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The contribution of heterogeneous ice nucleation to the formation of cirrus cloud ice crystals is still not well quantified. This results in large uncertainties when predicting cirrus radiative effects and their role in Earth's climate system. The goal of this case study is to simulate the composition, and thus activation conditions, of ice nucleating particles (INPs) to evaluate their contribution to heterogeneous cirrus ice formation in relation to homogeneous ice nucleation. For this, the regional model COSMO—Aerosols and Reactive Trace Gases (COSMO-ART) was used to simulate a synoptic cirrus cloud over Texas on 13 April 2011. The simulated INP composition was then compared to measured ice residual particle (IRP) composition from the actual event obtained during the NASA Midlatitude Airborne Cirrus Properties Experiment (MACPEX) aircraft campaign. These IRP measurements indicated that the dominance of heterogeneous ice nucleation was mainly driven by mineral dust with contributions from a variety of other particle types. Applying realistic activation thresholds and concentrations of airborne transported mineral dust and biomass-burning particles, the model implementing the heterogeneous ice nucleation parameterization scheme of Ullrich et al. is able to reproduce the overall dominating ice formation mechanism in contrast to the model simulation with the scheme of Phillips et al. However, the model showed flaws in reproducing the IRP composition. [ABSTRACT FROM AUTHOR]

A theoretical framework is presented that exposes the radiative-dynamical relationships that govern the subcritical destabilization of African easterly waves (AEWs) by Saharan mineral dust (SMD) aerosols. The framework is built on coupled equations for quasigeostrophic potential vorticity (PV), temperature, and SMD mixing ratio. A perturbation analysis yields, for a subcritical, but otherwise arbitrary, zonal-mean background state, analytical expressions for the growth rate and frequency of theAEWs. The expressions are functions of the domain-averaged wave activity,which is generated by the direct radiative effects of theSMD. Thewave activity is primarily modulated by the Doppler-shifted phase speed and the background gradients in PV and SMD. Using an idealized version of the Weather Research and Forecasting (WRF) Model coupled to an interactive dust model, a linear analysis shows that, for a subcritical African easterly jet (AEJ) and a background SMD distribution that are consistent with observations, the SMD destabilizes the AEWs and slows their westward propagation, in agreement with the theoretical prediction. The SMD-induced growth rates are commensurate with, and can sometimes exceed, those obtained in previous dust-free studies in which the AEWs grow on AEJs that are supercritical with respect to the threshold for barotropic-baroclinic instability. The clarity of the theoretical framework can serve as a tool for understanding and predicting the effects of SMD aerosols on the linear instability of AEWs in subcritical, zonal-mean AEJs. [ABSTRACT FROM AUTHOR]

A new scheme of water cloud optical properties is proposed for correlated k-distribution (CKD) models, in which the correlation in spectral distributions between the gaseous absorption coefficient and cloud optical properties is maintained. This is an extension of the CKD method from gas to cloud by dealing with the gas absorption coefficient and cloud optical properties in the same way. Compared to the results of line-by-line benchmark calculations, the band-mean cloud optical property scheme can overestimate cloud solar heating rate, with a relative error over 30%% in general. Also, the error in the flux at the top of the atmosphere can be up to 20 W m−2 at a solar zenith angle of 0°. However, the error is considerably reduced by applying the new proposed CKD cloud scheme. The physical explanation of the large error for the band-mean cloud scheme is the absence of a spectral correlation between the gaseous absorption coefficient and the cloud optical properties. The overestimation of the solar heating rate at the cloud-top layer could affect the moisture circulation and limit the growth of cloud. It is found that the error in the longwave cooling rate caused by the band-mean cloud scheme is very small. In the infrared, the local thermal emission strongly affects the spectral distribution of the radiative flux, which makes the correlation between the gaseous absorption coefficient and cloud optical properties very weak. Therefore, there is no obvious advantage in emphasizing the spectral correlation between gas and cloud. [ABSTRACT FROM AUTHOR]

Large-eddy simulation (LES) is used to explore the role of various processes in regulating the stratocumulus to cumulus transition (SCT). Simulations are based on a composite case derived from a Lagrangian analysis of 2 yr of data from the northeastern Pacific. The simulations reproduce well the observed transition from a compact stratocumulus layer to more broken fields of cumulus, simply as a response to increasing sea surface temperatures (SSTs) along the transition. In so doing they support earlier theoretical work that argued that the SCT was a response of boundary layer circulations to increased forcing by surface latent heat fluxes. Although the basic features of the SCT imposed by the increase in SST are robust, a variety of other factors affect the detailed character of the SCT. For example, enhanced precipitation or increased downwelling longwave radiative fluxes can accelerate the reduction in cloud cover that accompanies the SCT, while a gradual decrease in the large-scale divergence can make changes in cloud cover that accompany the SCT relatively more modest. The simulations also demonstrate that the pace of the SCT is mainly set by the strength of the temperature inversion capping the initial stratocumulus-topped boundary layer. [ABSTRACT FROM AUTHOR]

Investigates the sensitivity of model-produced cloud properties and radiative fluxes to the representation of cloud microphysics in the cloud radiation parameterizations. Model description; Point measurements versus areal averages; Model experiments and summary.

Part III. Presents information on a study which illustrated the role of cloud microphysical processes in the coupling among convection, radiative and surface processes and in establishing tropical atmosphere. Model on cloud resolving; Impact of cloud microphysics on convective available potential energy; Effect of cloud systems on radiative fluxes; Conclusions.

Provides information on a study of what controls stratocumulus radiative properties. Detailed information on the two groups hypotheses regarding the control of cloud radiative properties fall into; Information on the observation of boundary layer cloud evolution in a moving or Lagrangian frame of reference; Suggestions from the authors; Conclusions reached.