Examination of physical processes of convective cell evolved from a MCS — Using a different model initialization

The present study is focused on examination of the physical processes of convective cell evolved from a MCS occurred on 4 November 2011 over Genoa, Italy. The Quantitative Precipitation Forecasts (QPF) have been performed using WRF v3.6 model under different configurations and cloud permitting simulations. The results indicate underestimation of the amount of precipitation and spatial displacement of the area with a peak 24-h accumulated rainfall in (mm). Our main objective in the research is to test the cloud model ability and performance in simulation of this particular case. For that purpose a set of sensitivity experiments under different model initializations and initial data have been conducted. The results also indicate that the merging process apparently alters the physical processes through low- and middle-level forcing, increasing cloud depth, and enhancing convection. The examination of the microphysical process simulated by the model indicates that dominant production terms are the accretion of rain by graupel and snow, probabilistic freezing of rain to form graupel and dry and wet growth of graupel. Experiment under WRF v3.6 model initialization has shown some advantage in simulation of the physical processes responsible for production and initiation of heavy rainfall compared to other model runs. Most of the precipitation came from ice-phase particles-via accretion processes and the graupel melting at temperature T₀ ≥ 0°C. The rainfall intensity and accumulated rainfall calculated by the model closely reflect the amount of rainfall recorded. Thus, the main benefit is to better resolve convective showers or storms which, in extreme cases, can give rise to major flooding events. In such a way, this model may become major contributor to improvements in weather analysis and small-scale atmospheric predictions and early warnings of such subscale processes.     

Criteria for heat and cold wave duration indexes

Many areas of society are susceptible to the effects of extreme temperatures. Without an adequate definition of what constitutes heat and cold waves, it is impossible to assess either their changes in the past or their possible consequences for the future. The Intergovernmental Panel on Climate Change recommended criteria for heat wave duration indexes based on two arbitrarily defined constants. The principal weakness of this approach is that it does not yield comparable results for different geographical locations. This paper remedies the current lack of a meteorologically based definition of heat and cold waves and offers a preliminary test of its performance. Having previously shown that maximum daily temperature values follow normal frequency distribution, we derive statistical thresholds (e.g., below and above normal) from that distribution. These thresholds are thus climate specific and their change can be compared across geographical locations. These criteria are then tested on the homogeneous time series of maximum daily temperature observed for the period 1961–2008 with respect to three different geographical locations. The results obtained show an increase in the frequency of heat waves for the period 1991–2008 in comparison with the normal climatological period 1961–1990.     

Evaluation of the Physical and Chemical Properties of Eyjafjallajökull Volcanic Plume Using a Cloud-Resolving Model

The Eyjafjallajökull volcanic eruption, which occurred on April 14, 2010, caused many environmental, air traffic and health problems. An attempt has been made to demonstrate for the first time that certain improvements could be made in the quantitative prediction of the volcanic ash parameters, and in the accounting of the processes in the immediate vicinity of the volcano, using a cloud-resolving model. This type of explicit modeling by treatment of volcanic ash and sulfate chemistry parameterization, with input of a number parameters describing the volcanic source, is the way forward for understanding the complex processes in plumes and in the future plume dispersion modeling. Results imply that the most significant microphysical processes are those related to accretion of cloud water, cloud ice and rainwater by snow, and accretion of rain and snow by hail. The dominant chemical conversion rates that give a great contribution to the sulfate budget are nucleation and dynamic scavenging and oxidation processes. A three-dimensional numerical experiment has shown a very realistic simulation of volcanic ash and other chemical compounds evolution, with a sloping structure strongly influenced by the meteorological conditions. In-cloud oxidation by H₂O₂ is the dominant pathway for SO₂ oxidation and allows sulfate to be produced within the SO₂ source region. The averaged cloud water pH of about 5.8 and rainwater pH of 4.5 over simulation time show quantitatively how the oxidation may strongly influence the sulfate budget and acidity of volcanic cloud. Compared to observations, model results are close in many aspects. Information on the near field volcanic plume behavior is essential for early preparedness and evacuation. This approach demonstrates a potential improvement in quantitative predictions regarding the volcanic plume distribution at different altitudes. It could be a useful tool for modeling volcanic plumes for better emergency measures planning.     

Sensitivity study of the influence of cloud droplet concentration on hail suppression effectiveness

A cloud-resolving mesoscale model with a two-moment microphysical scheme has been used. The software package for cloud seeding and two categories of precipitation elements (graupel and frozen raindrops) are incorporated in the model. The aim of this study was to investigate the impact of cloud droplet concentration on hail suppression effectiveness. The concentration level of cloud droplets is prescribed in the model. We performed sensitivity tests of precipitation amounts (rain and hail) on the cloud droplet concentration in unseeded and seeded cases. We demonstrated for the unseeded case that increasing the concentration of cloud droplets created a reduction in rain accumulation, while the amount of hail accumulation increased. It is necessary to understand whether natural diversity in the cloud droplet concentration can affect the effectiveness of hail suppression. For operational cloud seeding activities, it would be helpful to determine whether it is possible to suppress hail if we know the optimal level of concentration for cloud droplets. Our study showed that hail suppression effectiveness had the greatest influence on lowering cloud droplet concentration levels; suppression effectiveness decreased as the cloud droplet concentration increased.     

Tendencies for the amounts of chemical material used for cloud seeding in Serbia

Weather modification activities are performed predominantly by cloud seeding. Some operational projects have been performed for more than half a century and cover planetary scales. These activities lead to a large amount of deposited chemical materials (seeding agents) at the ground level during precipitation. These deposits depend on the amount of the seeding agent. In the future, increased amounts of seeding agent deposits could be a serious problem due to various negative effects on the human environment. Therefore, the main intent of this paper is to determine trends for the seeding agent amount over certain areas of Serbia. Four areas covered by the Hail Suppression Project in Serbia are considered: the target area in central Serbia and areas in western and central Serbia, which are well-known hailfall regions. The annual seeding agent amounts show a slow decreasing trend because fewer seedings were performed during the last decade of the last century, which was due to economic reasons. In contrast, the annual seeding agent amounts of the other analysed areas indicate an increasing trend induced by the transfer of rockets to these hailfall regions. The main difference among small areas is the mean agent amount and its maximum time position as a consequence of the high spatial and time variability of the hail. However, a sharp decreasing trend that is influenced by the implementation of new methodologies, seeding agents and delivery tools may also be a factor in the implementation of cloud seeding projects. The given method is not only strictly applicable locally and may be applied to any other cloud seeding scenario and seeding area. Dominantly increasing trends in the agent amount indicate that the importance of weather modifications in the future will be greater than ever and will have both positive and negative effects.     

Late Pleistocene loess‐palaeosol sequences in the Vojvodina region,north Serbia

Late Pleistocene loess‐palaeosol sequences are widespread in the Vojvodina region, with thicknesses reaching a maximum of about 20 m. Our investigations include more than 40 of these loess sections. Geochronology of the last glacial loess‐palaeosol sequences, based on luminescence dating and amino acid racemisation, provides correlations between Upper Pleistocene loess‐palaeosol sediments in Vojvodina and comparable deposits at other European localities. Sedimentary logs of magnetic susceptibility, grain‐size measurements and carbonate content, combined with malacological evidence, indicate two main relatively cold and arid phases during the last glacial period, related to intensive accumulation of loess units L1L1 and L1L2, as well as many brief episodes of dry and windy climatic conditions, suggesting a possible relationship with cold events recorded in the North Atlantic region. Generally, late Pleistocene climate in the region was dry and relatively warm, compared with glacial period sites in central Europe, and was characterised by sharp differences between glacial and interglacial modes. New data and interpretations presented in this study emphasise the significance of loess‐palaeosol sequences in Vojvodina for the reconstruction of the temporal and spatial evolution of late Pleistocene palaeoclimate in this part of Europe. Copyright © 2007 John Wiley & Sons, Ltd.     

Examination of sulfate chemistry sensitivity in a mid-latitude and tropical storm using a cloud resolving model

We examine the sensitivities of heterogeneous sulfate chemistry in a mid-latitude and tropical storm using a cloud resolving model. Both thermodynamic environments show unstable conditions favorable for development of intensive convection, with more CAPE in tropical compared to mid-latitude storm. Compared with the observed severe storms, modeled results show a relatively good agreement with the radar and surface chemical observations. Microphysical evaluation indicates that the accretion and autoconversion appear to be most important processes in such considered clouds. This sensitivity simulation is an upper bound for conversion of S (IV) to sulfate. The tropical convective storm produces for about 2.5 times more sulfate compared to mid-latitude storm and converts more SO₂ to sulfate, increasing wet deposition of sulfur. The results for a midlatitude run indicate that aerosol nucleation and impact scavenging account for between 18.9% and 28.9% of the in-cloud sulfate ultimately deposited. As a result of greater rainfall efficiency, tropical storm shows about two times higher sub-cloud scavenging rate than mid-latitude storm. The oxidation of S (IV) to SO₄ ^?2 in cloud droplets and in precipitation is found to be dominant in both convective storms accounting almost with the same percentage contribution of 45.4% and 46.3% to sulfur deposition, respectively. In-cloud oxidation contribute a larger fraction of the total amount of sulfur deposited in tropical case (29.2%) when compared to the mid-latitude case (11.8), respectively. Neglecting aqueous-phase chemistry in ice-phase hydrometeors in both convective clouds led to overpredict deposition of about 40% to 33% relative to the base runs.     

An inadvertent transport of the seeding material as a result of cloud modification

Successful seeding of clouds in weather modification experiments essentially depends on the seeding time and dynamics, amount of seeding material and location of the initial seeding area. In the present study, we focus on the influence of the initial seeding zone location on the transport of seeding agent material into the target cloud. In addition, the inadvertent transport of seeding material is analysed. During weather modification activities, a lot of seeding material can be transferred far from the seeding zone in a downwind direction. The primary motivation for this research was to prove this statement. We use a three-dimensional, mesoscale cloud-resolving model to achieve our goal. We performed sensitivity tests with respect to the distance between the mass centres of the initial seeding area and the cloud. Different seeding scenarios are analysed. Our principal findings are as follows: (1) For distances between the mass centres of the initial seeding area and the cloud below 2.5 km, all seeding agent material would be activated after a short time. For distances above 10 km, most of the seeding agent would remain inactivated, because horizontal transport of the seeding agent becomes more important than transport induced by the main updraft. For these scenarios, the seeding agent is injected in the cold peripheral part of the cloud. (2) Sensitivity tests show that the inactivated seeding agent would remain close to the seeding area if the seeding is performed below cloud base. This effect occurs even for large distances between the seeding area and the target cloud (>20 km) due to low-level convergence. Thus, this seeding method suppresses the inert seeding material from being transferred far from the seeding zone. (3) The complete seeding material stays inactivated if the seeding is performed between the ?8 and ?12°C isotherms in front of the increased reflectivity zone. As a consequence, it would be transferred far from its initial area. The cloud would not be able to capture the seeding agent even during its greatest lateral extent.     

The Influence of the Acoustic-electric Coalescence on Phase and Mass Transfer of Supercooled Drops

The objective of this work is to study the influence of electrical discharge on the evolution of the cloud droplet spectra using a 1-D cloud model. It is shown that droplet motion and the electrical effects produced by lightning may cause a rapid and effective droplet coalescence process, with noticeably more phase transfer from liquid toward frozen water. As a measure of the drop spectra changes, we investigated variations of the radar reflectivity factor and the mass-weighted mean diameters at five temperature levels (0, −5, −10, −15, and −20C) with 15 combinations of the initial conditions. The model simulations suggest that about where the lightning occurred, after a few seconds, the initial unimodal spectra of supercooled water drops can be transferred to the bimodal spectra of unfrozen and frozen water drops. The number of newly created frozen drops is a few orders less than unfrozen water drops, but can still be very important for further transformations due to gravitational coagulation and other microphysical processes, e.g. glaciation. The results indicate that the procedure established to describe processes related to the electrical discharge and droplet spectra transformations can be used within a 3-D mesoscale model. It is concluded that the outcome can be also used to explain some of the physical characteristics inferred from polarimetric radar observations.     

Dependence of accumulated precipitation on cloud drop size distribution

Convective precipitation is the main cause of extreme rainfall events in small areas. Its primary characteristics are both large spatial and temporal variability. For this reason, the monitoring of accumulated precipitation fields (liquid and solid components) at the surface is difficult to carry out through the use of rain gauge networks or remote-sensing observations. Alternatively, numerical models seem to be the most powerful tool in simulating convective precipitation for various analyses and predictions. Due to a lack of comparisons between modelled and observed precipitation characteristics over a long period of time, we focus our research on comparisons between observations and three model samples of accumulated convective precipitation over a particular study area. We use a numerical cloud model with two model schemes involving the unified Khrgian–Mazin size distribution of cloud drops and a model scheme involving a monodisperse cloud droplet spectrum and the Marshall–Palmer size distribution for raindrops, respectively. For comparison, we have selected a study area with a sounding site. Our analysis shows that the model version with the Khrgian–Mazin size distribution exhibits a better agreement with the observed mean, median and range of extreme values of accumulated convective precipitation. Model simulations with the Khrgian–Mazin size distribution most closely match observations, with a correlation coefficient of 0.91. Use of the Marshall–Palmer size distribution, on the other hand, systemically underestimates the observed precipitation and has the lowest correlation coefficient among the methods, 0.83. Such an investigation is crucial to improve predictions of accumulated convective precipitation for various climatological and hydrological analyses and predictions.