Comprehensive T-Matrix Reference Database: A 2007-2009 Update

The T-matrix method is among the most versatile, efficient, and widely used theoretical techniques for the numerically exact computation of electromagnetic scattering by homogeneous and composite particles, clusters of particles, discrete random media, and particles in the vicinity of an interface separating two half-spaces with different refractive indices. This paper presents an update to the comprehensive database of T-matrix publications compiled by us previously and includes the publications that appeared since 2007. It also lists several earlier publications not included in the original database

Fast Radiative Transfer Approximating Ice Hydrometeor Orientation and its Implication on IWP Retrievals

The accurate simulation of microwave observations of clouds and precipitation are com-putationally challenging. A common simplification is the assumption of totally random orientation (TRO); however, studies have revealed that TRO occurs relatively infrequently in reality. A more appropriate assumption is that of azimuthally random orientation (ARO), but so far it has been a com-putationally expensive task. Recently a fast approximate approach was introduced that incorporates hydrometeor orientation into the assimilation of data from microwave conically scanning instruments. The approach scales the extinction in vertical (V) and horizontal (H) polarised channels to approximate ARO. In this study, the application of the approach was extended to a more basic radiative transfer perspective using the Atmospheric Radiative Transfer Simulator and the high-frequency channels of the Global Precipitation Measurement Microwave Imager (GMI). The comparison of forward simulations and GMI observations showed that with a random selection of scaling factors from a uniform distribution between 1 and 1.4–1.5, it is possible to mimic the full distribution of observed polarisation differences at 166 GHz over land and water. The applicability of this model at 660 GHz was also successfully demonstrated by means of existing airborne data. As a complement, a statistical model for polarised snow emissivity between 160 and 190 GHz was also developed. Combining the two models made it possible to reproduce the polarisation signals that were observed over all surface types, including snow and sea ice. Further, we also investigated the impact of orientation on the ice water path (IWP) retrievals. It has been shown that ignoring hydrometeor orientation has a significant negative impact (∼20% in the tropics) on retrieval accuracy. The retrieval with GMI observations produced highly realistic IWP distributions. A significant highlight was the retrieval over snow covered regions, which have been neglected in previous retrieval studies. These results provide increased confidence in the performance of passive microwave observation simulations and mark an essential step towards developing the retrievals of ice hydrometeor properties based on data from GMI, the Ice Cloud Imager (ICI) and other conically scanning instruments

Rain observations by a multifrequency dual-polarized radiometer

During the Convective and Orographically Induced Precipitation Study, advanced microwave radiometer for rain identification has continuously acquired measurements at the Atmospheric Radiation Measurement Mobile Facility in the Black Forest from the beginning of August until December 2007. The radiometer has six channels measuring in horizontal and vertical polarizations at 10.65, 21.0, and 36.5 GHz. Rainy events have been selected out of the entire database according to collocated gauges and, subsequently, analyzed. Measured brightness temperatures and (verticalhorizontal) polarization differences are interpreted by comparing with radiative transfer simulations, which account for the presence of nonspherical particles in preferential orientation. Measurements confirm the importance of the polarization signal for separating the effect introduced by non-Rayleigh scatterers and, therefore, the rain from the cloud component. More quantitative interpretation of the signal requires a better understanding of the role played by melting particles and an identification of the 3-D structure of the precipitating system under observation. Both aspects will be tackled in the near future by exploiting the synergy with a coinstalled micro rain radar. © 2009 IEEE

Polarized signals from oriented frozen hydrometeors using passive microwave radiometry

Ice clouds play a significant role in energy budget of the earth-atmosphere system, and they also participate in global hydrological cycle. Thick ice clouds which are associated with precipitation transfer energy and water between the atmosphere and the earth. The net effects of ice clouds on the earth-atmosphere system highly depend on their microphysical properties. However, the complex and variable structure of ice clouds makes it difficult to capture them well in models. The oversimplified microphysical properties of ice clouds in retrievals introduce significant uncertainties in weather and climate studies. The knowledge on the orientation of ice particles is very limited. The orientation of frozen hydrometeors which induces polarization signatures determines the magnitude of polarized signals. In order to investigate the potential polarized signatures induced by the oriented frozen hydrometeors, ground-based polarization observations have been performed at “Umweltforschungsstation Schneefernerhaus” (UFS) on Mount Zugspitze (German Alps) at 2650 m above sea level. In this study, the polarization observations carried out by a ground-based dual polarized microwave radiometer (DPR) at 150 GHz are investigated together with auxiliary instruments deployed at UFS, i.e., a second microwave radiometer (HATPRO) and a K-band micro rain radar (MRR). HATPRO measures liquid water path (LWP) and integrated water vapor (IWV) during snowfall, and MRR operating at 24.1 GHz provides indirect snow water path (SWP) information. Based on the observations, the analysis of a single snow case and one-year snowfall data show that the brightness temperature (TB) differences between the vertical and horizontal polarizations reach up to −10 K at an elevation angle of 34.8^o during snowfall. The polarized signals during snowfall can be explained well by the occurrence of oriented snow particles. The analysis of the synergic observations shows the effects of snowfall parameters on polarization differences (PDs) observed with DPR at 150 GHz. The dependencies of the measured PD and TB on MRR integrated radar reflectivity and independently derived LWP are discussed. It shows that the high SWP indicated by high values of MRR integrated reflectivity enhances both TB and PD due to the scattering effects of snow particles. Meanwhile, TBs are found to be enhanced during snowfall when supercooled liquid water is present, while PD resulting from oriented snow particles is damped by the increase of LWP. The polarization observations support the potential role of polarization measurements in improving retrievals of snowfall microphysical parameters. To evaluate the effects of SWP and LWP on PD and TB, radiative transfer (RT) simulations assuming horizontally aligned snow oblates using a polarized RT model have been performed. PD and TB observations can be captured well by the RT model with given reasonable assumptions on the microphysical parameters of oriented snow oblates. Additionally, the uncertainties of PD and TB caused by snow microphysical properties are fully examined in the RT simulations. The “damping (enhancing)” effects of supercooled liquid water on PD (TB) are further interpreted by a simple physical model where the height of cloud liquid varies with respect to the dichroic snow layer. From the ground-based observations, it is found that PD resulting from oriented snow particles is absorbed by supercooled liquid below snow layers. When supercooled liquid water is located above snow layers, PD is damped since it is compensated by the emission of supercooled liquid water penetrating the snow layer. TB is generally enhanced by the presence of LWP: the warmer the supercooled liquid water, the larger the TB. The polarization observations promote the design of new instruments further. Under an assumption that ice particles are oriented, RT simulations are performed for the space-borne satellite FengYun-4 (FY-4) channels to examine polarization information content for ice cloud characterization. The results show that polarization can be beneficial for ice cloud retrievals and additional information can be provided by polarized signals to quantify ice cloud parameters, especially at high frequency channels. Therefore, the present work strongly suggests the deployment of microwave polarization channels for ice cloud observations