REMOTE SENSING OF CLOUD-AEROSOL RADIATIVE EFFECTS FROM SATELLITE DATA: A CASE STUDY OVER THE SOUTH OF PORTUGAL
D. Santos(1), M. J. Costa(1,2), D. Bortoli(1,3) and A. M. Silva(1,2)
(1) Évora Geophysics Centre (CGE), University of Évora, Rua Romão Ramalho 59, 7000-671 Évora, Portugal,
Email: dinas@uevora.pt
(2)Department of Physics, University of Évora, Rua Romão Ramalho 59, 7000-671 Évora, Portugal, Email:
mjcosta@uevora.pt ; asilva@uevora.pt
(3) Institute of Atmospheric Sciences and Climate (ISAC-CNR), Via Gobetti 101, 40129 Bologna, Italy, Email:
d.bortoli@isac.cnr.it
ABSTRACT
The present work aims at characterizing cloud properties using combined measurements of multi- wavelength passive radiometers onboard satellites. The determination of cloud properties in different regions subject to the presence of aerosols provides information on the possible alterations that these properties may suffer due to the presence of an aerosol layer. The analysis presented proposes to understand if the changes found are real, or an apparent effect due to an incomplete description of the atmospheric characterization, critical to the radiative transfer calculations. The study refers to the end of May 2006, when a strong desert dust event originated in the Sahara Desert reached the south of Portugal. In the same period several stratocumulus clouds were detected in the nearby Atlantic Ocean.
1. INTRODUCTION
A great number of studies were conducted in the last years on the possible modification of cloud properties through the interaction with atmospheric aerosol particles, as this may lead to important changes of the Earth’s radiation budget. Portugal represents an interesting location in Europe for the study of cloud properties in the presence of typical and regular aerosol events. In the country, large unperturbed rural areas co- exist with dense industrial and urban agglomerates, which generate pollution. Furthermore, Portugal is affected by the long-range transport of anthropogenic aerosols emitted in northern Europe and is also on the pathway of the desert dust plumes advected from Africa at high levels (2000-4000m a.s.l.), particularly in spring and summer time. During summer, forest fire smoke is extensively produced mainly in the central mountainous regions of Portugal and transported across the mainland.
The aim of the present work is to determine the cloud optical properties, which can be taken as indicative properties of the cloud-aerosol interaction effect of continental and marine cloud layers. The analysis of the possible alterations undertaken by the clouds through
their interaction with aerosols, specifically with desert dust, is done for a selected episode of strong desert dust transport that occurred in the end of May, last year, in the area of Continental Portugal and nearby Atlantic Ocean. The cloud properties, namely the type, optical thickness, effective radius, top temperature and height, are derived from the inversion of MODIS radiation measurements in the visible and near infrared spectral regions. It was planned to use AATSR data but it was not possible to obtain it, therefore MODIS data was used instead. The cloud height retrievals obtained from MODIS data are compared with co-located ceilometer measurements.
2. METHOD
The methodology used to derived the cloud properties [1] consists of a first cloud detection and a subsequent step of particle phase determination (liquid water or ice), assuming that clouds are entirely constituted by either liquid water or ice particles (no mixed phase clouds are considered). The procedure relies on a bi- spectral technique that uses satellite measurements in the visible and infrared spectral regions centred at 0.65 µm and 11 µm, respectively. The satellite measurements are initially classified in terms of the underlying surface (land or water) using a land-sea mask. Subsequently, the histograms of the VIS radiances and IR brightness temperatures are analyzed to determine threshold values that define the limits between clear sky, water clouds and ice clouds. Such threshold classification is done at the pixel level and when the pixel is cloudy, four possible cases are distinguished: water clouds over the ocean, ice clouds over the ocean, water clouds over land, and ice clouds over land.
The visible, near infrared (centred at 3.75 µm) and infrared radiance measurements corresponding to the pixels classified in the four categories are used to retrieve cloud optical thickness, effective radius and cloud top temperature using the algorithm proposed by [2]. The four categories are treated separately because relevant differences in cloud and surface characterization must be taken into account. The algorithm relies on the comparison between the
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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)
modelled cloud radiances in the three spectral bands and the corresponding satellite radiance measurements. The latter are corrected for the undesirable components, such as the solar radiation reflected by the surface and the thermal radiation emitted from the cloud layer and the surface, in order to retain only the cloud signal. These corrections are based on the use of LookUp Tables (LUTs) calculated using a radiative transfer code [3], [4]. The LUTs contain the radiative quantities necessary for the cloud properties retrieval and are built for a grid of several values of cloud optical thickness, droplet effective radius, cloud top temperature, water vapour above and on the cloud layer, and geometric conditions.
It is possible to obtain the cloud optical thickness, effective radius, cloud top temperature. The cloud top height and pressure are also retrieved from the top temperature values by linear interpolation of the atmospheric vertical profile
3. RESULTS AND DISCUSSION
The effective radius is a property that may give indication of possible cloud modifications due to the aerosol indirect effect. On the other hand, an apparent radiative effect due to unrealistic atmospheric radiative transfer would also reflect in different effective radius values. In order to detect any changes (real or apparent) on the cloud properties, probably induced by the presence of Saharan dust aerosols, the analysis of the retrieved cloud droplet effective radius is carried out for 27, 28 and 29 May 2006. The cloud effective radius analysis is made for two selected situations: clouds in a dust-free atmosphere and clouds in the presence of dust particles (dusty atmosphere).
The 72-hour air mass backward trajectories ending over selected regions of the area of study, are calculated at several altitude levels, using HYSPLIT (HYbrid Single-
Particle Lagrangian Integrated Trajectory) model available from the U. S. National Oceanic and Atmospheric Administration (NOAA) [5], in order to determine the origin of the air masses arriving to the these regions.. The altitude levels chosen are 0.7, 1, 2, 2.5, 3 and 4km in order to take into account the long- range transport of particles (as Saharan desert dust).
Fig. 1 shows the 72h air mass backward trajectories for 27 May 2006 at 12:00 UTC, for some of the levels. The region on the North of the Iberian Peninsula can be considered typically maritime, since the air masses come directly from the ocean. As for the region southwest from Continental Portugal, the air masses are coming from the North of Africa, therefore this region was most probably contaminated by a desert dust plume. The 72-hour air mass backward trajectory analysis for 28 and 29 May 2006 showed similar results to 27 May (Fig. 1), therefore these are not presented here.
Fig. 2 shows the maps of the derived cloud top temperature for the three days of the study. The cloudy regions selected (red dashed lines), are apparently at the same level because cloud top temperature values are roughly the same. They differ from the fact that on the north, the backward trajectories indicate “clean” air masses, whereas on the southwest of Portugal they indicate the presence of desert dust. This is also corroborated by the visual inspection of RGB satellite images over the region. The cloudy regions considered on the Southwest region with respect to Portugal, seems then to be affected by the desert dust aerosols since the observed air mass backward trajectories, for all the study days, are originated in the North of Africa (Sahara desert). Although clouds are observed in the regions affected by the desert dust aerosols, it is still not clear if clouds and aerosols are at the same atmospheric levels.
.
Figure 1. Air mass backward trajectories obtained for 27 May 2006 at 12:00 UTC, from NOAA HYSPLIT model.
a) b) c)
Figure 2. Cloud top temperature derived from MODIS for: a) 27 May 2006, 12:00 UTC; b) 28 May 2006, 11:05 UTC;
c) 29 May 2006, 13:25 UTC.
It can be noted from the 72h air mass backward trajectories (Fig. 1) that for the region southwest of Portugal, the air masses coming directly from the north of Africa are approximately between 2000 and 4000 m a.g.l.. For the 28 and 29 May 2006, similar behaviours were observed in the backward trajectories, therefore, the dust layer was probably travelling between these altitudes during the days of the study. The analysis of the cloud top height values derived from the cloud top temperature values using the atmospheric vertical profile (see previous section), evidences that the cloud tops over the selected areas are apparently located
below the dust layer, with tops varying between 2 and 2.5 km, for the three days.
The cloud droplet effective radius values, over the regions selected, were subsequently examined. For these regions (red dashed lines in Fig. 2) it appears to be possible to distinguish between different type of particles, since different cloud droplet effective radius values are found for each selected region. This effect can be observed in the maps of Fig. 3 that illustrate the cloud effective radius retrieved from MODIS data, for the three days of the study.
a) b) c)
Figure 3. Cloud droplet effective radius derived from MODIS for: a) 27 May 2006, 12:00 UTC; b) 28 May 2006, 11:05 UTC; c) 29 May 2006, 13:25 UTC.
In order to quantify the cloud droplet effective radius differences between the selected regions, the frequency distribution functions of the effective radius are
presented for each region in Fig. 4. On the 27 May, in the dust free region (Fig. 4a), the cloud effective radius values are centred around two main peaks (7.0 and
11.6µm), nevertheless, the distinction between both is
not very clear. As for the region where dust was detected, the distribution presents very low dispersion, with the most frequent effective radius value of 6.8µm.
a)
0 5 10 15 20 25 30
0 200 400 600 800 1000
Frequency of occurrences
Cloud Effective Radius (µm)
Clouds in Dust Free Atmosphere 27 May 2006
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0
2000 4000 6000 8000 10000 12000 14000
16000 Clouds in Dusty Atmosphere 27 May 2006
Frequency of occurrences
Cloud Effective Radius (µm)
b)
0 5 10 15 20 25
0 200 400 600 800 1000 1200 1400 1600 1800
Cloud Effective Radius (µm)
Frequency of occurrences
Clouds in Dust Free Atmosphere 28 May 2006
0 2 4 6 8 10 12 14 16 18 20 22 24 0
500 1000 1500 2000
Clouds in Dusty Atmosphere
Frequency of occurrences
Cloud Effective Radius (µm)
28 May 2006
c)
0 5 10 15 20 25 30
0 200 400 600 800 1000 1200
29 May 2006
Frequency of occurrences
Cloud Effective Radius (µm) Clouds in Dust Free Atmosphere
0 2 4 6 8 10 12 14 16 18 20 22 24 0
200 400 600 800 1000 1200 1400
Clouds in Dusty Atmosphere 29 May 2006
Frequency of occurrences
Cloud Effective Radius (µm)
Figure 4. Frequency distribution functions of the cloud droplet effective radius for the two selected regions on: a) 27 May 2006, 12:00 UTC; b) 28 May 2006, 11:05 UTC; c) 29 May 2006, 13:25 UTC.
As for the 28 May (Fig. 4b), the region on the north of
the Iberian Peninsula presents values centred around two peaks: one at very low effective radius values (2.8 µm) and the other at 9.4µm. The region where desert
dust is present, is also characterized by two peaks: a first at 3.1 µm and another at 6.45µm.
On the 29 May, the selected regions are characterized by the distributions presented in Fig. 4c. The dust free region presents two very close peaks at 10.6 and 13.6µm. The region overpassed by the dust layer, the effective radius peak is at 6.2µm.
The histograms of Fig. 4 illustrate that, in general, a decrease of the satellite derived droplet effective radius values is observed, in the area where desert dust is present, with respect to the dust free region.
Nevertheless, it has been concluded before that most probably, the desert dust plume was overlying the cloud layer, therefore the diminution of the radius cannot be attributed to a real aerosol indirect effect on the clouds. This fact has to do with the assumptions done for the radiative transfer calculations, which do not consider an aerosol layer above clouds [6].
Moreover, this effect may be accentuated, depending on the aerosol properties, especially their absorbing capability [7] therefore, the improved radiative transfer results must be considered.
The cloud bottom height is another cloud parameter that has been retrieved, for the days of the study, from the cloud top height values directly derived from MODIS and the cloud geometrical thickness obtained from the radiative transfer calculations. These values are compared with measurements from a Vaisala LIDAR ceilometer, operating at 910 nm, installed in the CGE observatory in Évora (38°34' N, 7°54' W, 300m a.m.s.l.), south of Portugal, that provides the cloud bottom height. On 29 May there were no clouds around Évora however, an additional day was considered in this comparison - 6 June 2006. The cloud bottom height ceilometer measurements were averaged for a 10 minute period before and after the satellite overpass. As for the satellite derived cloud bottom heights, values are averaged in the area of 0.05ºx0.05º, centred on the ceilometer geographical location. The standard deviation values calculated reflect the temporal (ceilometer) and the spatial (satellite) heterogeneity of the values. Values are summarized in Tab. 1.
Cloud bottom height (km) Date and time
Ceilometer Satellite 27 May 2006
12:00 UTC 4.9 ± 2.3 4.1 ± 1.0 28 May 2006
11:05 UTC 5.6 ± 1.1 5.4 ± 0.7 6 June 2006
14:15 UTC 5.5 ± 1.9 5.2 ± 0.2 Table 1. Mean and standard deviation cloud bottom height values measured by the ceilometer and derived from the satellite data.
The ceilometer measurements are systematically higher than the satellite retrievals. This may be connected with the presence of the dust layer, which was also detected over Évora, at least in the end of May.
To note that the higher differences are found for 27 May, when the aerosol optical thickness in Évora was also higher. Nevertheless, all three days considered were characterized by few sparse clouds, this is evident from the high standard deviation values associated with the ceilometer measurements. Therefore, further comparisons for different cloud coverage situations must be performed, to draw any conclusions from such analysis.
4. CONCLUSION
The cloud properties are derived from satellite data, for a strong transport episode of desert dust that reached Continental Portugal and nearby Atlantic Ocean, during May 2006.
The case study revealed a decrease of cloud droplet effective radius when dust aerosol particles were present over the region considered. Nevertheless, it was found that the desert dust and the cloud layers were probably not located at the same atmospheric levels.
The apparent effect found is probably connected with the assumptions done in the radiative transfer calculations, which do not consider an aerosol layer above clouds.
The satellite derived cloud bottom height values are comparable with the same parameter measured by a ceilometer located in the south of Portugal.
Nevertheless, differences found should be further investigated, hence, more comparisons are needed.
5. ACKNOWLEDGEMENTS
The work was funded by the Portuguese FCT through projects PDCTE / CTA / 49826 / 2003 and POCI / CTE-ATM / 59773 / 2004.
The authors thank the MODIS Teams for maintaining and providing all necessary information.
The authors are grateful to the OpenCLASTR project (http://www.ccsr.u-tokyo.ac.jp/~clastr/) for making available the RSTAR (system for transfer of atmospheric radiation) and CAPCOM (Comprehensive Analysis Program for Cloud Optical Measurement) packages for use in this research.
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