Mladjen Ćurić

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.

Comparison of Modeled and Observed Accumulated Convective Precipitation in Mountainous and Flat Land Areas

Abstract 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 may be a useful tool to simulate convective precipitation for various analyses and predictions. This paper focuses on improving quantitative convective precipitation estimates that are obtained with a cloud-resolving model. This aim is attained by using the appropriate cloud drop size distribution and modified single sounding data. The authors perform comparisons between observations and three model samples of the areal-accumulated convective precipitation for a 15-yr period over mountainous and flat land areas with 45 and 29 convective events, respectively. They compare the results from a numerical...

The effects of a river valley on an isolated cumulonimbus cloud development

Abstract The mechanism of development, propagation and front-side cell regeneration of a three-dimensional isolated cumulonimbus (Cb) cloud is investigated by a cloud-resolving mesoscale model. Interactions of the simulated storm with orography including a river valley were studied. According to observational evidence, the mountainous environment of the Western Morava valley (Serbia) is an important place for the formation of isolated Cb clouds. Once formed, they move down into the valley and continue to propagate along it. The effects of orography on the development, propagation and regeneration of the model Cb cloud are recognized by comparison of its development with that simulated over flat terrain under the same conditions. In our study, two cases are considered: complex terrain case (referred to CT case) and flat terrain case (referred to FT case). It is found that: – Orography effects (CT case) play an important role on Cb cloud life; the cloud propagation and development are inhibited in lateral direction and its form is more compact. Cold air outflow near the ground remains in the valley with an increased depth compared to the FT case. Warm environmental air approaching the cold air nose from the opposite direction is forced aloft more frequently than in the FT case. As a consequence, the simulated cloud propagates faster in this case. – Warm environmental air forcing over cold air nose in the FT case is stronger initially than in the CT one, since the cloud development is not prevented by orography from the lateral direction. Consequently, the cold air outflow is more intense. In contrast, the cloud regenerates more slowly, since the cold air diverges in all directions, which in turn, makes the cold air nose thinner. The alternate reform and collapse of the cold air nose are more expressed in the CT case.

The Relative Importance of Scavenging, Oxidation, and Ice-Phase Processes in the Production and Wet Deposition of Sulfate

Abstract The relative importance of various processes to sulfate production and wet deposition is examined by using a cloud-resolving model coupled with a sulfate chemistry submodel. Results using different versions of the model are then compared and principal differences with respect to their dynamics, microphysics, and chemistry are carefully discussed. The results imply that the dominant microphysical and chemical conversions of sulfate in the 3D run are nucleation, scavenging, and oxidation. Due to the lower cloud water and rainwater pH, oxidation does not contribute as significantly to the sulfate mass in the 2D run as the 3D. Sensitivity tests have revealed that in-cloud scavenging in the 2D run for continental nonpolluted and continental polluted clouds accounted for 29.4% and 31.5% of the total sulfur deposited, respectively. The 3D run shows a lower percentage contribution to sulfur deposition for about 28.2% and 29.6%. In addition, subcloud scavenging for the 2D run contributed about 32.7% and 3...

Predictive Capabilities of a One-Dimensional Convective Cloud Model with Forced Lifting and a New Entrainment Formulation

Abstract A one-dimensional time-dependent nonhydrostatic convective cloud model, with an entrainment formulation that includes the combined effects of turbulent and organized dynamic processes and forced lifting, is used to improve the forecasting of maximum cloud-top heights for application in the hail suppression program of the Hydrometeorological Service of Serbia. The model is focused on the comparison between simulated and observed cloud-top heights for 50 nocturnal thunderstorms, using four different entrainment rates. It is shown that the model results are highly dependent on the entrainment formulation used. The correlation coefficient r between predicted cloud-top heights and radar echo tops varies from 0.48 to 0.90 depending on the entrainment rate. The most acceptable model results are obtained for the entrainment rate that dominates the dynamic process (r = 0.89). The forced lifting slightly increases the correlation coefficient (r = 0.90) since it tends to improve predicted cloud tops with he...

On the sensitivity of cloud microphysics under influence of cloud drop size distribution

Abstract The unique Khrgian–Mazin size distribution is suggested as an alternative approximation of the entire drop spectra besides the conventional approximation in which the monodisperse and the Marshall–Palmer size distributions for cloud droplets and raindrops are used, respectively. This approach is employed in bulk microphysics. It is shown that the effects of the changed drop size distribution are particularly pronounced in the case of the microphysical production terms with rain. This is a consequence of the fact that the Khrgian–Mazin size distribution produces more small raindrops and less large ones compared to the Marshall–Palmer one, where both have the same rain water mixing ratio. The new production terms with cloud water do not differ from those with the monodisperse size distribution used. In order to show the sensitivity of cloud microphysics with respect to the change of drop size distribution function, the experiments are perform with the two versions of a forced 1-D time-dependent model. The first one involves the microphysical production terms with the Khrgian–Mazin size distribution, while the other one uses the conventional approach. Experiments clearly show that cloud microphysics essentially depends on cloud drop size distribution. Some microphysical aspects of a model cloud with the new drop size distribution are in fair agreement with observations.

On the sensitivity of the continuous accretion rate equation used in bulk-water parameterization schemes

Abstract The analytical solutions of the continuous accretion rate equation are compared regarding two different size distributions for cloud and rain water, two hail terminal velocities for different turbulent regimes and two solution types. The Khrgian-Mazin and Marshall-Palmer size distributions are assumed for rain water fraction, while the Khrgian-Mazin and monodisperse ones are applied to cloud water. In all cases the exponential-type size distribution for hail is used. In the calculation procedure for hail terminal velocity we take into consideration that the drag coefficient may be fixed or dependent on Reynolds number. Both approximate and exact solutions of the accretion rate under the same other conditions are also compared. Results show that the Khrgian-Mazin size distribution makes the approximate accretion rate between hail and rain considerably higher compared to its former parameterization for both hail terminal velocity assumptions. On the other side, the new treatment of hail terminal velocity with a variable drag coefficient produces lower accretion rates compared to the fixed drag coefficient case for all size distributions used. The approximate solution of the accretion rate equation is mainly lower than the exact one. Only in the case of raindrops-cloud droplets interaction it is much higher than the exact one with the Khrgian-Mazin size distribution used.

The impact of the choice of the entire drop size distribution function on Cumulonimbus characteristics

The study compares two different ways to represent the size distributions of cloud droplets and raindrops in bulk microphysical schemes in the scope of cloud-resolving mesoscale modeling of cumulonimbus clouds. A unified Khrgian-Mazin size distribution for the entire liquid water is systematically compared to a widely applied approach using a Marshall-Palmer size distribution for rain and a fitted monodisperse distribution for cloud droplets. The impact of the distribution function on cloud microphysics, precipitation characteristics, cloud appearance and dynamics of one simulated cumulonimbus case is investigated. The agreement of the model values with typical observed cloud characteristics is discussed. The results of our study reveal that there are considerable differences between the two approaches, both with respect to the microphysical production terms and with respect to the cloud appearance while the differences are less pronounced in cloud dynamics. An important result is that the Khrgian-Mazin size distribution leads to more cumulative rain compared to the one composed of a monodisperse and a Marshall-Palmer distribution. There is observational evidence that a storm splitting, hail field characteristics and cumulative total precipitations are simulated more accurately by the Khrgian-Mazin distribution function.

A forced 1-D convective cloud model

SummaryIn order to improve prediction of some model output, an approach for the incorporation of dynamical entrainment and forced lifting into a 1-D cloud model is introduced.It is shown that:1.The new parameterization of entrainment processes causes oscillations in the magnitudes of the vertical velocities, rain water content and temperature perturbations.2.The forced lifting component nonlinearly amplifies vertical velocities, temperature perturbations and rain water production. It causes descents of the local maxima (positive and negative) of the parameters mentioned.3.Model values of cloud top heights and radar reflectivity maxima with new parameter of entrainment are in better agreement with observations than former ones. The rain water mixing ratio maxima under influence of forced lifting component coincide well with observed regions of rainfall maxima.

Some evidence on European monsoon existence

Large-scale European atmospheric circulation induced by temperature differences between the continent and the North Atlantic Ocean causes thermodynamic and climatic conditions that initiate a European monsoon. In Eastern Europe, the rainy season occurs in early summer, and the dry season occurs in winter. In Western Europe, the rainy season is in the early winter and the dry season is in the spring. This precipitation trend, as well as other climatic features, suggests the existence of a European monsoon.