Download Exploring the atmosphere with MERIS – water

Exploring the atmosphere with MERIS – water vapour content
and cloud top pressure
Jürgen Fischer, Rene Preusker, Peter Albert
Since its launch onboard ENVISAT in March 2002, the Medium Resolution Imaging
Spectrometer MERIS gives insight into the properties and dynamics of Earth, Atmosphere
and Ocean with unprecedented accuracy and resolution. With a maximum spatial resolution
of 290 m × 260 m, dynamic structures can be observed operationally on scales much smaller
than possible before.
The Institute for Space Science at the Free University of Berlin, Germany, has successfully
developed, tested and validated algorithms for the remote sensing of columnar water vapour
and cloud top pressure from MERIS measurements. Both properties play important roles in
the Earth-Atmosphere energy budget and are key variables in the local and global energy
transport and exchange.
With MERIS, water vapour and cloud top pressure can be monitored operationally on a
global scale with a spatial resolution of ~ 1km, with the opportunity of zooming into several
areas with the full resolution of ~ 300 m. The amount of detail in these scenes is illustrated in
a true colour image taken the 12th of August 2003 over Spain (figure 1).
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Figure 1: True colour image taken 12 of August 2003 over Spain.
Water Vapour Retrieval
Integrated columnar water vapour is derived over cloud free land surfaces from MERIS
measurements of backscattered solar radiation. An example is shown in figure 2 for a scene
taken the 12th Oct. 2002, 09:27 UTC over Sicily. The figure shows a true colour image (left)
and retrieved columnar water vapour (right). The bright feature visible on the ocean east of
Sicily is sun glint: sun light reflected at the ocean surface.
As the water vapour density usually decreases with increasing height, one expects smaller
values of columnar water vapour above elevated surfaces, which is well illustrated in the right
image, where the Etna is well indicated by smaller water vapour values than in its
surroundings.
This behaviour is also illustrated in figure 3, where the surface height along a cross-section is
plotted together with the appropriate retrieved water vapour values (left). The location of the
cross-section is shown in an image created from a digital elevation map (right).
For the period October 2002 to February 2003, 111 MERIS overpasses over Central Europe
were compared to radio soundings. For each radio sounding, the mean MERIS water vapour
was calculated from all cloud free pixels within 15 km of the radiosonde station. The result is
plotted in figure 4. The error bars represent 0.1 g/cm 2 for the radio soundings and the
standard deviation of MERIS measurements in the vicinity of each radiosonde station.
Figure 2: True colour image and retrieved columnar water vapour over land for a scene
taken 12th Oct. 2002 over Sicily
Figure 3: Cross-section of surface height and retrieved columnar water vapour for the scene shown in
figure 2. The location of the cross section is illustrated in the right figure.
Figure 4: Scatter plot of columnar water vapour measurements by radio soundings vs. MERIS measurements.
Data are taken over Central Europe between October 2002 and February 2003.
Columnar water vapour – a high resolution example
Figure 5a shows a subset of the high resolution image shown in figure 1. For the same area,
figures 5b and 5c show the surface height and the columnar water vapour derived from
MERIS. It is stunning how even small valleys are well visible as the derived water vapour is
higher compared to the surrounding higher surfaces!
Figure 5: (a)Subset of the true colour image shown in figure 1; (b) Surface height of a digital height;
c) Columnar water vapour
Columnar Water vapour above clouds
With MERIS, measurements of columnar water vapour are also possible in cloudy
atmospheres. In these cases, not the integrated water vapour between the surface and the
top of the atmosphere is retrieved but the water vapour content between the cloud top and
the top of the atmosphere. Together with measurements of the cloud top pressure, useful
information about atmospheric humidity can be obtained. In figure 6, a MERIS scene over
Great Britain and north-western France is shown. The upper images show the integrated
water vapour above the clouds (left) and cloud top pressure (right) from MERIS
measurements taken the 13th Oct. 2002, 10:36 UTC. The lower image shows the integrated
water vapour calculated from a radio sounding (green). The water vapour was integrated
from top of the atmosphere down to each pres sure level. The combined MERIS
measurements of water vapour and cloud top pressure in the vicinity of the radiosonde
station are displayed in black. The error bars indicate 0.2 g/cm 2 for water vapour and 30 hPa
for cloud top pressure.
Figure 6: MERIS measurements of columnar water vapour above clouds (WVC, upper left) and cloud
top pressure (CTP, upper right), taken the 13th Oct. 2002. The lower image shows a comparison of
combined WVC and CTP measurements with a radio sounding.
Columnar water vapour - a case study
In the next figures, a case study is shown where MERIS measurements of water vapour and
cloud top pressure nicely reflect the meteorological situation. On 12th of October, 2002, a
strong front was stretching from northern Great Britain over Germany towards Poland and
Russia. This is illustrated in figure 7 (right), showing the NCEP reanalysis of the 850 hPa
Temperature1. The true colour image created from a MERIS scene taken this day is mainly
covered by clouds, however, a small portion in northern Germany is cloud free, actually the
area where the strong temperature gradient occurs.
Figure 7: MERIS measurements of integrated water vapour. The right panel shows the reanalysis with
the green circle indicating the area of the MERIS measurements.
In figure 7, the columnar water vapour for this area is shown together with the reanalysis, the
green circle roughly indicating the area of the MERIS scene. As two radiosonde stations are
located within the cloud free area, it was possible to compare the MERIS measurements with
independent measurements. The result is illustrated in figure 8. The histograms of MERIS
measurements of columnar water vapour in the vicinity of the radiosonde stations are shown
in black, the radiosonde measurements are shown in green. As cold air can carry less water
vapour, one expects lower values of integrated water vapour for the northern sounding,
which is confirmed by the measurements. The MERIS measurements in the vicinity of both
stations agree very well with the radiosonde measurements.
Figure 8: Histograms of MERIS measurements of columnar water vapour in the vicinity of the two
radiosonde stations indicated in figure 8. The radiosonde measurements of columnar water vapour are
shown in green.
The front is also reflected in the combined MERIS measurements of columnar water vapour
above clouds and cloud top pressure, as shown in figure 9. On the first look, it seems wrong
1
Image provided by www.wetterzentrale.de
that the whole scene shows very low water vapour values, despite the fact that in the
northern part the clouds are much lower. But if one keeps in mind that the northern part is
dominated by colder and thus drier air, it is clear that the columnar water vapour above
clouds is low even for the low clouds. The southern, warmer and more moist part also shows
low water vapour values, however, these measurements are related to higher clouds. The
areas where the water vapour values increase are the areas where the clouds become
lower, which is what one would expect.
Figure 9: MERIS measurements of integrated water vapour above clouds and cloud top pressure for
the cloudy part of the MERIS scene shown in figure 7
Cloud Top Pressure Retrieval from Measurements in the O2-A Band
The operational cloud top pressure product of the MERIS is based on a differential
absorption algorithm using a 3.75nm wide channel in the oxygen A absorption band at
760nm. During the MERIS verification and validation phase in 2002 and 2003 images above
several sites in western Europe and the United States have been analysed. No artefacts or
non-physical values have been found during a visual inspection. Comparisons with cloud top
pressures estimated from 130 radio soundings and with cloud top pressures from cloud radar
data at the SGP ARM site are verifying ESA’s cloud top pressure product.
A MERIS image above the Canary Islands and the retrieved cloud top pressure are shown in
Figure 10. Passat clouds in this region are typically stratocumulus and the cloud top
pressures are usually between 900 and 700 hPa.
Figure 10: RGB and the cloud top pressure from one scene (12. 10. 2002)
in the north Atlantic trade wind region.
Comparison with radio soundings
For the validation of the MERIS cloud top pressure product the relative humidity profiles,
taken from the radiosondes, have been used, to estimate the profile of cloud. Time
differences between radiosonde launch and satellite overpass as well as imprecise
measurements of the relative humidity above clouds, limit the accuracy of this approach. Two
examples of MERIS cloud top pressure products are given in Figure 11. Variations of a few
hPa of a cloud field above Lindenberg have been estimated. Within the dryer air-mass above
Oppin scatterd clouds are observed with cloud tops between 850 and 800 hPa. In both cases
the humidity profiles, taken from the radiosonde, are confirming the satellite observations.
Figure 11: The MERIS cloud top pressure and the profile of relative humidity for a high level cloud
(upper; 21.10.2002 above Lindenberg, Germany) and for a low level cloud
(lower; 24. 10. 2002 above Oppin, Germany).
Comparison with cloud radar data
Comparison of the operational cloud top pressure from MERIS above the SGP-ARM site
(Southern Great Plains Site in Oklahoma, USA) with cloud top heights observed from the
millimeter wavelength cloud radar (www.arm.gov). Since now only a few overpasses are
available with an operating cloud radar and cloudy conditions. The results of this comparison
are within the expected range. For single layer clouds the accuracy is within 20 hPa (Figure
12). For multi- layer clouds with a thin top layer, the retrieved cloud top pressure is related to
an „effective“ penetration depth, somewhere between the upper and the lower layer (see Table
1).
Figure 12: The MERIS cloud top pressure and the general mode RADAR reflectivity profile
(www.arm.gov/docs/instruments/static/mmcr.html) for a single layer low level cloud (15.02.2003).
Table 1: MERIS and Radar cloud top pressures
Conclusions
The retrieval of atmospheric properties from MERIS measurements is successful. New kinds
of algorithms have been developed to take all the benefits from the MERIS instrument, that is
especially the combination of a spectral resolution of 2.5 nm in the oxygen A-band as well as
of 10 nm in the ρστ-H2O absorption band and a high spatial resolution of 300 m.
However, further validation is needed to render more precisely the accuracy of the
atmospheric products.
Publications
J. Fischer, 1988: High Resolution Spectroscopy for Remote Sensing of
Physical Cloud Properties and Water Vapour.- In: Current Problems in
Atmospheric Radiation, Ed. Lenoble and Geleyn, Deepak Publishing, 151-156.
Bennartz, R. and J. Fischer, 2001: Retrieval of columnar water vapour over land from
backscattered solar radiation using the Medium Resolution Imaging
Spectrometer. Remote Sensing of Environment, 78, 274-283
Albert, P., R. Bennartz and J. Fischer, 2001: Remote sensing of atmospheric
water vapour from backscattered sunlight in cloudy atmospheres. Journal of
Atmospheric and Oceanic Technology, 18 (6), 865-874
Fischer and Preusker, 2000: Cloud Top Pressure, ESA, Doc. No: PO-TN-MEL-GS-0005
(http://envisat.esa.int/instruments/meris/pdf/atbd_2_03.pd).