Comparison of AMSU Millimeter-Wave Satellite Observations, MM5/TBSCAT Predicted Radiances, and Electromagnetic Models for Hydrometeors

Abstract

This paper addresses the following: 1) millimeterwave scattering by icy hydrometeors and 2) the consistency between histograms of millimeter-wave atmospheric radiances observed by satellite instruments [Advanced Microwave Sounding Unit-A/B (AMSU-A/B)] and those predicted by a mesoscale numerical weather prediction (NWP) model (MM5) in combination with a two-stream radiative transfer model (TBSCAT). This observed consistency at 15-km resolution supports use of MM5/TBSCAT as a useful simulation tool for designing and assessing global millimeter-wave systems for remote sensing of precipitation and related parameters at 50-200 GHz. MM5 was initialized by National Center for Environmental Prediction NWP analyses on a 1 • grid approximately 5 h prior to each AMSU transit and employed the Goddard explicit cloud physics model. The scattering behavior of icy hydrometeors, including snow and graupel, was assumed to be that of spheres having an ice density F (λ) and the same average Mie scattering cross sections as computed using a discrete-dipole approximation implemented by DDSCAT for hexagonal plates and six-pointed rosettes, respectively, which have typical dimensional ratios observed aloft. No tuning beyond the stated assumptions was employed. The validity of these approximations was tested by varying F (λ) for snow and graupel so as to minimize discrepancies between AMSU and MM5/TBSCAT radiance histograms over 122 global storms. Differences between these two independent determinations of F (λ) were less than ∼0.1 for both snow and graupel. Histograms of radiances for AMSU and MM5/TBSCAT generally agree for 122 global storms and for subsets of convective, stratiform, snowy, and nonglaciated precipitation.

Improved scattering radiative transfer for frozen hydrometeors at microwave frequencies

“…Wiedner et al, 2004;Surussavadee and Staelin, 2006;Doherty et al, 2007;Johnson et al, 2012). However, the ECMWF attempts were not successful and there seems little point in detailing them here; as mentioned in the introduction, improving results at one frequency gives greater problems at another.…”

“…Though tuning schemes can be effective at one frequency, they can make results worse at others. To obtain good results from Mie theory Surussavadee and Staelin (2006) went as far as representing frozen particles with spheres that had a different size and density at each frequency. It would be preferable to use a particle model with a closer link to physical reality.…”

The impact of the melting layer on the passive microwave cloud scattering signal observed from satellites: A study using TRMM microwave passive and active measurements

“…It is important to keep in mind that modeling the optical properties of frozen particles is problematic even for pristine frozen particles that are not undergoing melting. The particle size distribution, the dielectric properties of the material, and the shape of the particle are all subject to large uncertainties that can translate into significant differences in terms of simulated brightness temperatures [e.g., Surussavadee and Staelin, 2006;Meirold-Mautner et al, 2006;Kulie et al, 2010].…”

Detection and Measurement of Snowfall from Space

“…The multi-sensor approach was first based on a logistic distribution to represent the probability of snowfall given the predictors and then using a Bayesian technique. A comparison was carried out with retrievals from the technique of the group at Massachusetts Institute of Technology [79][80][81][82] showing that both proposed methods discriminate snow and no-snow conditions in the polar regions with an overall reduction of the false alarms by at least 30% while considerably increasing the probability of detection. This would confirm the potential of using multisensor, multispectral approaches.…”