Publications Published in Journal of the Atmospheric Sciences

1. Can Anthropogenic Aerosols Decrease the Snowfall Rate?. Lohmann, U..
   Journal of the Atmospheric Sciences: 2004
   Notes
   Observations by Borys, Lowenthal, Cohn, and Brown in midlatitude orographic clouds show that for a given supercooled liquid water content, both the riming and the snowfall rates are smaller if the supercooled cloud has more cloud droplets as, for example, caused by anthropogenic aerosols. The climatic implication of this effect was studied in global climate model simulations by replacing the constant riming efficiency with a size-dependent one appropriate for planar crystals and aggregates, respectively. In the model simulations that use a size-dependent riming collection efficiency, the pollution-induced decrease in cloud droplet size causes a decrease in the riming rate in stratiform clouds despite larger liquid water contents in polluted clouds. Contrary to the above-mentioned observations, in all model simulations the snowfall rate increases because of feedbacks in the climate system. Anthropogenic aerosol particles increase the aerosol and cloud optical thickness, which reduces the solar radiation at the top of the atmosphere and the surface. This in turn causes a cooling in Northern Hemisphere midlatitudes that favors precipitation formation via the ice phase.
   
   
   
2. Deep Orographic Storms over the Sierra Nevada. Part II: The Precipitation Processes. Marwitz, John D..
   Journal of the Atmospheric Sciences: 1987
   Notes
   The thermodynamic and kinematic structure of two stable orographic storms were described in Part I based on instrumented aircraft data and single Doppler radar data. The precipitation processes in these storms are described in this paper. The storms were deep with cloud top temperatures of about -25 degrees celcius. Below the melting level the cloud droplet poplulation was continental with a mean droplet diamter < 10micrometers. Above the melting level the cloud droplet population was maritime with mean droplet diameters of 20 to 30 micrometers. Near the -5 degrees celsius level a peak in ice crystal concentration of 30 to 200 L^-1 was observed. Since most of the ice crystals were needles, a rime-splintering secondary ice crystal producation process as generally described by Hallett and Mossop was probably occuring. Calculations of the condesnation supply rates were compared with the depletion rates by deposition and accretion. The depletion rates by deposition were less than half the condensation supply rates, and the liquid water contents remained very low. Accretion is deduced to be the dominant process, which acts to deplete the condensate to near zero. Deep, stabel orographic storms over the Sierra barier, therefore, develop an efficient glaciation process.
   
   
   
3. Factors determining the impact of aerosols on surface precipitation from clouds: an attempt at classification. Khain, A.P.; BenMoshe, N.; Pokrovsky, A..
   Journal of the Atmospheric Sciences: 2008
   DOI: 10.1175/2007JAS2515.1
   Notes
   The simulation of the dynamics and the microphysics of clouds observed during the Large-Scale Biosphere– Atmosphere Experiment in Amazonia—Smoke, Aerosols, Clouds, Rainfall, and Climate (LBA– SMOCC) campaign, as well as extremely continental and extremely maritime clouds, is performed using an updated version of the Hebrew University spectral microphysics cloud model (HUCM). A new scheme of diffusional growth allows the reproduction of in situ–measured droplet size distributions including those formed in extremely polluted air. It was shown that pyroclouds forming over the forest fires can precipitate. Several mechanisms leading to formation of precipitation from pyroclouds are considered. The mechanisms by which aerosols affect the microphysics and precipitation of warm cloud-base clouds have been investigated by analyzing the mass, heat, and moisture budgets. The increase in aerosol concentration increases both the generation and the loss of the condensate mass. In the clouds developing in dry air, the increase in the loss is dominant, which suggests a decrease in the accumulated precipitation with the aerosol concentration increase. On the contrary, an increase in aerosol concentration in deep maritime clouds leads to an increase in precipitation. The precipitation efficiency of clouds in polluted air is found to be several times lower than that of clouds forming in clean air. A classification of the results of aerosol effects on precipitation from clouds of different types developing in the atmosphere with high freezing level (about 4 km) is proposed. The role of air humidity and other factors in precipitation’s response to aerosols is discussed. The analysis shows that many discrepancies b ween the results reported in different observational and numerical studies can be attributed to the different atmospheric conditions and cloud types analyzed.
   
   
   
4. Sensitivity studies of aerosol-cloud interactions in mixed-phase orographic precipitation. Andreas Muhlbauer, Ulrike Lohmann.
   Journal of the Atmospheric Sciences: 2009
   DOI: 10.1175/2009JAS3001.1
   Notes
   Anthropogenic aerosols serve as a source of cloud condensation nuclei (CCN) as well as ice nuclei (IN) and affect microphysical properties of clouds. Increasing aerosol number concentrations is assumed to retard the cloud droplet coalescence and the riming process in mixed-phase orographic clouds thereby decreasing orographic precipitation. In this study, idealized 3D simulations are conducted to investigate aerosol-cloud interactions in mixed-phase orographic clouds and the possible impact of anthropogenic and natural aerosols on orographic precipitation. Two different types of aerosol anomalies are considered which are naturally occuring mineral dust and anthropogenic black carbon. In the simulations with a dust aerosol anomaly the dust aerosols serve as efficient ice nuclei in the contact mode leading to an early initiation of the ice-phase in the orographic cloud. As a consequence, the riming rates in the cloud are increased leading to an increased precipitation efficiency and to an enhancement of orographic precipitation. The simulations with an anthropogenic aerosol anomaly suggest that the mixing state of the aerosols plays a crucial role since coating and mixing may cause the aerosols to initiate freezing in the less efficient immersion mode than by contact nucleation. It is found that externally mixed black carbon aeroso s increase riming in orographic clouds and enhance orographic precipitation. In contrast, internally mixed black carbon aerosols decrease the riming rates which in turn leads to a decrease in the orographic precipitation.
   
   
   
5. Sensitivity studies of the role of aerosols in warm-phase orographic precipitation in different dynamical flow regimes. Muhlbauer, Andreas; Lohmann, Ulrike.
   Journal of the Atmospheric Sciences: 2008
   DOI: 10.1175/2007JAS2492.1
   Notes
   Aerosols serve as a source of cloud condensation nuclei (CCN) and influence the microphysical properties of clouds. In the case of orographic clouds, it is suspected that aerosol–cloud interactions reduce the amount of precipitation on the upslope side of the mountain and enhance the precipitation on the downslope side when the number of aerosols is increased. The net effect may lead to a shift of the precipitation distribution toward the leeward side of mountain ranges, which affects the hydrological cycle on the local scale. In this study aerosol–cloud interactions in warm-phase clouds and the possible impact on the orographic precipitation distribution are investigated. Herein, simulations of moist orographic flow over topography are conducted and the influence of anthropogenic aerosols on the orographic precipitation formation is analyzed. The degree of aerosol pollution is prescribed by different aerosol spectra that are representative for central Switzerland. The simulations are performed with the Consortium for Small-Scale Modeling’s mesoscale nonhydrostatic limited-area weather prediction model (COSMO) with a horizontal grid spacing of 2 km and a fully coupled aerosol–cloud parameterization. It is found that an increase in the aerosol load leads to a downstream shift of the orographic precipitation distribution and to an increase in the spillover factor. A reduction of warm-phase orographic precipitation is observed at the upslope side of the mountain. The downslope precipitation enhancement depends critically on the width of the mountain and on the flow dynamics. In the case of orographic precipitation induced by stably stratified unblocked flow, the loss in upslope precipitation is not compensated by leeward precipitation enhancement. In contrast, flow blocking may lead to leeward precipitation enhancement and eventually to a compensation of the upslope precipitation loss. The simulations also indicate that latent heat effects induced by aerosol–cloud–precipitation interactions may considerably affect the orographic flow dynamics and consequently feed back on the orographic precipitation development.