Yefim L. Kogan

A New Cloud Physics Parameterization in a Large-Eddy Simulation Model of Marine Stratocumulus

Abstract A new bulk microphysical parameterization for large-eddy simulation (LES) models of the stratocumulus-topped boundary layer has been developed using an explicit (drop spectrum resolving) microphysical model as a data source and benchmark for comparison. The liquid water is divided into two categories, nonprecipitable cloud water and drizzle, similar to traditional Kessler-type parameterizations. The cloud condensation nucleus (CCN) count, cloud/drizzle water mixing ratios, cloud/drizzle drop concentrations, and the cloud drop integral radius are predicted in the new scheme. The source/sink terms such as autoconversion/accretion of cloud water into/by drizzle are regressed using the cloud drop size spectra predicted by an explicit microphysical model. The results from the explicit and the new bulk microphysics schemes are compared for two cases: nondrizzling and heavily drizzling stratocumulus-topped boundary layers (STBLs). The evolution of the STBL (characterized by such parameters as turbulence...

The Simulation of a Convective Cloud in a 3-D Model With Explicit Microphysics. Part I: Model Description and Sensitivity Experiments

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Controls on precipitation and cloudiness in simulations of trade-wind cumulus as observed during RICO

Twelve large-eddy simulations, with a wide range of microphysical representations, are compared to each other and to independent measurements. The measurements and the initial and forcing data for the simulations are taken from the undisturbed period of the Rain in Cumulus over the Ocean (RICO) field study. A regional downscaling of meteorological analyses is performed so as to provide forcing data consistent with the measurements. The ensemble average of the simulations plausibly reproduces many features of the observed clouds, including the vertical structure of cloud fraction, profiles of cloud and rain water, and to a lesser degree the population density of rain drops. The simulations do show considerable departures from one another in the representation of the cloud microphysical structure and the ensuant surface precipitation rates, increasingly so for the more simplified microphysical models. There is a robust tendency for simulations that develop rain to produce a shallower, somewhat more stable cloud layer. Relations between cloud cover and precipitation are ambiguous.

Drizzle Suppression in Ship Tracks

Abstract Although drizzle was a relatively infrequent occurrence during the Monterey Area Ship Track study, diverse measurements from several sources produced data signals consistent with a reduction in drizzle drops in stratus clouds affected by ship effluents. Concurrent increases in liquid water in the cloud droplet size range, due to redistribution from the drizzle mode, were not always observed, possibly because of the relatively small and often negligible amounts of water in the drizzle mode. Significant changes in cloud droplet size distribution, as well as reductions in drizzle flux and concentrations of drops >50-μm radius, were observed in ship tracks when drizzle was more uniformly present in the ambient cloud. Radiometric measurements showed that increased droplet concentrations in ship tracks, which resulted in reduced droplet sizes, can significantly alter the liquid water path. Radar observations indicated that the reduced reflectivities of ship tracks compared with ambient clouds may be du...

A Large Eddy Simulation Model with Explicit Microphysics: Validation against Aircraft Observations of a Stratocumulus-Topped Boundary Layer

Abstract A new dynamical framework for the Cooperative Institute for Mesoscale Meteorological Studies large eddy simulation model (CIMMS LES) with an explicit microphysics scheme is developed. It is shown that simulation results are very sensitive to the drop spectrum remapping technique used in condensation calculations; however, the results are almost insensitive to doubling of the spectrum resolution used in the CIMMS LES model. It is also shown that the drop coagulation procedure conserves the liquid water content as long as the predominant radius of the drop size spectrum, defined as the cube root of the ratio of the drop radar reflectivity to the liquid water content, is below a threshold value of 250 μm. Finally, it is demonstrated that for typical maritime conditions this threshold radius is exceeded only in 0.1% of all cloudy points. Realism of the model is evaluated by a direct comparison of its predictions with the aircraft observations of a stratocumulus-topped boundary layer. The first simula...

An Investigation of Ice Production Mechanisms in Small Cumuliform Clouds Using a 3D Model with Explicit Microphysics. Part I: Model Description

Abstract A new cloud model that combines a three-dimensional nonhydrostatic dynamical framework with explicit liquid- and ice-phase microphysics and a detailed treatment of ice nucleation and multiplication processes is presented. The use of 28 size bins to describe each cloud drop and ice particle spectra allows the authors to account for all major microphysical processes. A detailed scavenging model has been implemented to calculate the collection rate of contact ice nuclei by cloud drops. In addition to contact nucleation, the model accounts for deposition, condensation-freezing, and immersion ice nucleation mechanisms, as well as for secondary ice production via rime splintering. The model performance is illustrated by a simulation of a New Mexican cumulus cloud. Combination of high 100-m spatial resolution with a new initialization procedure that promotes development of small eddies results in a cloud with a more realistic distribution of liquid water content compared to simulations initialized by th...

An Investigation of Ice Production Mechanisms in Small Cumuliform Clouds Using a 3D Model with Explicit Microphysics. Part II: Case Study of New Mexico Cumulus Clouds

Abstract A new 3D model with explicit liquid- and ice-phase microphysics and a detailed treatment of ice nucleation and multiplication processes is applied to study ice formation and evolution in cumulus clouds. Simulation results are compared with in situ observations collected by the National Center for Atmospheric Research King Air aircraft in a cloud over the Magdalena Mountains in New Mexico on 9 August 1987. The model reproduces well the observed cloud in terms of cloud geometry, liquid water content, and concentrations of cloud drops and ice particles (IP). Primary ice nucleation is shown to produce IP in concentrations on the order of 103 m−3 (1 L−1) once the cloud top reaches −10° to −12°C. At mature and early dissipating stages of cloud development, ice production is dominated by the rime-splintering (Hallett–Mossop) mechanism, which in some regions generates up to 5 × 104 m−3 (50 L−1) IP in about 10 min. The predicted maximum of IP concentration is in agreement with observations. The sampling t...

A Cumulus Cloud Microphysics Parameterization for Cloud-Resolving Models

AbstractA microphysical parameterization for shallow cumulus and boundary layer stratocumulus clouds has been developed. Similar to the Khairoutdinov and Kogan parameterization for stratocumulus clouds, the new parameterization is based on an explicit microphysical large-eddy simulation (LES) model as a data source and benchmark for comparison. The predictions of the bulk model using the new parameterization were tested in simulations of shallow cumulus and boundary layer stratocumulus clouds; in both cases the new parameterization matched the predictions of the explicit microphysics LES quite accurately. These results show the importance of the choice of the dataset in parameterization development and the need for it to be balanced by realistic dynamic conditions. The strong sensitivity to representation of rain evaporation is also demonstrated. Accurate formulation of this process, tuned for the case of cumulus convection, has substantially improved precision of rain production.

The effect of CCN regeneration on the evolution of stratocumulus cloud layers

Abstract The paper presents the results from a numerical simulation of a stratocumulus cloud-topped boundary layer (CTBL) based on a newly developed model that includes 3-D LES dynamical framework and explicit formulation of cloud microphysics. The presented results reveal such interesting features of CTBL evolution as decoupling of the stratiform cloud layer and formation of bimodal spectra due to the processes of droplet evaporation and reactivation. The effect of CCN regeneration on cloud properties has been studied in a set of four experiments. It is shown that the process of CCN regeneration may be quite important in formation and evolution of cloud microphysical and radiative properties, while cloud dynamical structure appears to be less affected. We also conclude that the parameters of stratocumulus cloud are rather insensitive to the exact form of CCN spectrum regenerated after droplet evaporation. The conclusion may be of significance in prioritizing the processes for explicit cloud modeling.

Simulating the Transition from Drizzling Marine Stratocumulus to Boundary Layer Cumulus with a Mesoscale Model

A case of coastal California summer season boundary layer cloud has been simulated with the U.S. Navy Coupled Ocean‐Atmosphere Mesoscale Prediction System and the results analyzed in the context of consistency with conclusions derived from large eddy simulation‐based (LES) studies. Results show a pronounced diurnal cycle and fair agreement with satellite-derived observations of liquid water path. When drizzle processes are included, a significant degree of mesoscale organization emerges in the form of cloud bands, accompanied by a transition from a well-mixed boundary layer topped by unbroken stratocumulus cloud into a more potentially unstable, convective boundary layer regime. The transition and the subsequent development of mesoscale variability is analogous to the drizzle-induced cloud breakup produced in large eddy simulation studies. The dynamics of the pure stratocumulus cloud are dictated by the model’s subgrid parameterization, while the more convective regime exhibits appreciable vertical velocities characteristic of an ensemble of cumulus updrafts. The existence of convective updrafts is tied to a weak drizzle-induced decoupling of the cloud and subcloud layer, after which air of higher equivalent potential temperature (ue) can pool at the surface. Some similarities to the propagation of deep convection are also noted.