Mikhail Ovtchinnikov

Cloud Sky Cover versus Cloud Fraction: Whole-Sky Simulations and Observations

Abstract The relationship between hemispherical sky cover and nadir-view cloud fraction is examined by using both model simulations and surface observations. Monte Carlo simulations of ground-based hemispherical measurements are based on four-dimensional cloud fields produced by a large-eddy simulation model. Surface hemispherical observations are performed during the Atmospheric Radiation Measurement Program’s Cloudiness Intercomparison Intensive Operational Period. It is shown that (i) 15-min averages of frequently sampled (30 s) sky cover provide a reasonable estimation of the cloud fraction for limited fields of view and that (ii) this estimation can be substantially improved (for cumulus clouds) if additional information about the cloud aspect ratio is incorporated into the retrieval process.

Evaluation of the Multiscale Modeling Framework Using Data from the Atmospheric Radiation Measurement Program

In a recently developed approach to climate modeling, called the multiscale modeling framework (MMF), a two-dimensional cloud-resolving model (CRM) is embedded into each grid column of the Community Atmospheric Model (CAM), replacing traditional cloud and radiation parameterizations. This study presents an evaluation of the MMF through a comparison of its output with the output from the CAM and with data from two observational sites operated by the Atmospheric Radiation Measurement Program, one at the Southern Great Plains (SGP) in Oklahoma and one at the island of Nauru in the tropical western Pacific (TWP) region. Two sets of one-year-long simulations are considered: one using climatological sea surface temperatures (SSTs) and another using 1999 SST. Each set includes a run with the MMF as well as a CAM run with traditional or standard cloud and radiation treatments. Time series of cloud fraction, precipitation intensity, and downwelling solar radiation flux at the surface are analyzed. For the TWP site, the distributions of these variables from the MMF run are shown to be more consistent with observation than those from the CAM run. This change is attributed to the improved representation of convective clouds in the MMF compared to the conventional climate model. For the SGP, the MMF shows little to no improvement in predicting the same quantities. Possible causes of this lack of improvement are discussed.

Stochastic Radiative Transfer in Multilayer Broken Clouds. Part II: Validation Tests

In the second part of our two-part paper, we estimated the accuracy and robustness of the approximated equations for the mean radiance that were derived in Part I. In our analysis we used the three-dimensional (3D) cloud fields provided by (i) the stochastic Boolean model, (ii) large-eddy simulation model and (iii) satellite cloud retrieval. The accuracy of the obtained equations was evaluated by comparing the ensemble-averaged radiative properties that were obtained by the numerical averaging method (reference) and the analytical averaging method (approximation). The robustness of these equations was estimated by comparing the domain-averaged radiative properties obtained by using (i) the full 3D cloud structure (reference) and (ii) the bulk cloud statistics (approximation). It was shown that the approximated equations could provide reasonable accuracy (∼15%) for both the ensemble- and domain-averaged radiative properties.

Satellite multiangle cumulus geometry retrieval: Case study

Most satellite-based analyses have been conducted using near nadir-viewing sensors. The Multi-angle Imaging SpectroRadiometer (MISR), recently launched on the National Aeronautics and Space Administration (NASA) Terra platform, provides high-resolution measurements of reflectance at nine different viewing angles. In this study, we examine the possible retrieval of detailed cumulus geometry using the new and unique MISR datasets. We suggested one approach and apply it to an early MISR dataset of small marine cumulus clouds. This paper also presents validation analysis of this technique with both independent ground-based radar measurements and a model-output inverse problem. Collocated and coincident MISR data and ground-based observations at the Atmospheric Radiation Measurement (ARM) Tropical Western Pacific (TWP) site form the basis of this validation. Future work will attempt to test the suggested approach with additional MISR scenes.

Evaluation of radar retrieval algorithms in stratiform clouds using large‐eddy simulations

The performance of algorithms retrieving cloud liquid water content W from radar reflectivity factor Z is evaluated using data from a three-dimensional large-eddy simulation model with detailed size-resolving microphysics. On the basis of case studies of a marine stratocumulus cloud observed during the Atlantic Stratocumulus Transition Experiment in June of 1992 and a continental stratus cloud observed over north central Oklahoma on April 30, 1994, it is shown that retrieval algorithms based only on Z are very sensitive to cloud microphysics and do not reproduce adequately the horizontally averaged vertical profile of W. Additional independent measurements are needed to constrain the retrieved W profile. Surprisingly, algorithms show little improvement when the exact concentrations of cloud droplets obtained from the model are included in the calculations. Vertically integrated liquid water path P is found to be a robust constraint that ensures a more accurate retrieval. In the two studied cases an algorithm based on the Z-W relation of the form Z = a W b gives an optimal performance when a is inversely proportional to P and b = 1.32. The horizontal averaging of P over a computational domain of several kilometers does not affect the accuracy of the retrieval of the horizontally averaged profile of cloud liquid water content, which adds to the versatility of the algorithm.

A New Retrieval of Aerosol Optical Depth under Partly Cloudy Conditions with Multi-Spectral Measurements of Reflectance

Estimating the aerosol optical depth (AOD) in the vicinity of clouds is a long‐standing problem. A few recent studies have provided important demonstration of how three‐dimensional (3D) effects can increase substantially the clear sky reflectance and, therefore, lead to significant overestimation (up to 140%) of AOD. We first assess the importance of the 3D cloud effects on the reflectance ratios for pixels located far away from clouds and their shadows (clear pixels) and show that for clear pixels the reflectance ratios for two pairs of wavelengths (660, 470 nm and 870, 470 nm) are less sensitive to the 3D cloud effects than the reflectances themselves. We also propose an approach for converting reflectance ratios to the AOD for the clear pixels. Finally, we use Monte Carlo radiative transfer calculations and 3D fields of cumulus clouds and aerosol generated by a large eddy simulation (LES) model to illustrate the performance of the retrieval with the model‐output inverse problem. Results of the model‐in...