The Microphysics of the Warm-rain and Ice Crystal Processes of Precipitation in Simulated Continental Convective Storms (original) (raw)
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Atmospheric Research, 2014
An improved approach for cloud droplet activation process parameterization is proposed that can utilize the empirically determined hygroscopicity information and practically limit the sizes of newly activated droplets. With the implementation of the improved approach in a cloud model, the aerosol effects on ice microphysics in convective cloud and precipitation development under different thermodynamic conditions is investigated. The model is run for four different thermodynamic soundings and three different aerosol types, maritime (M), continental (C) and polluted (P). Warm rain suppression by increased aerosol (i.e., CCN) is clearly demonstrated when weakly convective warm clouds are generated but the results are mixed when relatively stronger convective warm clouds are generated. For one of the two soundings that generate strong convective cold clouds, the accumulated precipitation amount is larger for C and P than for M, demonstrating the precipitation enhancement by increased CCN. For the maritime cloud, precipitation is initiated by the warm rain processes but ice hydrometeor particles form fast, which leads to early but weak cloud invigoration. Another stronger cloud invigoration occurs later for M but it is still weaker than that for C and P. It is the delayed accumulation of more water drops and ice particles for a burst of riming process and the latent heat release during the depositional growth of rimed ice particles that invigorate the cloud strongly for C and P. For the other sounding where freezing level is low, ice particles form fast for all three aerosol types and therefore warm rain suppression is not clearly shown. However, there still is more precipitation for C and P than for M until the accumulated precipitation amount becomes larger for M than for C near to the end of the model run. The results demonstrate that the precipitation response to aerosols indeed depends on the environmental conditions.
Monthly Weather Review, 2004
A revised approach to cloud microphysical processes in a commonly used bulk microphysics parameterization and the importance of correctly representing properties of cloud ice are discussed. Several modifications are introduced to more realistically simulate some of the ice microphysical processes. In addition to the assumption that ice nuclei number concentration is a function of temperature, a new and separate assumption is developed in which ice crystal number concentration is a function of ice amount. Related changes in ice microphysics are introduced, and the impact of sedimentation of ice crystals is also investigated.
Effect of aerosols on freezing drops, hail and precipitation in a mid-latitude storm
Journal of the Atmospheric Sciences, 2015
A midlatitude hail storm was simulated using a new version of the spectral bin microphysics Hebrew University Cloud Model (HUCM) with a detailed description of time-dependent melting and freezing. In addition to size distributions of drops, plate-, columnar-, and branch-type ice crystals, snow, graupel, and hail, new distributions for freezing drops as well as for liquid water mass within precipitating ice particles were implemented to describe time-dependent freezing and wet growth of hail, graupel, and freezing drops. Simulations carried out using different aerosol loadings show that an increase in aerosol loading leads to a decrease in the total mass of hail but also to a substantial increase in the maximum size of hailstones. Cumulative rain strongly increases with an increase in aerosol concentration from 100 to about 1000 cm−3. At higher cloud condensation nuclei (CCN) concentrations, the sensitivity of hailstones’ size and surface precipitation to aerosols decreases. The phys...
Is the dependence of warm and ice precipitation on the aerosol concentration monotonic
2008
Effects of aerosols on microphysics and precipitation of deep convective clouds was simulated in many numerical studies. Most of them were related to clouds with warm cloud base and high (~4 km) freezing level. Only a few studies investigated aerosol effects on precipitation from convective clouds developing in the atmosphere with a comparatively low (~2-3 km) freezing level. The difference between these cases is quite significant: while clouds with high freezing level produce warm rain at low aerosol concentration, and melted (cold) precipitation at high aerosol concentration, in case of low freezing level precipitation is formed by melted ice at any aerosol concentrations. Effects of aerosols on ice microphysics and especially on hail formation are not well known. In this presentation we perform preliminary results of simulations of deep convective clouds under different thermodynamic and aerosol conditions. The main focus of the study is investigation of aerosol effects on hail formation process, hail content and size. The investigation is performed using a modified version of the Hebrew University Cloud model (HUCM) with spectral (bin) microphysics.
Journal of the Atmospheric Sciences, 2007
Simulations of one maritime and four continental observed cases of deep convection are performed with the Hebrew University Cloud Model that has spectral bin microphysics. The maritime case is from observations made on 18 September 1974 during the Global Atmospheric Research Program’s Atlantic Tropical Experiment (GATE). The continental storm cases are those of summertime Texas clouds observed on 13 August 1999, and green-ocean, smoky, and pyro-clouds observed during the Large-Scale Biosphere–Atmosphere Experiment in Amazonia–Smoke, Aerosols, Clouds, Rainfall, and Climate (LBA–SMOCC) campaign on 1–4 October 2002. Simulations have been performed for these cases with a detailed melting scheme. This scheme allows calculation of liquid water fraction within each mass bin for the melting of graupel, hail, snowflakes, and crystals, as well as alteration of the sedimentation velocity of ice particles in the course of their melting. The results obtained with the detailed melting scheme are ...
A wintertime case study on the impact of ice particle habits on simulated clouds and precipitation
Atmospheric Research, 2006
Ice particles are considered in prevalent cloud microphysical parametrisation schemes in a simplified way by assigning them certain fixed forms, whereas in the atmosphere they occur in various different shapes. In this study, the sensitivity of cloud and precipitation properties to the selected interpretation of the ice particle shape is investigated with the help of simulations of a wintertime weather episode using the mesoscale weather forecast model 'Lokal-Modell' of the DWD. The results show that the interpretation of the particular ice particle type has a clear impact on the local surface precipitation rates while the area mean surface precipitation is only slightly affected. Most striking, however, is the influence on the simulated masses of cloud water in the atmosphere. To avoid the prescription of a fixed ice particle shape, a parametrisation variant is presented to account for the variation of ice particles' shape according to their growth history.
Aerosol Impact on Precipitation from Convective Clouds
Measuring Precipitation From Space, 2007
Mechanisms through which atmospheric aerosols affect cloud microphysics, dynamics and precipitation are investigated using a spectral microphysics two-dimensional cloud model. A significant effect of aerosols on cloud microphysics and dynamics has been found. Maritime aerosols lead to a rapid formation of raindrops that fall down through cloud updraughts increasing the loading in the lower part of a cloud. This is, supposedly, one of the reasons for comparatively low updraughts in maritime convective clouds. An increase in the concentration of small cloud condensation nuclei (CCN) leads to the formation of a large number of small droplets with a low collision rate, resulting in a time delay of raindrop formation. Such a delay prevents a decrease in the vertical velocity caused by the falling raindrops and thus increases the duration of the diffusion droplet growth stage, increasing latent heat release by condensation. The additional water that rises to the freezing level increases latent heat release by freezing. As a result, clouds developing in continental-type aerosol tend to have larger vertical velocities and to attain higher levels. The results show that a decrease in precipitation efficiency of single cumulus clouds arising in microphysically continental air is attributable to a greater loss of the precipitating mass due to a greater sublimation of ice and evaporation of drops while they are falling from higher levels through a deep layer of dry air outside cloud updraughts. By affecting precipitation, atmospheric aerosols influence the net heating of the atmosphere. Simulations show that aerosols also change the vertical distribution of latent heat release, increasing the level of the heating peak. Clouds arising under continental aerosol conditions produce as a rule stronger downdraughts and stronger convergence in the boundary layer. Being triggered by larger dynamical forcing, secondary clouds arising in microphysically continental air are stronger and can, according to the results of simulations, form a squall line. The squall line formation was simulated both under maritime (GATE-74) and continental (PRE-STORM) thermodynamic conditions. In the maritime aerosol cases, clouds developing under similar thermodynamic conditions do not produce strong downdraughts and do not lead to squall line formation. Thus, the 'aerosol effect' on precipitation can be understood only in combination with the 'dynamical effect' of aerosols. Simulations allow us to suggest that aerosols, which decrease the precipitation efficiency of most single clouds, can contribute to the formation of very intensive convective clouds and thunderstorms (e.g. squall lines, etc.) accompanied by very high precipitation rates. Affecting precipitation, net atmospheric heating and its vertical distribution, as well as cloud depth and cloud coverage, atmospheric aerosols (including anthropogenic ones) influence atmospheric motions and radiation balance at different scales, from convective to, possibly, global ones.
ATMOSPHERIC CHEMISTRY AND PHYSICS
This study focuses on the effects of aerosol parti-cles on the formation of convective clouds and precipitation in the Eastern Mediterranean Sea, with a special emphasis on the role of mineral dust particles in these processes. We used a new detailed numerical cloud microphysics scheme that has been implemented in the Weather Research and Forecast (WRF) model in order to study aerosol–cloud interaction in 3-D configuration based on 1 • × 1 • resolution reanalysis me-teorological data. Using a number of sensitivity studies, we tested the contribution of mineral dust particles and differ-ent ice nucleation parameterizations to precipitation devel-opment. In this study we also investigated the importance of recycled (regenerated) aerosols that had been released to the atmosphere following the evaporation of cloud droplets. The results showed that increased aerosol concentration due to the presence of mineral dust enhanced the formation of ice crystals. The dynamic evolution of the clou...
In the present study, the Weather Research and Forecasting (WRF) model was used to simulate the features associated with a severe thunderstorm over India while examining the sensitivity of the simulation to three microphysical (MP) schemes (WDM6, Thompson and Morrison). The model simulated results (e.g., surface temperature, relative humidity, pressure, reflectivity and rainfall) for all sensitivity experiments are compared with observations (e.g., AWS, TRMM and DWR). There are major differences in the simulations of the thunderstorm among the MP schemes. The Morrison scheme simulates CAPE, surface properties, wind speed, vertical velocity, reflectivity and precipitation reasonably well, compared to other MP schemes, though there are some uncertainties. Therefore, an attempt is made to improve the simulation through modifications in the Morrison scheme. Different heterogeneous ice nucleation formulations have been tested into the Morrison double-moment bulk cloud MP scheme. We hypothesize that the improvement in cloud ice generation and its subsequent influence in cloud microphysics and dynamics through latent heat release may eventually lead to an improvement in thunderstorm simulation. The results demonstrate that the modification in the microphysical scheme better reproduces CAPE, wind speed, maximum reflectivity, vertical velocity and cloud hydrometeors (ice and mixed-phase processes) than the default Morrison and other schemes and compared to observations. The modified MP-scheme produces greater latent heating due to deposition in the upper troposphere and gives rise to increased updraft. This seems to be one of the most responsible processes that enhance the intensity of the storm compared to existing microphysical schemes. This study therefore provides a framework for the improvement of thunderstorm simulation through the modification of the cloud ice parameterization of the model.