Retrieval of atmospheric attenuation using ground-based and airborne millimeter-wave cloud radar measurements
Abstract (Summary)Cloud measurements at millimeter-wave frequencies are affected by attenuation due to atmospheric gases, clouds and precipitation. Estimation of the true equivalent radar reflectivity, Ze , is complicated because extinction mechanisms are not well characterized at these short wavelengths. This dissertation discusses cloud radar calibration and intercomparison of airborne and ground-based radar measurements, and describes two attenuation retrieval algorithms. The first one is the dual-radar method, which is based on dual 95 GHz radar measurements of the same cloud and precipitation volumes collected from opposing viewing angles. True radar reflectivity is retrieved by combining upward-looking and downward-looking radar profiles. This method reduces the uncertainty in radar reflectivity and attenuation estimates since it does not require a priori knowledge of the microphysical properties of hydrometeors. The second one is the modified Hitschfeld and Bordan (HB) algorithm, which uses single radar measurements with path integrated attenuation (PIA) as a constraint. Case studies are performed using data collected during the Summer 1998 NASA DC-8 Cloud Radar Experiment. Atmospheric attenuation and true radar reflectivity are retrieved by applying the dual-radar method. The results are then compared with those obtained using the modified HB algorithm. The analysis shows that the dual-radar method directly calculates the atmospheric attenuation and true radar reflectivity without knowledge of the relationship between clouds/precipitation attenuation rate (k ) and true radar reflectivity ( Ze ). It also provides a PIA reference to refine the HB algorithm. On the other hand, appropriate k - Z e relationships are essential to improve the performance of the modified HB algorithm. Ice cloud attenuation retrieved from the dual-radar method is related to the particle median volume diameter, which is estimated from MHz/95GHz dual wavelength measurements using theoretical models. Analysis of ice cloud measurements indicates that W-band attenuation is mostly caused by the loss due to scattering from large particles. Based on an IWC - Ze relationship, ice water content (IWC) is obtained. The analysis results show that appropriate attenuation correction significantly improves the accuracy of the IWC retrieval.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/2000