The Down-welling Long-wave Radiation (DLR) flux (W.m-2)
is defined as the thermal irradiance reaching the surface in the
thermal infrared spectrum (4-100 µm). It is determined by the
radiation that originates from a shallow layer close to the surface,
about one third being emitted by the lowest 10 meters and 80% by the
500-meter layer.
DLR is a critical parameter since it controls the net radiation budget and the
amount of water in the soil. Therefore, it plays a crucial role in the
evaluation of soil moisture and evapotranspiration processes that, in turn,
require an adequate assessment of type, level and state of vegetation as well
as on a correct estimation of photosynthetic activity.
ONC requires
DLR because it is necessary to quantify the atmospheric forcing in the carbon
cycle modelling.
DLR flux product from METEOSAT-7 14.08.2000
- 06:00 UTC
In clear cky conditions, DLR flux depends on
the vertical profiles of
temperature and gaseous absorbers, primarily H2O followed by
CO2. Clouds have a strong effect on the long-wave radiation transfer
because they modify the atmospheric emissivity in the atmospheric window
(8 - 13 µm) and their contribution depends on cloud base properties (height,
temperature, and emissivity), mainly. The DLR flux retrieval is difficult
since only atmospheric window TOA radiances can bear information on the
near-surface radiation field. Consequently, the development of empirical
formulas becomes complicated. One problem is that the methods are developed
empirically at one location and with one set of instruments, but it is
difficult to generalize over larger areas.
Then, the
Institute of
Meteorology (IM) of Portugal has developed a strategy which consists in
using conjointly satellite images and Numerical Weather Prediction (NWP) data.
The algorithm
to compute the DLR for clear-shy conditions consists of an hybrid method based
upon a bulk parameterisation proposed by Dilley and O'Brien (1998). This
scheme depends on two-meter air temperature and precipitable water content.
For cloudy sky conditions, the DLR is computed using the parameterisation
scheme proposed by Josey et al. (2003). It is based on the relationship of the
effective temperature of a blackbody with a radiative flux equivalent to that
from the atmosphere in terms of two-meter air temperature, total cloud amount
and two-meter dew point temperature. Two-meter air temperature, two-meter
dew point temperature and precipitable water content fields are
obtained from ECMWF model forecast and the total cloudiness data are obtained
from METEOSAT processing. IM Portugal has developed similar approaches in the
frame of the LSA
(Land Surface Analysis) SAF
to retrieve DLR from MSG data in an operational way.
Automatic Quality Control (QC) is performed on DLR data and the quality
information is provided on a pixel by pixel basis. DLR QC contains general
information about input data quality and information about DLR confidence
level. The DLR confidence level was defined based upon the following
parameters: screen-level temperature and total column water vapor for clear
sky conditions and on total cloud fraction for cloudy sky conditions. The
three considered levels of confidence (i.e. above nominal, nominal and below
nominal) correspond to estimated uncertainties on DLR values (respectively
less than 5%, between 5 and 10%, and above 10%).
The validation of the algorithms includes feasibility analyses,
sensitivity studies, assessment of prototypes. The validation of the DLR
product primarily
relies on regular measurements to be carried out on existing permanent ground
stations from CarboEurope, BSRN, SURFRAD and ARM networks. The DLR retrievals
may also be compared with satellite results obtained in the framework of other
projects or with the outputs from numerical weather prediction models. The
validation is performed in close cooperation with the
University of Karlsruhe (IMK).
References
Dilley, A. C. and D. M. O'Brien, Estimating downward clear sk long-wave
irradiance at the surface from screen temperature and precipitable water,
Q. J. R. Meteorolo. Soc., 124,1391-1401, 1998.
Josey, S. A., R. W. Pascal, P. K. Taylor, M. J. Yelland, A new formula
for determining the atmospheric longwave flux at ocean surface at mid-high
latitudes. Journal of Geophysical Research - Oceans, 108, 3108,
2003.
