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Guosheng Liu

Abstract

As satellite high-frequency passive microwave data have recently become available, there is an increasing demand for an accurate and computationally efficient method to calculate the single scattering properties of nonspherical ice particles, so that it may be used in radiative transfer models for physical retrievals of ice water path and snowfall rate. In this study, two such approximations are presented for calculating the single scattering properties of three types of large ice particles: bullet rosettes, sector snowflakes, and dendrite snowflakes, for the frequency range of 85 to 220 GHz, based on results of discrete-dipole approximation (DDA) modeling. By analyzing the DDA modeling results, it is noted that, for nonspherical ice particles, the scattering and absorption cross sections and the asymmetry parameter have a magnitude between those of the two imaginary equal-mass spheres. One is a solid sphere, and the other is an ice–air mixed soft sphere whose diameter equals the particle's maximum dimension. Therefore, the first approximation involves substituting the single scattering properties of a nonspherical ice particle with those of an equal-mass sphere, which can be calculated by Lorenz–Mie theory, with an effective dielectric constant derived by mixing ice and air using the Maxwell–Garnett formula. The diameter of such an equal-mass sphere D is bigger than the diameter of the solid sphere D 0, but smaller than the particle's maximum dimension D max. Defining a softness parameter SP = (DD 0)/(D maxD 0), it is found that the best-fit equal-mass sphere has an SP value of 0.2 ∼ 0.5 for calculating the volume scattering coefficient, depending on frequency and particle shape. At 150 GHz, the best-fit softness parameter is found to be ∼1/3 when averaging over all particle shapes. For calculating the asymmetry parameter, the DDA model results show that the best-fit softness parameter is close to 0 (i.e., the same as the solid sphere) for frequencies higher than 150 GHz, while it is about 0.3 for 85.5 GHz. The second approximation presented is a polynomial fit to the scattering and absorption cross sections and the asymmetry parameter using the particle size parameter as an independent variable. For the scattering cross section, three fitting curves are derived for, respectively, rosettes, sector snowflakes, and dendrite snowflakes. For the absorption cross section, a single curve is used to fit all particle shapes. For the asymmetry parameter, two curves are derived, one for rosettes and one for snowflakes. The best-fit softness parameter for three particular frequencies (85.5, 150, and 220 GHz) and for three particle shapes in the first approximation, as well as the coefficients of the polynomial fit in the second approximation, are presented. After implementing these approximations in a radiative transfer model, radiative transfer simulations are carried out for a snowfall and an ice cloud case. The simulated brightness temperatures based on the two approximations agree with each other within 3 K, but are significantly different from those based on the solid- and the soft-sphere approximations.

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Guosheng Liu

As satellite observations at high microwave frequencies have recently become available, there is an increasing demand for methods that accurately evaluate the single-scattering properties of nonspherical ice particles at these frequencies. Algorithm developers can use the single-scattering datasets in the retrievals of cloud ice water content and snowfall rate. However, the methods that correctly handle the scattering of complex nonspherical particles are computationally inefficient and impractical for physical retrieval algorithms, in which scattering needs to be evaluated many times for particles with various sizes and shapes. As a remedy, during the past several years we have computed the scattering properties—scattering and absorption cross sections, phase functions, asymmetric parameters, and backscattering cross sections —using an accurate discrete dipole approximation method and arranged the results into an easy-to-access database. The database contains the scattering properties at frequencies from 15 to 340 GHz, with temperatures from 0° to −40°C, of particle sizes (maximum dimension) from 50 to 12,500 μm, and for 11 particle shapes. The database along with an easy-to-use reading program is now made available to interested investigators. This article explains how this database is derived and how it can be used.

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Kazumasa Aonashi and Guosheng Liu

Abstract

The Baiu front is a subtropical convergence zone that is formed over east Asia in early summer (hereinafter referred to as the Baiu period). In this study, an overocean precipitation retrieval algorithm is developed to retrieve precipitation for the Baiu period from brightness temperatures (TBs) supplied by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). The basic idea of the algorithm is to find the optimal precipitation that gives radiative transfer model (RTM)-calculated, field-of-view–averaged TBs that fit best with the TMI TBs at 10.7, 19.7, and 85.5 GHz with vertical polarization. For the RTM calculation, spatial precipitation inhomogeneity and freezing-level height are estimated from TMI TBs. The optimal precipitation with 10-km resolution is obtained by solving the gradient equation of a cost function that is a weighted sum of squares of TB differences between the TMI observation and the RTM calculation. Precipitation retrieved by this algorithm was validated using TRMM precipitation radar (PR) data from the western part of Japan during June–July of 1998. The results indicate the following.

  1. Mesoscale (∼100 km) structures of precipitation disturbances were retrieved successfully with the algorithm. However, there were discrepancies in position and strength of individual rain cells between the precipitation retrievals and PR data.
  2. Precipitation retrieved by the algorithm agreed well with PR data within the precipitation range of 1–25 mm h−1, irrespective of precipitation type.
Experimental algorithms were applied to some cases during this period to examine the effect of improvements made to the algorithm, as compared with the authors’ previous work. The results show that use of TBs at 10.7 GHz largely improved heavy precipitation retrievals, and that correction using estimated spatial precipitation inhomogeneity alleviated underestimation of heavy precipitation caused by beam-filling error. It was also found that estimating freezing-level height slightly reduced precipitation retrieval errors.

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Yunfei Fu and Guosheng Liu

Abstract

Precipitation radar and microwave radiometer data collected by the Tropical Rainfall Measuring Mission (TRMM) satellite are used to study the variability of precipitation profiles and the relationship between precipitation profile and microwave brightness temperature. The variability has been examined using empirical orthogonal function (EOF) analysis and microwave emission and scattering signatures. Precipitation profiles are divided into three groups according to emission signatures at 19.4 GHz and three groups according to scattering signatures at 85.5 GHz. For stratiform rain, the differences of vertical precipitation profiles among these groups are small and are mainly seen in the slope of profiles below the freezing level. However, clear differences in vertical precipitation profiles can be found among the deep-convective rain groups. The maximum rainfall rate occurs at a considerably lower altitude when low liquid-emission or low ice-scattering signatures are observed. When emission or scattering signatures are high, precipitation profiles peak near the freezing level, a feature that is similar to the one in stratiform precipitation profiles. The three patterns of the vertical profiles derived from microwave signatures are very similar to the three patterns derived by EOF analysis. This similarity suggests that the three patterns derived by microwave signatures represent the most significant variability in vertical precipitation profiles. Results also show that, for the same near-surface rainfall rate, the pixel group with anomalously high microwave emission also shows anomalously high microwave scattering, and vice versa, suggesting that the liquid and ice water amounts in tropical rains are correlated over scales the size of a satellite pixel. It is also found that, for a given surface rainfall rate, the brightness-temperature differences among the pixel groups are large, highlighting the importance of vertical precipitation profile in determining upwelling microwave radiation and, therefore, the need to incorporate realistic precipitation profile information in rain retrieval algorithms.

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Hongfei Shao and Guosheng Liu

Abstract

The relative change in cloud droplet number concentration with respect to the relative change in aerosol number concentration, α, is an indicator of the strength of the aerosol indirect effect and is commonly used in models to parameterize this effect. Based on Twomey’s analytical expression, the values of α derived from measurements of an individual cloud (i.e., αT) can be as large as 0.60–0.90. In contrast, the values of α derived from direct measurements of polluted and clean clouds (i.e., α Δ) typically range from 0.25 to 0.85, corresponding to a weaker but more uncertain cooling effect. Clearly, reconciling α Δ with αT is necessary to properly calculate the indirect aerosol forcing. In this study, the terms that are involved in determining αT and α Δ are first analytically examined. Then, by analyzing satellite data over subtropical oceans, the satellite-observed α Δ can be successfully related to Twomey’s analytical solution. It is found that except for the dust-influenced region of the northeastern Atlantic Ocean, injecting continental aerosols into a marine background may significantly reduce the average aerosols’ ability to act as cloud condensation nuclei. Taking this competing effect into account may reduce the cooling effect proposed by Twomey from 0.76 to 0.28. It is also found that the variability of the adiabaticity (i.e., the cloud dilution state with respect to adiabatic cloud) among different clouds accounts for ∼50% uncertainty in α Δ. Based on these results, the authors explain the claimed discrepancies in the first aerosol indirect effect (AIE) from different methods and on different scales and present an improved parameterization of the first AIE that can be used in global climate models.

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Yulan Hong and Guosheng Liu

Abstract

The characteristics of ice clouds with a wide range of optical depths are studied based on satellite retrievals and radiative transfer modeling. Results show that the global-mean ice cloud optical depth, ice water path, and effective radius are approximately 2, 109 g m−2, and 48 , respectively. Ice cloud occurrence frequency varies depending not only on regions and seasons, but also on the types of ice clouds as defined by optical depth values. Ice clouds with different values show differently preferential locations on the planet; optically thinner ones ( < 3) are most frequently observed in the tropics around 15 km and in midlatitudes below 5 km, while thicker ones ( > 3) occur frequently in tropical convective areas and along midlatitude storm tracks. It is also found that ice water content and effective radius show different temperature dependence among the tropics, midlatitudes, and high latitudes. Based on analyzed ice cloud frequencies and microphysical properties, cloud radiative forcing is evaluated using a radiative transfer model. The results show that globally radiative forcing due to ice clouds introduces a net warming of the earth–atmosphere system. Those with < 4.0 all have a positive (warming) net forcing with the largest contribution by ice clouds with ~ 1.2. Regionally, ice clouds in high latitudes show a warming effect throughout the year, while they cause cooling during warm seasons but warming during cold seasons in midlatitudes. Ice cloud properties revealed in this study enhance the understanding of ice cloud climatology and can be used for validating climate models.

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Yunfei Fu and Guosheng Liu

Abstract

Rain-type statistics derived from Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) standard product show that some 70% of raining pixels in the central Tibetan Plateau summer are stratiform—a clear contradiction to the common knowledge that rain events during summer in this region are mostly convective, as a result of the strong atmospheric convective instability resulting from surface heating. In examining the vertical distribution of the stratiform rain-rate profiles, it is suspected that the TRMM PR algorithm misidentifies weak convective rain events as stratiform rain events. The possible cause for this misidentification is believed to be that the freezing level is close to the surface over the plateau, so that the ground echo may be mistakenly identified as the melting level in the PR rain classification algorithm.

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Atul K. Varma and Guosheng Liu

Abstract

The horizontal distribution of rain rates within an area comparable to the pixel size of satellite microwave radiometers and the grid size of numerical weather prediction models has been studied over the global Tropics using three years of the Tropical Rainfall Measuring Mission satellite precipitation radar (PR) data. The global distribution of rain-rate standard deviation derived from the PR data suggests that the horizontal variability of rain rates is largely influenced by two factors: surface type (land or ocean) and latitudinal location (tropical or extratropical). Except for light stratiform rain, the land–ocean contrast seems to be the dominant feature for the differences in conditional probability density functions (PDFs) of rain rate. That is, oceanic rain-rate distribution is narrower when the rain rate is low, but becomes broader when the rain rate is high. For light stratiform rain, there is no clear difference among the rain-rate PDFs for rain events over land and ocean. The latitudinal variation of rain-rate PDFs seems to be greater for heavy rain than for light rain. In particular, there is no measurable difference in overland convective rain-rate PDFs between the Tropics and extratropics. Based on three years of observational data, two attributes, fractional rain cover and conditional rain-rate PDFs, are parameterized as a function of 0.25° × 0.25° areal rain rate. These parameterizations are particularly useful in satellite microwave rainfall retrieval and assimilation of satellite microwave radiance data in numerical weather prediction models.

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Guosheng Liu and Judith A. Curry

Abstract

To provide guidance for the development of satellite microwave rainfall-retrieval algorithms, the basic relationships between emission and scattering signals in natural clouds must be understood. In this study, the relationship between two parameters observed from microwave satellite data—the polarization difference at 19 GHz D and the polarization-corrected temperature PCT—is investigated over the global ocean on a monthly and 5° (lat) × 5° (long) mean basis. Using data from January and July 1993, the occurrence frequencies and latitudinal variation and horizontal distribution of the D–PCT relationships are investigated. The D–PCT slope is studied by dividing the entire weather range into three regimes: nonprecipitation, light precipitation, and heavy precipitation. The analysis shows that small variation of PCT in the nonprecipitation regime could be achieved by employing a variable coefficient in the PCT definition equation. The slopes in the light precipitation regime are latitude dependent. Although the interpretation is inconclusive, it is felt that the differences in the fractional coverage and the rain layer depth in different latitudes is responsible for the latitudinal dependence. No clear latitudinal dependence of slopes in the heavy precipitation regime is found.

The connection of the D–PCT relationship to the performances of an emission-based and a scattering-based rainfall algorithm are investigated using the Second WetNet Precipitation Intercomparison Project rainfall cases. The results of this study emphasize the necessity of incorporating the scattering signal in rainfall rate retrieval algorithms. Additionally, the D–PCT slope information can be used to help categorize precipitation types, which may be useful in determining the specific algorithm best used for a certain precipitation type and/or regime.

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Guosheng Liu and Judith A. Curry

Abstract

An analysis of satellite microwave brightness temperatures at 85 GHz (37 GHz) shows that these temperatures sometimes vary by more than 30 K (15 K) within 1 or 2 days at a single location over Arctic sea ice. This variation can be seen in horizontal brightness temperature distributions with spatial scales of hundreds of kilometers, as well as in brightness temperature time series observed at a single location. Analysis of satellite observations during winter shows that such brightness temperature warming frequently occurs in the Arctic Ocean, particularly in regions over which low pressure systems often pass. By comparing the observed microwave brightness temperature warming with ground-based measurements of geophysical variables collected during the Surface Heat Budget of the Arctic (SHEBA) experiment and with numerical prediction model analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF), it is found that brightness temperature anomalies are significantly correlated with clouds and precipitation. This finding raises the possibility of using satellite microwave data to estimate cloud liquid water path and precipitation in the Arctic. Factors contributing to the brightness temperature warming were examined, and it was found that the primary contributors to the observed warming were cloud liquid water and surface temperature change.

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