In an effort to evaluate scattering models for particle size distributions of ice crystals within cirrus clouds, simultaneous data was collected in March 2000 during the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Cloud Intensive operational period (Cloud IOP) at the Cloud and Radiation Testbed (CART) site in Lamont, Oklahoma. In situ measurements of ice particles were collected using the National Center for Atmospheric Research (NCAR) Video Ice Particle Sampler (VIPS), which flew on the University of North Dakota Citation research aircraft. Ground-based vertical radar profiles were collected using the University of Massachusetts (UMass) 33GHz/95GHz Cloud Profiler Radar System (CPRS). Data from both sensors was used to retrieve and compare the equivalent radar reflectivity at Ka band (33GHz). The equivalent radar reflectivity measured by the ground-based, zenith-looking, CPRS radar at Ka band and compared to the reflectivity computed from the airborne VIPS samples of particle size distribution, N(D), using Mie theory. As anticipated the equivalent reflectivity of the radar and VIPS were similar at the time the UND Citation overflew the radar.
Various methods and techniques to estimate ice crystals radar response have been developed to study the structure of cirrus clouds. Most methods assume a spherical shape for the ice crystals. This assumption leads to mistakes on the parameter estimation related to the particles' size. In this work, we modeled the shape of ice particles found in cirrus cloud as measured by airborne instruments, specifically ice bullets. These can be found depending on the temperature and cloud altitude, isolated or in groups of two or more bullets, called bullet rosettes. The model of the bullets was developed using the parameters obtained by airborne measurements from the National Center for Atmospheric Research (NCAR) Video Ice Particle Sampler (VIPS). This is an airborne instrument that takes samples of the cirrus cloud particles sizes. With these sample parameters we created a bullet function in DDSCAT with the actual shape of the bullets. This software allows us to create irregular models of particles using the Discrete Dipole Approximation method. With this model we can analyze the backscattering produced by the bullet and rosette model or reflectivity and compute the total volume backscattering coefficient from the cirrus clouds. Various models of ice crystal habits are compared.
Characterization of the microphysical properties of non-precipitating stratus clouds including their suspended-water droplet size distribution and the cloud's liquid water content are estimated in this work. The dual wavelength ratio, DWR, and the differential extinction, DE, were computed at two millimeter frequencies, 33 GHz and 95 GHz, using UMass Cloud Profiling Radar System (CPRS) to estimate the drop size distribution. Data from radiosonde observations (Raob) is used as input in a recently calibrated model for estimation of the gaseous attenuation at Ka.-band and Liebe's model at W-band. Integrated specific humidity from a radiometer is used to constrain the radiosonde specific humidity. The radar reflectivity is corrected to take into account the effect of the wind speed, the difference of beamwidth at both frequencies and the difference in sampled range cells. Radar reflectivity and ancillary data are combined to obtain the differential extinction and the estimated cloud's liquid water density. Profiles of the processed data, such as DE, the DWR and the cloud's liquid water density are presented. Cloud's water density and radar reflectivity were used for the size distribution estimation of the suspended water droplets and the median drop diameter.
The use of multi-frequency radar's Doppler Spectrum to study different aspects of precipitation has demonstrated its utility as an accurate profiling rain-gauge method. Recent studies used this concept to retrieve the drop-size distribution and vertical air motion in rain using dual-frequency Cloud Profiling Radar System, which operates at 33 GHz (Ka-band) and 95 GHz (W-band). This study was performed for low to moderate rain-rates because the use of the Ka-band frequency limited the accuracy of the measurements for high rain-rates due to the attenuation this signal suffers while it passes through the cloud. In this work we use a non-attenuating frequency, 2.8 GHz, instead of the Ka-band, to obtain measurements over a wider dynamic range of rain conditions, extending the active rain-gauge concept to heavier rain-rates. The W-band signal provides accurate measurement of the vertical air motion in rain. The actual drop's shapes must be corrected for heavy rain in which case large non-spherical raindrops exist. Data will be processed as suggested by Firda et al., 1999 considering the drop's shape corrections. This research's goal is to develop IDL codes to align, process, and analyze the collected data to retrieve several cloud characterization parameters, such as drop size distribution and vertical air motion that would be used to study the inner processes of rain. Rain-rate approximations and the vertical air motion retrieval will be presented.
In this work, we examine the relevance of an improved model for the microwave brightness temperature over calm ocean to the TOPEX/Poseidon satellite. The model is divided into two sub-models, the atmospheric absorption model and the ocean surface emissivity model. Both models are developed from a comparison via Newthon-Raphson iterative method of ancillary data as required by the radiative transfer equation, with well calibrated radiometer data. Finally, the contribution of both models to the overall error budget for the wet troposphere propagation path delay estimation algorithm is presented. The path delay is used to correct the altimeter sensor on board of the satellite utilized to measure global sea surface topography. The improvement in the accuracy of the path delay is found to be 37% which render more accurate measurements from this satellite mission.