Abstract |
A deterministic spectral shortwave radiative transfer model was used for
the computation of the Earth's atmospheric radiation budget, based on high
temporal and spatial resolution satellite data of aerosols and atmospheric climatic
parameters from the Moderate Resolution Imaging Spectroradiometer
(MODIS) sensor.
The study focused on the evaluation of the aerosol direct radiative effect
(DRE) on the radiation budget components. Due to the high spatial
and temporal variability of aerosols, the DRE, which constitutes a crucial
component of the overall effect of aerosols on climate, is thus also highly
variable.
The aerosol direct effect on the tropospheric ozone photolysis rate, J(O1D),
was also examined, being a dominant sink of tropospheric ozone. We note
that tropospheric ozone contributes to the global greenhouse effect. Thus,
J(O1D) is an important climatic parameter, which needs to be studied using
modelling approaches, due to the scarcity of measuring stations, and
because it takes place primarily below 330 nm, a spectral region where the
aerosol effect is a key operating factor.
The aerosol direct effect on potential evaporation was also assessed. Potential
evaporation equals actual evaporation in shallow lakes, and constitutes
a crucial parameter of the hydrological cycle. The aerosol DRE decreases
potential evaporation by decreasing the solar radiation reaching the Earth's
surface.
The model runs were performed for the period 2000-2010 over several
sites in Greece, which are characterised by high aerosol loads, with unique
characteristics in terms of seasonal variation and origin. Two research stations
in Crete (HCMR/AERONET and Finokalia), were selected due to the
appropriateness of the island for studying Saharan dust episodes, which are
frequent in the wider Eastern Mediterranean, and the availability of ground-based data for both model supplementary input and validation. The model
was also run over four lakes in Central Greece, which constitute the main
water supply reservoirs of the city of Athens, for the evaluation of the aerosol
effect on potential evaporation.
MODIS Level 2 data of aerosols, clouds and atmospheric parameters were
analyzed and processed, and used as input to the model. These data are
available since 2000, on a daily basis and at 10kmx10km and 5kmx5km
spatial resolution. The model takes into account all physical parameters and
processes that affect significantly the solar radiation transfer. The aerosol
DRE is determined at the Earth's surface, within the atmosphere and at the
top of the atmosphere.
The model output downwelling shortwave radiation was successfully validated
against ground{based measurements at the HCMR and Finokalia stations
and at the four lakes in Central Greece. The model output J(O1D) was
successfully validated against Finokalia station measurements. The analysis
of the aerosol DRE on the model radiation budget, J(O1D) and potential
evaporation was performed on an instantaneous/daily mean, seasonal and
inter{annual basis. Dust event effects were also quantified, and trends during
the period examined were assessed and evaluated in terms of corresponding
trends and effects of operating factors, including aerosols, clouds and total ozone.
Results show a decreasing trend in aerosols and the corresponding DRE
over all sites examined. Changes in the radiation budget components, however,
are also controlled by other factors; an increase in cloud fraction over
HCMR station counterbalanced the effect that the DRE reduction would
have caused. Similarly, although the DRE on J(O1D) has decreased, J(O1D)
has not increased as was expected, due to an increase in total atmospheric
ozone. The presence of aerosols reduces potential evaporation by about 0.5
mm on a mean daily basis, reaching up to 2 mm in summer. However, a
decreasing trend in the aerosol load and DRE was found over all lakes during
the period 2001-2010.
Depending on the availability of model input data, the methodology developed
in this study is applicable to any region of specific interest over the globe.
|