S. C. van den Heever

Representation of microphysical processes in cloud-resolving models: Spectral (bin) microphysics versus bulk parameterization

Most atmospheric motions of different spatial scales and precipitation are closely related to phase transitions in clouds. The continuously increasing resolution of large-scale and mesoscale atmospheric models makes it feasible to treat the evolution of individual clouds. The explicit treatment of clouds requires the simulation of cloud microphysics. Two main approaches describing cloud microphysical properties and processes have been developed in the past four and a half decades: bulk microphysics parameterization and spectral (bin) microphysics (SBM). The development and utilization of both represent an important step forward in cloud modeling. This study presents a detailed survey of the physical basis and the applications of both bulk microphysics parameterization and SBM. The results obtained from simulations of a wide range of atmospheric phenomena, from tropical cyclones through Arctic clouds using these two approaches are compared. Advantages and disadvantages, as well as lines of future development for these methods are discussed. (Less)

Observations of aerosol‐induced convective invigoration in the tropical east Atlantic

Four years of CloudSat data have been analyzed over a region of the east Atlantic Ocean in order to examine the influence of aerosols on deep convection. The satellite data were combined with information about aerosols taken from the Global and Regional Earth-System Monitoring Using Satellite and In Situ Data model. Only those profiles fitting the definition of deep convective clouds were analyzed. Overall, the cloud center of gravity, cloud top, and rain top were all found to increase with increased aerosol loading. These effects were largely independent of the environment, and the differences between the cleanest and most polluted clouds sampled were found to be statistically significant. When examining an even smaller subset of deep convective clouds likely to be part of the convective core, similar trends were seen. These observations suggest that convective invigoration occurs with increased aerosol loading, leading to deeper, stronger storms in polluted environments.

Trimodal cloudiness and tropical stable layers in simulations of radiative convective equilibrium

[1] In this paper, we examine the tropical environment at radiative convective equilibrium using a large-domain cloud system resolving numerical model. As in observed studies of convectively active periods over warm tropical oceans (in particular the tropical western Pacific), we find a trimodal cloud structure that is closely associated with the presence of three distinct stable layers, including a prominent stable layer located near the zero-degree Celsius level. In addition, the simulation exhibits three separate large scale zonal overturning circulations, with two of these circulations located above the trade wind inversion and separated by the freezing level stable layer. At equilibrium, latent heat release associated with freezing and melting processes is dwarfed by that of vapor transitions, and simulation results suggest that this stable layer can be maintained by subsidence in the presence of longwave radiative cooling above the zero-degree level. Citation: Posselt, D. J., S. C. van den Heever, and G. L. Stephens (2008), Trimodal cloudiness and tropical stable layers in simulations of radiative convective equilibrium, Geophys. Res. Lett., 35, L08802, doi:10.1029/ 2007GL033029.

Aerosol Effects on the Anvil Characteristics of Mesoscale Convective Systems

Simulations of two mesoscale convective systems (MCSs) that occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E) were performed to examine the impact of aerosol number concentration on the vertical distributions of liquid and ice condensate and the macrophysical, microphysical, and radiative properties of the cirrus-anvil cloud shield. Analyses indicate that for an increase in aerosol concentration from a clean continental to a highly polluted state there was an increase in the rime collection rate of cloud water, which led to less lofted cloud water. Aerosol-induced trends in the cloud mixing ratio profiles were, however, non-monotonic in the mixed phase region, such that a moderate increase in aerosol concentration produced the greatest reduction in cloud water. Generally, less lofted cloud water led to less anvil ice mixing ratio but more numerous, small ice crystals within the anvil. In spite of reduced anvil ice mixing ratio, the anvil clouds exhibited greater areal coverage, increased albedo, reduced cloud top cooling, and reduced net radiative flux, which led to an aerosol-induced warming (reduced cooling) effect in these squall lines.

The microphysical contributions to and evolution of latent heating profiles in two MC3E MCSs

The shapes and magnitudes of latent heating profiles have been shown to be different within the convective and stratiform regions of mesoscale convective systems (MCSs). Properly representing these distinctions has significant implications for the atmospheric responses to latent heating on various scales. This study details (1) the microphysical process contributions to latent heating profiles within MCS convective, stratiform, and anvil regions and (2) the time evolution of these profiles throughout the MCS lifetime, using cloud-resolving model simulations. Simulations of two MCS events that occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E) are conducted. Several features of the simulated MCSs are compared to a suite of observations obtained during the MC3E field campaign, and it is concluded that the simulations reasonably reproduce the MCS events. The simulations show that condensation and deposition are the primary contributors to MCS latent warming, as compared to riming and nucleation processes. In terms of MCS latent cooling, sublimation, melting, and evaporation all play significant roles. It is evident that throughout the MCS lifecycle, convective regions demonstrate an approximately linear decrease in the magnitudes of latent heating rates, while latent heating within stratiform regions is associated with transitions between MCS flow regimes. Such information regarding the temporal evolution of latent heating within convective and stratiform MCS regions could be useful in developing parameterizations representing convective organization.

Saharan dust, convective lofting, aerosol enhancement zones, and potential impacts on ice nucleation in the tropical upper troposphere

Dry aerosol size distributions and scattering coefficients were measured on 10 flights in 32 clear air regions adjacent to tropical storm anvils over the eastern Atlantic Ocean. Aerosol properties in these regions were compared with those from background air in the upper troposphere at least 40 km from clouds. Median values for aerosol scattering coefficient and particle number concentration >0.3 μm diameter were higher at the anvil edges than in background air, showing that convective clouds loft particles from the lower troposphere to the upper troposphere. These differences are statistically significant. The aerosol enhancement zones extended ~10-15 km horizontally and ~ 0.25 km vertically below anvil cloud edges, but were not due to hygroscopic growth since particles were measured under dry conditions. Number concentrations of particles >0.3 μm diameter were enhanced more for the cases where Saharan dust layers were identified below the clouds with airborne lidar. Median number concentrations in this size range increased from ~100 l-1 in background air to ~400 l-1 adjacent to cloud edges with dust below, with larger enhancements for stronger storm systems. Integration with satellite cloud frequency data indicates that this transfer of large particles from low to high altitudes by convection has little impact on dust concentrations within the Saharan Air Layer itself. However, it can lead to substantial enhancement in large dust particles and, therefore, heterogeneous ice nuclei in the upper troposphere over the Atlantic. This may induce a cloud/aerosol feedback effect that could impact cloud properties in the region and downwind.