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Judith A. Curry

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Judith A. Curry

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The Arctic Stratus Experiment, conducted during June 1980 over the Beaufort Sea, produced an extensive set of simultaneous measurements of boundary layer structure, radiation fluxes, and cloud microphysical properties. In this paper these data are used to determine the interactions between mixing, radiative transfer, and cloud microphysics for four cloud decks. The thermodynamic structure and fluxes of the thermodynamic quantities in the cloudy boundary layer are examined, including liquid water fluxes. Net radiative heating profiles are also determined. A detailed analysis of the fine-scale structure of the cloud microphysics is presented, including correlations between the cloud microphysical parameters (droplet concentration, liquid water content, mean radius, spectral dispersion, and the 95% volume liquid water drop radius), which are used to infer the nature of the mixing processes and the local effects of radiative heating/cooling. A comparison is then made with other observations and existing model conceptions of the cloudy boundary layer and cloud microphysical processes.

Due to the large static stability and frequent occurrence of a humidity inversion, these clouds are not maintained by surface fluxes of moisture. The net radiative cooling at the cloud top is balanced differently for each of the cases examined, although in all four cases at least a portion of the radiative cooling was found to promote mixed-layer convection. The effects of turbulent entrainment do not penetrate beyond 50 m below mean cloud top, therefore not directly affecting the evolution of the drop spectra except for right near cloud top. Significant liquid water production due to radiative cooling is indicated by the profiles of buoyancy flux, entropy flux, water fluxes, and vertical velocity variance, and also by the large drop spectral dispersions and the correlations between the cloud microphysical parameters. Liquid water fluxes are determined to be nearly as large as the vapor fluxes. The liquid water flux divergences introduce significant structure into the profiles of liquid water content and drop spectra, and also enhance coalescence processes in the lower portion of the clouds.

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Judith A. Curry and Guosheng Liu

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This paper explores the potential of using satellite data to develop a climatology of aircraft icing probability in oceanic regions. The datasets employed in this analysis are: the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) microwave radiances; the U.S. Air Force Three-Dimensional Nephanalysis (3DNEPH); the European Centre for Medium-Range Weather Forecasts (ECMWF) initialized analyses; and the High-Resolution Infrared Sounder 2-Microwave Sounding Unit (HIRS2-MSU) satellite radiances. This analysis focuses on the midlatitude regions of the North Atlantic Ocean, encompassing the paths of most transatlantic flights between the United States and Europe.

Estimates of cloud temperature, cloud horizontal extent, liquid water path, cloud depth, cloud liquid water content, drop-size characteristics, and precipitation characteristics are obtained from these datasets. A total of 14% of low-level clouds are found to have average temperatures between −2° and −36°C, while 99% of midlevel clouds fall in this temperature range. More than half of the cloud decks have a horizontal areal extent of less than (46 km)2, and no cloud decks exceed (442 km)2 in horizontal extent. The mean low-cloud depth is determined to be about 1000 m and the midcloud depth to be about 1950 m. A maximum value of 800 g m−2 supercooled liquid water path is obtained considering single-layer cloud decks with 100% areal coverage. The average values of the liquid water path for the supercooled middle and low clouds are 62 and 92 g m−2, respectively. Using an assumed shape for the vertical profile of liquid water content, average values of 0.095 g m−3 for low-level clouds and 0.043 g m−3 for midlevel clouds are determined. The maximum value of liquid water content is determined to be 1 g m−3.

Assessment of aircraft icing potential for these clouds is made by first determining the probability of icing and then determining the ice accretion per icing encounter. The joint probability of an icing encounter with a specified mass of ice accretion can then be used to quantify the icing threat. This analysis shows maximum icing probability of 4.3% occurring at altitudes between 1.3 and 2.5 km, with icing probabilities less than 1% at altitudes below 200 m and above 5 km. Assuming a collection efficiency of unity, the average ice accretion per icing encounter is determined to be about 5 kg m−2, with a maximum average value of 13 kg m−2. A maximum value of ice accretion as large as 250 kg m−2 is determined near the 1400-m altitude. Under average conditions, the maximum icing threat is seen to occur at an altitude of 1700 m, corresponding to a 4.3% probability of an ice accumulation of 13.5 kg m−2. A discussion of errors and uncertainties associated with this methodology is given.

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Guosheng Liu and Judith A. Curry

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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

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An ice water path retrieval algorithm, using airborne Millimeter-Wave Imaging Radiometer brightness temperatures at 89, 150, and 220 GHz, is developed for tropical clouds. This algorithm is based on the results of radiative transfer model simulations, using in situ ice particle properties measured from aircraft as model inputs. The scattering signatures at the 150- and 220-GHz channels are the primary inputs into the algorithm, while 89-GHz data are used for determining the nonice background radiation. The ice water path is first calculated from each of the 150- and 220-GHz scattering signatures, and then a combination of the two channels is used for the final retrieval, based on the consideration of the different channel sensitivities to the magnitude of the ice water path. The algorithm is evaluated by comparing the retrieved with in situ measured ice water paths for seven cases observed during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Theoretical analysis shows that the uncertainty due to particle size could be the largest error in the retrievals and this error could be as large as plus or minus 50%. As an application of this algorithm, the ice water path characteristics during TOGA COARE are studied, including assessment of the mean of ice water path, its frequency distribution, and its relationships with cloud-top temperature and liquid water amount. Although tropical clouds are the target of this study, this algorithm could be modified and extended to other climatological regions.

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Guosheng Liu and Judith A. Curry

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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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Guosheng Liu and Judith A. Curry

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An over-ocean ice water path (IWP) algorithm, using satellite Special Sensor Microwave Water Vapor Sounder (SSM/T-2) data, is presented for clouds during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. In developing the retrieval algorithm, clouds are first divided into 10 classes based on their top temperatures and microwave radiative properties. Radiative transfer model simulations are then performed for the different classes to establish a relation between IWP and the depression of 150-GHz brightness temperature. Correction to the effect of supercooled liquid water is done by incorporating data of liquid water path (LWP) retrievals from Special Sensor Microwave/Imager (SSM/I) and relative humidity profiles from the European Centre for Medium-Range Weather Forecasts analyses. The algorithm retrievals are compared with the analyses in the International Satellite Cloud Climatology Project (ISCCP) dataset. By using collocated SSM/T-2, SSM/I, and ISCCP data, the relations among IWP and other atmospheric hydrological properties including cloud-top temperature, LWP, rainfall rate, and precipitable water are investigated. The results indicate that IWP tends to increase with the decrease of cloud-top temperature and this correlation is particularly evident for precipitating clouds. LWP retrieved for nonprecipitating clouds has a similar tendency but only for those with top temperatures warmer than 0°C. There is no clear relation between IWP and LWP. The ratio of IWP to total condensed water (IWP + LWP) for nonprecipitating clouds seems to be negatively correlated with cloud-top temperature on an average of a large data volume, but this relationship differs substantially among individual cases. Rainfall rate has a strong correlation with IWP. High values of IWP and LWP are always associated with high precipitable water although high precipitable water does not automatically correspond to high IWP or high LWP.

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Guosheng Liu and Judith A. Curry

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The method of simultaneously retrieving ice water path and mass median diameter using microwave data at two frequencies is examined and implemented for tropical nonprecipitating clouds. To develop the retrieval algorithm, the authors first derived a bulk mass–size relation for ice particles in tropical clouds based on microphysical data collected during the Central Equatorial Pacific Experiment. This relation effectively allows ice particle density to decrease with particle size. In implementing the retrieval algorithm, 150- and 220-GHz Millimeter-Wave Imaging Radiometer data collected during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment were used. Ice water path and mass median diameter are determined based on a lookup table generated by a radiative transfer model. The lookup table depends on cloud type, cloud liquid water path, and atmospheric temperature and humidity profiles. Only nonprecipitating clouds are studied in this paper. Error analyses were performed by a Monte Carlo procedure in which atmospheric profiles, ice cloud height, liquid water content, surface temperature, and instrument noise vary randomly within their uncertainty range through a Latin hypercube sampling scheme. The rms error in the retrievals is then assessed and presented in a two-dimensional diagram of ice water path and mass median diameter. It is shown that the simultaneous retrieval method using 150 and 220 GHz may be used for clouds with ice water path larger than 200 g m−2 and mass median diameter larger than 200 μm. To obtain meaningful retrievals for “thinner” clouds, higher microwave frequencies are needed. It is also shown that liquid water clouds that are at the same altitude as ice clouds interfere with the retrievals to a significant degree. To obtain reasonable ice water path and mass median size retrievals, it is necessary first to group clouds into several classes, then to apply separate algorithms to the different classes. The accuracy of the retrievals also depends on cloud type, with the best accuracy for cirrus and the worst for the midtop mixed-phase cloud among the clouds investigated in this study.

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Branko Kosović and Judith A. Curry

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Using the large eddy simulation (LES) technique, the authors study a clear-air, stably stratified atmospheric boundary layer (ABL) as it approaches a quasi-steady state. The Beaufort Sea Arctic Stratus Experiment (BASE) dataset is used to impose initial and boundary conditions. The authors explore the parameter space of the boundary layer by varying latitude, surface cooling rate, geostrophic wind, inversion strength, and surface roughness. Recognizing the critical dependence of the results of LES on the subgrid-scale (SGS) model, they test and use a nonlinear SGS model, which is capable of reproducing the effects of backscatter of turbulent kinetic energy (TKE) and of the SGS anisotropies characteristic for shear-driven flows. In order to conduct a long-term LES so that an ABL can reach a quasi-steady state, a parallel computer code is developed and simulations with a spatial domain of up to 963 grid points are performed.

The authors analyze the evolution of the mean wind, potential temperature, and turbulence profiles as well as the turbulence budgets. In their simulations, they observe the development of features that are characteristic of a stably stratified ABL: a two-layer ABL structure, an elevated inversion, and an associated inversion wind maxima. Good agreement is found between the LES results and the observations and with Nieuwstadt’s analytical model. The authors study the dependence of the boundary layer height on the flow parameters and determine model coefficients for a truncated Zilitinkevich–Mironov model.

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Vitaly I. Khvorostyanov and Judith A. Curry

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In order to understand the mechanisms of formation of broad size spectra of cloud droplets and to develop a basis for the parameterization of cloud microphysical and optical properties, the authors derive a general kinetic equation of stochastic condensation that is applicable for various relationships between the supersaturation relaxation time τ f and the timescale of turbulence τ L. Supersaturation is considered as a nonconservative variable, and thus additional covariances and a turbulent diffusion coefficient tensor that is dependent on the supersaturation relaxation time, k ij(τ f), are introduced into the kinetic equation. This equation can be used in cloud models with explicit microphysics or can serve as a basis for development of parameterizations for bulk cloud models and general circulation models.

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