Istvan Geresdi

Freezing Drizzle Formation in Stably Stratified Layer Clouds: The Role of Radiative Cooling of Cloud Droplets, Cloud Condensation Nuclei, and Ice Initiation

Abstract This study evaluates the role of 1) low cloud condensation nuclei (CCN) conditions and 2) preferred radiative cooling of large cloud drops as compared to small cloud drops, on cloud droplet spectral broadening and subsequent freezing drizzle formation in stably stratified layer clouds. In addition, the sensitivity of freezing drizzle formation to ice initiation is evaluated. The evaluation is performed by simulating cloud formation over a two-dimensional idealized mountain using a detailed microphysical scheme implemented into the National Center for Atmospheric Research–Pennsylvania State University Mesoscale Model version 5. The height and width of the two-dimensional mountain were designed to produce an updraft pattern with extent and magnitude similar to documented freezing drizzle cases. The results of the model simulations were compared to observations and good agreement was found. The key results of this study are 1) low CCN concentrations lead to rapid formation of freezing drizzle. This ...

Effects of Aerosol Solubility and Regeneration on Warm-Phase Orographic Clouds and Precipitation Simulated by a Detailed Bin Microphysical Scheme

This study evaluates the possible impact of aerosol solubility and regeneration on warm-phase orographic clouds and precipitation. The sensitivity evaluation is performed by simulating cloud formation over two identical 2D idealized mountains using a detailed bin microphysical scheme implemented into the Weather Research and Forecasting model (WRF) version 3. The dynamics, thermodynamics, topography, and microphysical pathways were designed to produce precipitating clouds in a linear hydrostatic mountain wave regime.Thecloud over thesecond mountainis affectedby regeneratedaerosolsadvectedfrom thecloud over the first mountain. Effects of aerosol solubility and regeneration were investigated with surface relative humidity of 95% and 85% for both clean and polluted background aerosol concentrations. Amongthefindingsare thefollowing:1)The totalnumberof clouddrops decreasesas theaerosolsolubility decreases, and the impacts of aerosol solubility on cloud drops and precipitation are more significant in polluted clouds than in clean clouds. 2) Aerosol regeneration increases cloud drops and reduces the precipitation by 2%‐80% in clouds over the second mountain. Regenerated aerosol particles replenish one-third to two-thirds of the missing particles when regeneration is not considered. 3) Different size distributions of regenerated aerosol particles have negligible effect on clouds and precipitation except for polluted clouds with high aerosol solubility. 4) When the solubility of initial aerosol particles decreases with an increasing size of aerosol particles, the modified solubility of regenerated aerosol particles increases precipitation over the second mountain.

Effects of Aerosol Solubility and Regeneration on Mixed-Phase Orographic Clouds and Precipitation

AbstractA detailed bin aerosol-microphysics scheme has been implemented into the Weather Research and Forecast Model to investigate the effects of aerosol solubility and regeneration on mixed-phase orographic clouds and precipitation. Two-dimensional simulations of idealized moist flow over two identical bell-shaped mountains were carried out using different combinations of aerosol regeneration, solubility, loading, ice nucleation parameterizations, and humidity. The results showed the following. 1) Pollution and regenerated aerosols suppress the riming process in mixed-phase clouds by narrowing the drop spectrum. In general, the lower the aerosol solubility, the broader the drop spectrum and thus the higher the riming rate. When the solubility of initial aerosol increases with an increasing size of aerosol particles, the modified solubility of regenerated aerosols reduces precipitation. 2) The qualitative effects of aerosol solubility and regeneration on mixed-phase orographic clouds and precipitation ar...

Freezing Drizzle Formation in Stably Stratified Layer Clouds. Part II: The Role of Giant Nuclei and Aerosol Particle Size Distribution and Solubility

Abstract This paper investigates how the characteristics of aerosol particles (size distribution and solubility) as well as the presence of giant nuclei affect drizzle formation in stably stratified layer clouds. A new technique was developed to simulate the evolution of water drops from wet aerosol particles and implemented into a detailed microphysical model. The detailed microphysical model was incorporated into a one-dimensional parcel model and a two-dimensional version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). Sensitivity experiments were performed with the parcel model using a constant updraft speed and with the two-dimensional model by simulating flow over a bell-shaped mountain. The results showed that 1) stably stratified clouds with weak updrafts (<10 cm s−1) can form drizzle relatively rapidly for maritime size distributions with any aerosol particle solubility, and for continental size distributions with hi...

Severe storms and nowcasting in the Carpathian basin

Summer weather can cause severe situations in the Carpathian basin. Convective events such as thunderstorms (sometimes tornado-producing supercells) and squall lines occur frequently during the summer. In the first part of this paper, some typical convective storm events and their atmospheric conditions are presented. Most of the convective storms are associated with cold fronts but the most dangerous phenomena can be connected to waves on frontal systems or cutoff cyclones. To solve the problem of very short range forecasts of these phenomena, the Hungarian Meteorological Service (HMS) runs a project to develop a nowcasting system (the MEANDER system), which is described in the second part of the paper. This system supports a framework for several nowcasting procedures and produces complex objective analyses and forecasts as long as 3 h. The MEANDER system has two main segments: an analysis segment and a forecasting segment. Input and processed parameters are divided into basic parameters (such as pressure, temperature, geopotential, etc.) and derived parameters (such as visibility, present weather) which are calculated from the basic parameters. In the objective analysis, radar and satellite data are also involved. In the forecasting procedure, the objective analysis at the beginning time, and the (practically at 3 h forecast) fields of the background dynamical model at the final time are considered as nowcasted fields. Between these two times, basic parameters follow the tendency of the numerical model, but gradually approach the forecasted values of the background numerical model. To forecast radar parameters (precipitating systems), so-called replacement vectors are applied. These are calculated from the nowcasted basic parameters. The MEANDER system has both a special update segment to catch rapidly growing cloud systems and a warning segment to issue automated weather warnings as well.

Severe convective storms and associated phenomena in Hungary

Convective activity dominates the weather of Hungary in the summer. Especially during the first part of the summer, the frequency of severe thunderstorms grows and associated phenomena such as wind storms, hail, sometimes even tornadoes cause serious damage. In this paper, an overview of the severe thunderstorm situation in the Carpathian Basin is presented with a focus on the most frequent phenomena: squall lines. To understand the physical background of these kinds of severe thunderstorms, the terminology of convective components is introduced. Making use of case studies, the roles of convective components are shown for different types of thunderstorm systems. Case studies also show that most of the tornado events are associated with organized thunderstorm systems, especially prefrontal squall lines.

Evaluation of Orographic Cloud Seeding Using a Bin Microphysics Scheme: Two-Dimensional Approach

AbstractA new version of a bin microphysical scheme implemented into the Weather Research and Forecasting (WRF) Model was used to study the effect of glaciogenic seeding on precipitation formation in orographic clouds. The tracking of silver iodide (AgI) particles inside of water drops allows the proper simulation of the immersion nucleation. The ice formations by deposition, condensational freezing, and contact nucleation of AgI particles are also simulated in the scheme. Cloud formation—both stably stratified and convective—and the spread of AgI particles were simulated by idealized flow over a two-dimensional (2D) bell-shaped mountain. The results of numerical experiments show the following: (i) Only the airborne seeding enhances precipitation in stably stratified layer clouds. Seeding can reduce or enhance precipitation in convective clouds. AgI seeding can significantly affect the spatial distribution of the surface precipitation in orographic clouds. (ii) The positive seeding effect is primarily due...

Idealized simulations of a squall line from the MC3E field campaign applying three bin microphysics schemes: Dynamic and thermodynamic structure

AbstractThe squall-line event on 20 May 2011, during the Midlatitude Continental Convective Clouds (MC3E) field campaign has been simulated by three bin (spectral) microphysics schemes coupled into the Weather Research and Forecasting (WRF) Model. Semi-idealized three-dimensional simulations driven by temperature and moisture profiles acquired by a radiosonde released in the preconvection environment at 1200 UTC in Morris, Oklahoma, show that each scheme produced a squall line with features broadly consistent with the observed storm characteristics. However, substantial differences in the details of the simulated dynamic and thermodynamic structure are evident. These differences are attributed to different algorithms and numerical representations of microphysical processes, assumptions of the hydrometeor processes and properties, especially ice particle mass, density, and terminal velocity relationships with size, and the resulting interactions between the microphysics, cold pool, and dynamics. This study...