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Agnieszka A. Mrowiec, O. M. Pauluis, A. M. Fridlind, and A. S. Ackerman

Abstract

Application of an isentropic analysis of convective motions to a simulated mesoscale convective system is presented. The approach discriminates the vertical mass transport in terms of equivalent potential temperature. The scheme separates rising air at high entropy from subsiding air at low entropy. This also filters out oscillatory motions associated with gravity waves and isolates the overturning motions associated with convection and mesoscale circulation. The mesoscale convective system is additionally partitioned into stratiform and convective regions based on the radar reflectivity field. For each of the subregions, the mass transport derived in terms of height and an isentropic invariant of the flow is analyzed. The difference between the Eulerian mass flux and the isentropic counterpart is a significant and symmetric contribution of the buoyant oscillations to the upward and downward mass fluxes. Filtering out these oscillations results in substantial reduction of the diagnosed downward-to-upward convective mass flux ratio. The analysis is also applied to graupel and snow mixing ratios and number concentrations, illustrating the relationship of the particle formation process to the updrafts.

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Y. Cheng, V. M. Canuto, A. M. Howard, A. S. Ackerman, M. Kelley, A. M. Fridlind, G. A. Schmidt, M. S. Yao, A. Del Genio, and G. S. Elsaesser

Abstract

We formulate a new second-order closure turbulence model by employing a recent closure for the pressure–temperature correlation at the equation level. As a result, we obtain new heat flux equations that avoid the long-standing issue of a finite critical Richardson number. The new, structurally simpler model improves on the Mellor–Yamada and Galperin et al. models; a key feature includes enhanced mixing under stable conditions facilitating agreement with observational, experimental, and high-resolution numerical datasets. The model predicts a planetary boundary layer height deeper than predicted by models with low critical Richardson numbers, as demonstrated in single-column model runs of the GISS ModelE general circulation model.

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J. Rémillard, A. M. Fridlind, A. S. Ackerman, G. Tselioudis, P. Kollias, D. B. Mechem, H. E. Chandler, E. Luke, R. Wood, M. K. Witte, P. Y. Chuang, and J. K. Ayers

Abstract

A case study of persistent stratocumulus over the Azores is simulated using two independent large-eddy simulation (LES) models with bin microphysics, and forward-simulated cloud radar Doppler moments and spectra are compared with observations. Neither model is able to reproduce the monotonic increase of downward mean Doppler velocity with increasing reflectivity that is observed under a variety of conditions, but for differing reasons. To a varying degree, both models also exhibit a tendency to produce too many of the largest droplets, leading to excessive skewness in Doppler velocity distributions, especially below cloud base. Excessive skewness appears to be associated with an insufficiently sharp reduction in droplet number concentration at diameters larger than ~200 μm, where a pronounced shoulder is found for in situ observations and a sharp reduction in reflectivity size distribution is associated with relatively narrow observed Doppler spectra. Effectively using LES with bin microphysics to study drizzle formation and evolution in cloud Doppler radar data evidently requires reducing numerical diffusivity in the treatment of the stochastic collection equation; if that is accomplished sufficiently to reproduce typical spectra, progress toward understanding drizzle processes is likely.

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Peter J. Marinescu, Susan C. van den Heever, Max Heikenfeld, Andrew I. Barrett, Christian Barthlott, Corinna Hoose, Jiwen Fan, Ann M. Fridlind, Toshi Matsui, Annette K. Miltenberger, Philip Stier, Benoit Vie, Bethan A. White, and Yuwei Zhang

Abstract

This study presents results from a model intercomparison project, focusing on the range of responses in deep convective cloud updrafts to varying cloud condensation nuclei (CCN) concentrations among seven state-of-the-art cloud-resolving models. Simulations of scattered convective clouds near Houston, Texas, are conducted, after being initialized with both relatively low and high CCN concentrations. Deep convective updrafts are identified, and trends in the updraft intensity and frequency are assessed. The factors contributing to the vertical velocity tendencies are examined to identify the physical processes associated with the CCN-induced updraft changes. The models show several consistent trends. In general, the changes between the High-CCN and Low-CCN simulations in updraft magnitudes throughout the depth of the troposphere are within 15% for all of the models. All models produce stronger (~+5%–15%) mean updrafts from ~4–7 km above ground level (AGL) in the High-CCN simulations, followed by a waning response up to ~8 km AGL in most of the models. Thermal buoyancy was more sensitive than condensate loading to varying CCN concentrations in most of the models and more impactful in the mean updraft responses. However, there are also differences between the models. The change in the amount of deep convective updrafts varies significantly. Furthermore, approximately half the models demonstrate neutral-to-weaker (~−5% to 0%) updrafts above ~8 km AGL, while the other models show stronger (~+10%) updrafts in the High-CCN simulations. The combination of the CCN-induced impacts on the buoyancy and vertical perturbation pressure gradient terms better explains these middle- and upper-tropospheric updraft trends than the buoyancy terms alone.

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Robert Wood, Michael P. Jensen, Jian Wang, Christopher S. Bretherton, Susannah M. Burrows, Anthony D. Del Genio, Ann M. Fridlind, Steven J. Ghan, Virendra P. Ghate, Pavlos Kollias, Steven K. Krueger, Robert L. McGraw, Mark A. Miller, David Painemal, Lynn M. Russell, Sandra E. Yuter, and Paquita Zuidema
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