DETERMINATION OF CLOUD TOP HEIGHT USING METEOROLOGICAL SATELLITE
AND RADAR DATA
R. Randriamampianina*, J. Nagy**, Gy. Gyarmati** and J. Kerényi*
* research fellows, Satellite Research Laboratory of the Hungarian Meteorological Service
** research fellow, Division of Aerology and Radarmeteorology of the Hungarian Meteorological Service
Commission VII
KEY WORDS: MRL-5 Weather Radar, Digital
Meteosat, Cloud Detection, Cloud Top Height
ABSTRACT
At the Hungarian Meteorological Service we receive the
digital Meteosat images and operate a radar network
which consist of three MRL-5 weather radars. In this
study we derive the cloud top height (CTH) from
Meteosat images and compare it with echo top values
calculated from RHI vertical radar pictures.
Cloud detection method has been developed using a
threshold technique based on infrared and visible data of
Meteosat. After cloud detection the cloud top height is
calculated operationally from the infrared brightness
values and ECMWF predicted temperature profiles at 12
UTC. For the comparison vertical radar pictures
measured at 3 and 10 cm wavelength in different ranges
(32, 64, 128 and 256 km) are used.
Comparable results were found in those cases when one
cloud type was observed.
INTRODUCTION
The nowcasting system recently worked out at the
Hungarian Meteorological Service requires cloud top
height (CTH) data as recent as possible. For this reason a
reliable calculation scheme to determine the CTH is being
worked out. The geostationary satellite (in our case
Meteosat) provides consecutive images that allow us to
know the cloud motion. At the Satellite Research
Laboratory we receive digital pictures from Meteosat 3
hourly.
Many methods were developed to assign the CTH from
the remotely sensed data, such as the CO, absorption
method (Menzel et al. 1983), the histogram method
(Derrien et al, 1988), the water vapour absorption
method (Schmetz et al., 1993; Nieman et al., 1993 and
Nieman et al, 1997) and the infrared (IR) window
method (Schmetz et al., 1993).
We use the IR window method for the CTH assignment at
this stage. This paper describes the cloud detection and
CTH assignment schemes used and discusses the first
results by comparing them with data derived from radar
measurement. A conclusion has been drawn from the
preliminary results.
MATERIALS AND METHODS
Database
For the CTH assignment using the IR window method
ancillary data are required (Schmetz et al., 1993). In our
case the forecasts of the European Centre for Medium-
Range Weather Forecasts (ECMWT) are used. Since
November 1996, we receive the products (forecasts and
analyses) of ECMWT, unfortunately with delay. So we
are obliged to use the forecasts for the CTH assignment.
The ECMWF data are archived according to their
acquisitions.
We receive digital images (in the visible, the infrared
window and the water vapour absorption channels) of the
Meteosat every 3 hour. This study is carried out for the 12
UTC data.
The Hungarian radarmeteorological network is
constituted of 3 radars. They are situated in Budapest-
Lórinc, Nyiregyhaza-Napkor and in Szentgothard-
Farkasfa. For the comparison with the detected CTH,
vertical radar pictures (measured at 3 and 10 cm
wavelength) in different ranges (32, 64, 128 and 256 km)
are measured hourly. The radar vertical measurement has
only one direction.
The period from January until April 1998 was used for
this estimation.
Cloud detection
The cloud detection technique is close to the one
published by Tánczer (1997). The method is based on
threshold method using the image from visible channel.
The visible threshold is computed as follows:
n= f. f. min
f. max fr min : (1)
Where f, is the brightness value from the visible spectral
channel at 12 UTC already corrected for the solar
elevation angle. fin and fin. are the global minimum
and maximum of f, after filtration of the erroneous values.
The cloud detection scheme is presented in Fig.l. This
scheme is simplified according to the aim of the study - to
derive the CTH. Thus, it should be more complicate for
cloud classification and for cloud type specification cases.
742 International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998
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