instituut; Ministerie van Verkeer en Waterstaat

KNMI Cabauw Observations
 
Remote Sensing
Introduction

The remote sensing program aims to deliver high resolution, high quality thermodynamic profiles throughout the troposphere and information on clouds and aerosols and boundary layer height. The data are used amongst others for model evaluation, satellite retrieval verification, process studies and development of retrieval algorithms. Also improvements in techniques and signal processing, assessment of new active and passive ground-based remote-sensing systems are performed. Most of the systems are operated semi-operational and measure continuously. Remote sensing systems operated by other members of the CESAR consortium complement the KNMI observations, like the scanning IDRA radar on top of the tower and GPS receivers operated by the University of Delft. The program participates in international networks like Earlinet, E-WINPROF and Cloudnet.

In the sections below the remote sensing systems operated at Cabauw by the Regional Climate department are briefly described.

Lidars

Lidar is an acronym for LIght Detecting And Ranging, which is a technique to measure atmospheric properties over a large distance, often from the surface, occasionally also from airplanes and balloons and even from space. KNMI operates two types of ground based lidar systems at Cabauw.

read more about our lidars

Caeli in operation at Cabauw

A fully automated UV-backscatter lidar equipped with a depolarization channel (Leosphere ALS-450) is used to measure backscatter profiles. The depolarization channel provides additional information about the shape of atmospheric particles. A so-called depolarisation lidar can distinguish whether scatterers are almost spherical (e.g. droplets), yielding low depolarisation, or non-spherical (mineral dust, volcanic ash, ice crystals), giving high depolarisation.

Caeli (CESAR Water Vapour, Aerosol and Cloud Lidar) is also stationed at Cabauw. Caeli is a high-performance multi-wavelength Raman lidar, capable of simultaneously observing vertical profiles of water vapour mixing ratio, aerosol backscatter and extinction and cloud properties. Since aerosol and cloud properties are observed at multiple wavelengths, information about the particle microphysical properties can be obtained. Caeli measurements are used for climate process studies, as well as climatological observations.

Ceilometers

Two near infrared ceilometers are located in Cabauw. One is a research type ceilometer, the CT75. The other is an LD40 ceilometer which is also used in the operational network of the KNMI. A ceilometer is low power lidar that transmits short light pulses and detects the atmospheric return from clouds and aerosols as function of height. The ceilometers can detect up to three cloud bases every 30 en 15 sec. respectively. The backscatter profiles are also stored and are used e.g. in the CloudNet processing suite in the target classification product. The LD40 backscatter profiles are also used to retrieve the atmospheric boundary layer height using a wavelet detection algorithm developed at KNMI.

Cloud Radar

A 35 GHz Doppler cloud radar is operational in Cabauw since August 2001. The radar is pointed in zenith direction. The vertical resolution in the operational mode is 90 m. A profile of backscatter (dBZ), vertical velocity and Doppler spectral width is created from a combination of an uncoded and coded mode approximately every 15 seconds.

Windprofiler/RASS

A 1290 MHz LAP3000 windprofiler was installed in Cabauw in July 1994. The profiler is located approximately 300 m. south of the tower. The profiler transmits short radar pulses and the return signal from the atmosphere is processed to retrieve the Doppler velocity in the beam direction. Combining the radial velocity of at least three beams it is possible to measure the horizontal wind speed and direction at a each range gate of the profiler. Height coverage depends on radar characteristics and atmospheric conditions.

read more about the profiler

The radar pulse is backscattered by so-called clear-air Bragg scatter and also by hydrometeors. Bragg scatter is caused by radio refractive index fluctuations in the atmosphere due to turbulence, with a typical length scale of the half the radar wavelength. The RASS (Radio-Acoustic-Sounding-System) is an addition to the profiler which transmits sound waves. The radar pulses reflect from these sound waves and the speed of the sound waves can be measured in this way. The speed of sound is related to the air-temperature, hence measuring the sound speed also provides the air temperature.

The routine operational acquisition program consists of measuring in wind mode for 55 min. and in RASS mode for 5 min. each hour resp. In wind mode one cycle consists of 10 measurements each: 5 in low mode and 5 in high mode. Low and high mode measurements are interleaved. One cycle takes approximately 5 min. of measuring and processing time. Since the installation in 1994 the profiler has been operated continuously. However in the early years of operation data acquisition was frequently interrupted due to problems with software and storage on optical disk. Since the system is connected to the network of the KNMI early 2000, a high reliability of the measurements is achieved.

The 1290 MHz profiler/RASS

The Doppler spectra are processed in real-time and the first three moments of the strongest peak of each spectrum are calculated. The velocity of the strongest peaks are used to compute an hourly consensus averaged wind speed, direction and temperature. A quality analysis is applied to the consensus averaged winds and temperatures to remove outliers. In spring 2000 cup-anemometers have been installed in the tower. Reflections of the cup-anemometers sometimes disturb the wind measurements of the profiler between 300 and 400 agl.

Hourly averaged wind data are send on to the GTS in near realtime for assimilation in operational weather forecast models.

Microwave Radiometer

A 14 channel microwave radiometer (HATPRO) was installed at Cabauw in April 2006. This instrument measures the sky brightness temperature in the water vapor band (20-30 GHz) and the oxygen band (50-60 GHz). With a set of statistical retrievals the column integrated liquid water (LWP) and the column integrated water vapor (IWV) are calculated. In January 2011 the radiometer is equipped with an azimuth controller. This allows to make 3-D scans of the sky brightness temperature which can be used to study inhomogeneities in the atmospheric liquid water and water vapour fields.

read more about the HATPRO radiometer

The HATPRO microwave radiometer

The radiometer measures in zenith direction with a sample time of 1 sec. The zenith observations are interrupted for a boundary layer scan each half hour. The boundary layer scan provides detailed temperature profiles in the first one to two kilometers of the atmosphere. Beside LWP and IWV also temperature and humidity profiles up to 10 km. height are retrieved from the zenith observations. These profiles, especially the humidity profile, have a rather smoothed appearance due to the poor vertical resolution of the retrieval methods. During rain the measurements of the radiometer are unreliable.

Imagers

Imagers are passive remote sensing sensors. At Cabauw two imagers are in operation. A scanning infrared imager, the NubiScope, and a Total Sky Imager, which records during day time images of the sky with a CCD array camera. The recorded images are analysed by the system software and provide total cloud cover data at 10 and 1 minute intervals respectively. Also a webcam, recording the sky in northward direction, is operated continuously. During daytime images are stored every 5 sec. From these images a movie is created for each day.

Links to quicklooks

Quicklooks of data are available from a number of sites. Some data are already included in near real time in the CESAR data portal, others are presented on dedicated websites.


Page last modified on 1 April 2015