T. Maestri

EAQUATE: An International Experiment For Hyperspectral Atmospheric Sounding Validation

The international experiment called the European Aqua Thermodynamic Experiment (EAQUATE) was held in September 2004 in Italy and the United Kingdom to validate Aqua satellite Atmospheric Infrared Sounder (AIRS) radiance measurements and derived products with certain groundbased and airborne systems useful for validating hyperspectral satellite sounding observations. A range of flights over land and marine surfaces were conducted to coincide with overpasses of the AIRS instrument on the Earth Observing System Aqua platform. Direct radiance evaluation of AIRS using National Polar-Orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounder Testbed-Interferometer (NAST-I) and the Scanning High-Resolution Infrared Sounder has shown excellent agreement. Comparisons of level-2 retrievals of temperature and water vapor from AIRS and NAST-I validated against high-quality lidar and dropsonde data show that the 1-K/l-km and 10%/1-km requirements for temperature and water vapor (respectively) are ge...

Breadboard of a Fourier-transform spectrometer for the Radiation Explorer in the Far Infrared atmospheric mission

In preparation for a possible space mission, a breadboard version named REFIR-BB of the Radiation Explorer in the Far Infrared (REFIR) instrument has been built. The REFIR is a Fourier-transform spectrometer with a new optical layout operating in the spectral range 100-1100 cm(-1) with a resolution of 0.5 cm(-1), a 7-s acquisition time, and a signal-to-noise ratio of better than 100. Its mission is the spectral measurement in the far infrared of the Earths outgoing emission, with particular attention to the long-wavelength spectral region, which is not covered by either current or planned space missions. This measurement is of great importance for deriving an accurate estimate of the radiation budget in both clear and cloudy conditions. The REFIR-BB permits the trade-off among all instrument parameters to be studied, the optical layout to be tested, and the data-acquisition strategy to be optimized. The breadboard could be used for high-altitude ground-based campaigns or could be flown for test flights on aircraft or balloon stratospheric platforms. The breadboards design and the experimental results are described, with particular attention to the acquisition strategy and characterization of the interferometer. Tests were performed both in laboratory conditions and in vacuum. Notwithstanding a loss of efficiency above 700 cm(-1) caused by the poor performance of the photolithographic polarizers used as beam splitters, the results demonstrate the feasibility of using the spectrometer for space applications.

Scattering properties of modeled complex snowflakes and mixed-phase particles at microwave and millimeter frequencies

A microphysically based algorithm (named Snow Aggregation and Melting (SAM)) that models snowflakes composed of a collection of hexagonal columns by simulating a random aggregation process is presented. SAM combines together pristine columns with multiple dimensions to derive complex aggregates constrained to size-mass relationship obtained by data collected from in situ measurements. The model also simulates the melting processes occurring for environmental temperatures above 0°C and thus define the mixed-phase particles structure. The single-scattering properties of the modeled snowflakes (dry and mixed phased) are computed by using a discrete dipole approximation (DDA) algorithm which allows to model irregularly shaped targets. In case of mixed-phased particles, realistic radiative properties are obtained by assuming snow aggregates with a 10% of melted fraction. The single-scattering properties are compared with those calculated through Mie theory together with Maxwell-Garnett effective medium approximation using both a homogeneous sphere and a layered-sphere models. The results show that for large-size parameters there are significant differences between the radiative properties calculated using complex microphysical and optical algorithms (i.e., SAM and DDA) and those obtained from simplified assumptions as the layered-sphere models (even when the radial ice density distribution of the aggregated snowflakes is perfectly matched). Finally, some applications to quantitative precipitation estimation using radar data are presented to show how the resulting differences in the basic optical properties would propagate into radar measurable. Large discrepancies in the derivation of the equivalent water content and snowfall rate from radar measurements could be observed when large-size parameters are accounted for.

Retrieval of Cloud Optical Properties From Multiple Infrared Hyperspectral Measurements: A Methodology Based on a Line-by-Line Multiple-Scattering Code

A methodology to retrieve cloud optical properties using high spectral resolution (HSR) infrared (IR) measurements is presented. This new method has the ability to easily adapt to multiple instruments and viewing angles. The retrieval uses a line-by-line multiple-scattering simulation and HSR IR measurements to retrieve spectrally resolved cloud optical depths (ODs). The spectral ODs are compared to a precomputed OD database generated from an ensemble of cloud particle-size distributions and precomputed single-scattering and single-particle optical properties for a variety of ice-crystal habits. Cloud microphysics are retrieved by finding the closest fit to the database. Results are independent of first-guess optical property assumptions on size and habit. The retrieval method has been applied to aircraft, satellite, and uplooking HSR measurements with results evaluated against coincident HSR lidar and radar measurements. Analysis of retrieval errors produced by assumptions and uncertainties in the atmospheric state demonstrates different sensitivities to atmospheric parameters when uplooking or downlooking data are analyzed. For both viewing geometries, the retrieval is most sensitive to the uncertainties in the assumed cloud boundaries. It is also found that nonuniform vertical distribution of cloud OD can result in significant biases in the IR retrieved cloud ODs.

Spectral infrared analysis of a cirrus cloud based on Airborne Research Interferometer Evaluation System (ARIES) measurements

[1] The study compares radiance measurements taken by the Airborne Research Interferometer Evaluation System (ARIES) above a cirrus cloud over the sea with multiple scattering line-by-line simulations. The complete data set allows only a partial reconstruction of the main microphysical and geometrical features of the cirrus. However, spectral standard deviation of the radiance signal, which includes the spatial variability of the signal, is also available and is of great importance when comparing the simulations with the measurements. Two measurement sections are analyzed where the cloud shows different transmissivity and the comparison of simulations with measurements shows deviations that are below 1 standard deviation from 600 to 2250 cm -1 , except in small spectral intervals. The quality of the simulation results has justified an in-depth theoretical examination of the diabatic heating of the atmosphere. The main result is that different geometrical configurations, maintaining the cloud top elevation and varying cloud depth but keeping the ice water path constant, produce very similar results in the radiance and fluxes above the cloud and at the surface and, at the same time, very different diabatic heating of the atmosphere inside and below the cloud, which implies very different effects on the evolution of the cloud and its microphysics. From these results the use of additional remote-sensing tools to define the cloud geometry (for example, a lidar sensor) appears of fundamental importance when the dynamical and microphysical evolution of a cloud layer needs to be investigated.