Elsa Cattani

Simulations of time‐coincident, co‐located measurements from ENVISAT‐1 instruments for the characterization of tropospheric aerosols: a sensitivity study including cloud contamination effects

Abstract A sensitivity test of the aerosol properties retrieval is conducted using nadir viewing instruments on board the ENVISAT-1 platform trying to reproduce the natural variability of the aerosol field. The analysis highlights problems arising in the retrieval when aerosol loading, relative humidity, and cloud coverage are simultaneously varied.

Retrieval of aerosol properties over the ocean using Global Ozone Monitoring Experiment measurements: Method and applications to test cases

Satellite monitoring of aerosol properties using passive techniques is widely considered a crucial tool for the study of climatic effects of atmospheric particulate [Kaufman et al., 1997]. Only space-based observations can provide the required global coverage information on spatial distribution and temporal variation of the aerosol field. This paper describes a method for deriving aerosol optical thickness at 500 nm and aerosol type from Global Ozone Monitoring Experiment (GOME) data over the ocean under cloud-free conditions. GOME, flying on board the second European Remote Sensing satellite (ERS 2) since April 1995, is a spectrometer that measures radiation reflected from Earth in the spectral range 240–793 nm. The features of the instrument relevant to the aerosol retrieval task are its high relative radiometric accuracy (better than 1%), its spectral coverage, and its spectral resolution, which allows wavelengths in spectral regions free of molecular absorption (atmospheric windows) to be selected. The method presented is based on a pseudo-inversion approach in which measured reflectance spectra are fitted to simulated equivalents computed using a suitable radiative transfer model. The crucial aspects of this method are the high accuracy and the nonapproximate nature of the radiative transfer model, which simulates the spectra during the fitting procedure, and careful selection of candidate aerosol classes. A test application of the proposed method to a Saharan dust outbreak which occurred in June 1997 is presented, showing that in spite of the instruments low spatial resolution, information on both optical thickness and spectral characterization of the aerosol can be retrieved from GOME data. Preliminary comparisons of the results with independent estimates of the aerosol content show that a good correlation exists, encouraging planning of a systematic application of the method.

CONSIDERATIONS ON DAYLIGHT OPERATION OF 1.6-VERSUS 3.7-µm CHANNEL ON NOAA AND METOP SATELLITES

Abstract The transition from the Advanced Very High Resolution Radiometer (AVHRR)/2 to AVHRR/3 on NOAA polar orbiters was associated with a switching from daylight operations of the 3.7- to 1.6-µm wave band, while retaining 3.7 µm for nighttime operations. Investigations of the daylight applicability of the two channels suggest that the 1.6-µm wave band for daylight operations does not prove to be the better choice, at least for cloud applications. The 3.7-µm wave band is much less affected by surface contamination, and measures more faithfully and unambiguously the particle effective radius near cloud tops. The 1.6-µm radiation penetrates deeper into the cloud, supplying an integrated signal throughout the inner portions of the cloud, including surface contribution. Therefore, a synergetic use of the two wave bands can provide an improved retrieval of cloud microstructure and precipitation than from any of the channels alone. However, when one channel must be selected for the AVHRR/3, 3.7 µm performs muc...

Detection and Measurement of Snowfall from Space

Snowfall detection and measurement represent highly difficult problems in modern hydrometeorology. Ground measurements are complicated due to detection technology limitations, snow drift and accumulation issues, and error definition. The snowfall detection from space is in turn affected by all detection limitations that characterize the measurement of rainfall with the addition of several complications, such as the indirect character of remote sensing precipitation estimation, the presence of frozen or snow-covered terrain, and the unknown vertical distribution of hydrometeors in the cloud column. Several methods for the retrieval of snowfall intensity from satellite have been proposed in recent times using passive and active sensors. No satisfactory answer to the general problem of quantitative snowfall intensity determination has been found to date, but several studies contribute to delineate a working framework for the future operational retrieval algorithms.

Evaluation of Monthly Satellite-Derived Precipitation Products over East Africa

AbstractEast Africa experienced in the 2001–11 time period some of the worst drought events to date, culminating in the high-impact drought of 2010/11. Long-term monitoring of precipitation is thus essential, and satellite-based precipitation products can help in coping with the relatively sparse rain gauge ground networks of this area of the world. However, the complex topography and the marked geographic variability of precipitation in the region make precipitation retrieval from satellites problematic and product validation and intercomparison necessary. Six state-of-the-art monthly satellite precipitation products over East Africa during the 2001–09 time frame are evaluated. Eight areas (clusters) are identified by investigating the precipitation seasonality through the Global Precipitation Climatology Centre (GPCC) climatological gauge data. Seasonality was fully reproduced by satellite data in each of the GPCC-identified clusters. Not surprisingly, complex terrain (mountain regions in particular) re...

Effectiveness of the MS-method for computation of the intensity field reflected by a multi-layer plane-parallel atmosphere

Abstract A radiative transfer code based on the coupling of the currently labeled MS method (MS refers to the separation of the multiply and singly scattered radiation), with the reliable and widely used radiative transfer package DISORT is presented. We show that this code can be used to compute the intensity field reflected by a plane-parallel, non-emitting, aerosol loaded atmosphere with the same accuracy as a non-approximate model but maintaining a high computational speed. Results obtained for a two-layer atmosphere show that the single scattering features are clearly visible in the radiative field in the range from moderate to high aerosol optical thicknesses. Tests carried out in a reasonable range of viewing geometries, restricted to dark surfaces, and considering a significant set of aerosol optical properties, have shown that the present code is capable of attaining the same accuracy as DISORT but using a greatly decreased number of angular discretizations (streams), thereby reducing the computational time by a factor of between 2 and 10 with respect to DISORT , depending on the complexity of the scenario.

Aerosol optical thickness and classification: use of METEOSAT, GOME, and modeled data

Satellite measurements at high spectral resolution and span that avoid gas absorption bands and determine aerosol spectral optical properties are necessary for obtaining aerosol optical thickness values at the reference wavelength of 550 nm (hereinafter AOT). GOME (Global Ozone Monitoring Experiment on board the ERS-2 spacecraft) measurements fit such requirements, with a suitable spectral resolution over the region between UV and near IR while presenting a low spatial (320 X 40 Km2) and temporal resolution. The present new method overcomes these limitations by combining aerosol optical characteristics retrieved from GOME with METEOSAT visible data; the latter allow for monitoring aerosol events with adequate temporal resolution over wide cloud-free oceanic areas. The AOT results from fitting the measured broad-band visible METEOSAT radiance with the simulated radiance from radiative transfer calculations; aerosol properties estimated from GOME data are the essential input parameters. Several parameterizations of aerosol microphysical quantities have been tested to improve the AOT retrievals.

Satellite and Numerical Model Investigation of Two Heavy Rain Events over the Central Mediterranean

AbstractTwo heavy rain events over the Central Mediterranean basin, which are markedly different by genesis, dimensions, duration, and intensity, are analyzed. Given the relative low frequency of this type of severe storms in the area, a synoptic analysis describing their development is included. A multispectral analysis based on geostationary multifrequency satellite images is applied to identify cloud type, hydrometeor phase, and cloud vertical extension. Precipitation intensity is retrieved from (i) surface rain gauges, (ii) satellite data, and (iii) numerical model simulations. The satellite precipitation retrieval algorithm 183-Water vapor Strong Lines (183-WSL) is used to retrieve rain rates and cloud hydrometeor type, classify stratiform and convective rainfall, and identify liquid water clouds and snow cover from the Advanced Microwave Sounding Unit-B (AMSU-B) sensor data. Rainfall intensity is also simulated with the Weather Research and Forecasting (WRF) numerical model over two nested domains w...

Characterization of plumes on top of deep convective storm using AVHRR imagery and radiative transfer simulations

Abstract Cirrus clouds often form on top of intense convective storms and influence the radiation exchange at the top of the atmosphere. From time to time these cirrus assume a typical plume form, whose origin is yet to be clearly explained. Investigation on plumes structure and composition is necessary using satellite observations and radiative transfer modeling, since in situ measurements are generally not available for the storms extreme scenarios. Plumes of ice crystals produce a significant increase of cloud top reflectivity in channel 3 (3.55–3.93 μm) of National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR), due to the small size of the crystals (of the order of the channel wavelength). AVHRR measurements over a severe storm are compared with radiative transfer simulations through a 1-D, plane-parallel radiative transfer model (RTM). The hydrometeor vertical distribution of a cumulonimbus tower is assumed, composed of a water droplet layer underlying an ice crystal layer. The presence of the plume on top of the cumulonimbus is simulated by an ice crystal layer whose optical depth and ice crystal size and habit are varied. The results of the comparisons show that channel 3 conveys relevant information on the ice crystal size and habit.

Potential vorticity patterns in Mediterranean “hurricanes”

The potential vorticity (PV) anomalies due to the intrusion of dry stratospheric air and those generated by the tropospheric diabatic latent heating are qualitatively analyzed for five Mediterranean tropical-like cyclones (also known as Medicanes or TLCs). Model simulations show the presence of an upper level PV streamer in the early stages of the cyclone, located on the left exit of a jet stream, and a middle-low level PV anomaly generated by the convection developing around the low-level vortex. In the mature stage, the upper level PV anomaly around the cyclone evolves differently for each case and appears somehow dependent on the lifetime. Only for the 2006 Medicane, the PV anomalies form an intense PV tower extending continuously from the upper troposphere to the lower stratosphere.