Patrick Chazette

Simulation of the mineral dust emission frequencies from desert areas of China and Mongolia using an aerodynamic roughness length map derived from the POLDER/ADEOS 1 surface products

To determine the location of the potential dust source regions and to evaluate the dust emission frequencies over the arid and semiarid areas of China and Mongolia (35.5°N-47°N; 73°E-125°E), we established a map of Z 0 from the composition of protrusion coefficient (PC) derived from the POLDER-1 bidirectional reflectance distribution function (BRDF). Using a 1/4° x 1/4° resolution Z 0 data set, we derived a map of the 10-m erosion threshold wind velocity for the Chinese and Mongolian deserts. The retrieved erosion thresholds range from 7 m s -1 in the sandy deserts (Taklimakan, Badain Jaran, and Tengger deserts) to up to 20 m s -1 in the Gobi desert. They are comparable to the minimum wind velocity measured in meteorological stations during dust storms in the Taklimakan (6-8 m s -1 ) and in the Gobi desert (11 -20 m s -1 ). These erosion thresholds were combined with surface wind fields, soil moisture, and snow cover to simulate the dust emission frequencies of the eastern Asian deserts over 3 years (1997-1999). The simulations point out the Taklimakan desert and the deserts of northern China as the most frequent sources of dust emissions. The simulated seasonal cycle is characterized by a maximum in late spring and a minimum in late autumn and winter. The comparison with climatologic studies of dust storms derived from synoptic observations confirms the importance of these two source areas and the reliability of the simulated seasonal cycle. It reveals an underestimation of the dust emission frequency in the Gobi desert, because of a low frequency of high wind velocities. Our results also suggest that soil moisture and snow cover are not the main factors controlling the seasonal cycle or the interannual variability of the dust emission frequencies. We finally compared the simulated dust event frequencies to occurrences of Total Ozone Mapping Spectrometer (TOMS) Absorbing Aerosol Index (AAI) higher than 0.7 over the Taklimakan desert, where mineral dust is expected to be the dominant absorbing aerosol. A very good agreement is obtained between the simulated frequencies and the TOMS AAI frequencies on monthly and seasonal timescales.

Surface and aerodynamic roughness in arid and semiarid areas and their relation to radar backscatter coefficient

Surface roughness is a key parameter for surface-atmosphere exchanges of mass andenergy. Only a few field measurements have been performed in arid or semiarid areaswhere it is an important control of the aeolian erosion threshold. An intensive fieldcampaign was performed in southern Tunisia to measure the lateral cover, Lc, and theaerodynamic roughness length, Z0, over 10 sites with different surface roughnesses. Lcwas determined by combining field measurements of the geometry of the roughnesselements and simple assumptions on their shapes. Z0was experimentally determined fromhigh-precision wind velocity and air temperature profiles. The resulting data were found tobe in good agreement with the existing relationships linking the geometric and theaerodynamic roughness. This suggests that for natural surfaces, Z0can be estimated onthe basis of the geometric characteristics of the roughness elements. This data set wasthen used to investigate the capabilities of radar backscatter coefficients, s0, to retrieve Lcand/or Z0. Significant relationships were found between s0and both Lcand Z0. TheSAR/ERS data set is in agreement with the SIR-C SLR data set from Greeley et al. (1997).On the basis of these two data sets including data from different arid and semiarid areas(North Africa, South Africa, North America), we propose an empirical relationship toretrieve Z0using radar observations in the C band from operational sensors.

Mapping the aerodynamic roughness length of desert surfaces from the POLDER/ADEOS bi-directional reflectance product

Surface roughness is a key parameter for computing the emissions and for simulating the atmospheric cycle of mineral dust. However its assessment on the basis of field measurements from source areas scattered round the globe requires much effort. Here we investigate the retrieval of the aerodynamic roughness length of arid areas using surface bi-directional reflectance products derived from passive multi-directional measurements in the solar spectrum of the POlarization and Directionality of the Earths Reflectances (POLDER) sensor. The so-called protrusion coefficient (PC) of the surface derived from the POLDER bi-directional reflectance distribution function (BRDF) is well suited to estimate surface roughness. From an appropriate selection of POLDER data, a composite PC dataset has been established over the Sahara and the Arabian Peninsula. We have investigated the relationship between aerodynamic roughness length and PC, and have derived a statistically significant empirical relationship between these two parameters. This relationship is applied to the POLDER-derived PC to map the aerodynamic roughness length of arid areas in northern Africa and the Arabian Peninsula at the spatial resolution of POLDER (∼1/16°). When degrading these data for global models, we show that the information is essentially preserved at coarser resolutions up to ¼°. This map of roughness length derived from the POLDER instrument, and a corresponding map derived from a geomorphologic classification, have been tested by comparing the predicted dust event frequencies obtained using them to dust indices (IDDI) derived from Meteosat IR observations over the Sahara desert. The agreement using the POLDER derived roughness length is at least as good as using the map of roughness length derived from the geomorphologic approach. Our results show promising new prospects for regional and global scale simulations of mineral dust emissions from arid regions.

Optical properties of urban aerosol from airborne and ground-based in situ measurements performed during the Etude et Simulation de la Qualité de l'air en Ile de France (ESQUIF) program

[1]xa0Urban aerosol microphysical and optical properties were investigated over the Paris area coupling, for the first time, with dedicated airborne in situ instruments (nephelometer and particle sizers) and active remote sensor (lidar) as well as ground-based in situ instrumentation. The experiment, covering two representative pollution events, was conducted in the framework of the Etude et Simulation de la Qualite de lair en Ile de France (ESQUIF) program. Pollution plumes were observed under local northerly and southerly synoptic wind conditions on 19 and 31 July 2000, respectively. The 19 July (31 July) event was characterized by north-northwesterly (westerly) advection of polluted (clean) air masses originating from Great Britain (the Atlantic Ocean). The aerosol number size distribution appeared to be composed mainly of two modes in the planetary boundary layer (accumulation and nucleation) and three modes in the surface layer (accumulation, nucleation, and coarse). The characteristics of the size distribution (modal radii and geometric dispersion) were remarkably similar on both days and very coherent with the aerosol optical parameters retrieved from lidar and nephelometer measurements. The city of Paris mainly produces aerosols in the nucleation mode (modal radius of ∼0.03 μm) that have little influence on the aerosol optical properties in the visible spectral range. The latter are largely dominated by the scattering properties of aerosols in the accumulation mode (modal radius of ∼0.12 μm). When the incoming air mass is already polluted (clear), the aerosol in the accumulation mode is shown to be essentially hydrophobic (hydrophilic) in the outgoing air mass.

Retrieval and monitoring of aerosol optical thickness over an urban area by spaceborne and ground-based remote sensing

We used an instrumental synergy of both ground-based (sunphotometer) and spaceborne [POLDER (polarization and directionality of the Earths reflectances) and Meteosat] passive remote-sensing devices to determine the aerosol optical thickness over the suburban area of Thessaloniki, Greece, from April 1996 to June 1997. The POLDER spaceborne instrument measures the degree of polarization of the solar radiance reflected by the Earth-atmosphere system. Aerosol optical thickness (AOT) retrieval needs an accurate estimate of the contribution of the ground surface to the top of atmospheres polarized radiance. We tested existing surface reflectance models and fitted their parameters to find the best model for the Thessaloniki area. The model was constrained and validated by use of independent data sets of coincident sunphotometer and POLDER measurements. The comparison indicated that the urban AOT over Thessaloniki was retrieved by the POLDER instrument with an accuracy of +/-0.05. From analysis of Meteosat data we found that approximately 40% of the days with high AOT (>0.18) are associated with African dust transport events, all observed in the period March-July. Excluding dust events, the 15-month AOT averages 0.12 +/- 0.04. During the 15-month period that the study was conducted, we observed aerosol pollution peaks with an AOT of >0.24 on 15 of the 164 days on which measurements were possible.

Principle and Physics of the LiDAR Measurement

In this chapter, a brief overview of the LiDAR measurement used for Earth and space observation is first presented. This is subsequently followed by an overview of the LiDAR instrument, describing its various optical constituent elements. The LiDAR equation is then described with emphasis on its key parameters. Finally, the main causes of LiDAR error on surface observations are specified, with a particular emphasis on airborne or spaceborne measurements.

Use of ERS/SAR measurements for soil geometric and aerodynamic roughness estimation in semi-arid and arid areas

This paper discusses the potential of radar signal to characterise the bare surface roughness in arid or semi-arid regions. The used microwave sensor is the SAR of ERS. Ground truth measurements were acquired over different arid sites in the South of Tunisia. An empirical approach is proposed to derive the surface roughness from SAR measurements. The relationships with two different kinds of roughness have been studied: the geometric roughness, which is characterised by a rather new parameter called Zs, and the classical aerodynamic roughness Z/sub 0/.

Measurements of stratospheric volcanic aerosol optical depth from NOAA TIROS Observational Vertical Sounder (TOVS) observations

[1]xa0We show that the infrared optical depth of stratospheric volcanic aerosols produced by the eruption of Mount Pinatubo in June 1991 may be retrieved from the observations of the High-Resolution Infrared Radiation Sounder (HIRS-2) on board the polar meteorological satellites of the National Oceanic and Atmospheric Administration (NOAA). Evolution of the concentration in time and in space, in particular the migration of the aerosols from the tropics to the Northern and Southern Hemispheres, is found to be consistent with our knowledge of the consequences of this eruption. The method relies on the analysis of the differences between the satellite observations and simulations from an aerosol-free radiative transfer model using collocated radiosonde data as the prime input. Thus aerosol optical depths are retrieved directly without making assumptions about the aerosol size distribution or absorption coefficient. The aerosol optical depths reached a maximum in August 1991 in the tropical zone (0.055 at 8.3 μm, 0.03 at 4.0 μm, and 0.02 at 11.1 μm). The peak occurred in November 1991 in the southern midlatitudes and in March/April 1992 in the northern midlatitudes. A reanalysis of the almost 25 year archive of NOAA TIROS-N Operational Vertical Sounder (TOVS) observations holds considerable promise for improved knowledge of the atmosphere loading in volcanic aerosols.

Development of a small portable eyesafe unattended scanning lidar for analysis of the structural and optical properties of tropospheric aerosols

Structural and optical properties of aerosols and clouds can be retrieved by active remote sensing systems, such as lidars. Such parameters are of importance in the study of dynamics and radiation budget of the atmosphere. In that respect, a small, portable, eyesafe, unattended, elastic-backscatter lidar is being developed at Cimel Electronique, in collaboration with CNRS. It sues a compact, low-energy laser in the visible. The detection is made by a high-gain, high-speed PMT, and a single electronic card for fast acquisition. The aim of the system is also to be tunable to various pointing angles. A variational method was developed to make use of the multiangle measurements and tested on data collected during the INDOEX campaign in March 1999. The optical thickness and backscatter coefficient profiles were retrieved up to 1 km with a total uncertainty of 18 percent. The system has been assembled and first measurements have been made beginning of 2000 for comparison with the theoretical predictions. The system has shown it was satisfactory and the signal profiles obtained are in agreement with the ones simulated with the system parameters.

Assessment of the Oceanic Surface Reflectance by Airborne Lidar to Improve a Stable Inversion Technique

Open ocean surface reflectance has been determined from inverted lidar measurements taken with LEANDRE I during campaigns over the Azores. Aerosol phase function has been calculated using a model of mixed sulphate and sea salt particles including humidity dependence and wind production. Results obtained are in good agreement with modelling and can be used to improve a stable inversion.