Dejan Janc

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.

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.

Dependence of the simulated seeding effects of Cb cloud on the types of the AgI agents

SummaryA one-dimensional kinematic model is used to investigate the effects of silver iodide seeding in the region of a Cb cloud between isotherms of −8°C and −12°C. The agent interaction with cloud atmosphere is simulated by an improved microphysical model version which includes phoretic processes. The behaviour of the different agent types is investigated using the maximum values of agent mixing ratios and corresponding agent particle masses and radii.It is shown that the agent residence time in the seeding zone significantly depends on vertical velocity. The residence time is comparable to that previously reported. On the other side, the final graupel production decreases slightly when vertical velocity increases, while the corresponding graupel production is quite different for agents used. The main agent nucleation mechanisms are the Brownian coagulation of cloud droplets, inertial impact of cloud droplets and deposition nucleation.

On the influence of entrainment and forced lifting on some products of a 1-D model of a Cb cloud

Abstract A new parameterization of the entrainment process and forced lifting of air is incorporated into a 1-D model of a convective cloud. The entrainment is treated as an organized dynamical inflow/outflow caused by the horizontal gradient of pressure perturbation. Different sensitivity experiments are conducted. Orographically induced forced lifting of air is included in the model after consideration of the characteristics of the cold-air outflow from the cloud base and the warm-air motion toward the cloud base. This process is incorporated into a 1-D model of a convective cloud (although such a cloud is 3-D in nature). It is shown that the new entrainment parameterization causes the nonstationarity of vertical motion fields which is in agreement with numerous observations. The zone of downward motions below the cloud base is well correlated with the precipitation at ground level. The forced lifting of air nonlinearly intensifies upward and downward motions and the amount of precipitation.

Numerical study of the cloud seeding effects

SummaryThe microphysical model with the bulk water parameterization is applied to simulated both contact and deposition nucleation as well as the imersion freezing for unseeded cases and the cases immediately after seeding performed for the cold continental Cb clouds with small cloud droplets. The injection of agent AgI is performed in temperature region between −8°C and −12°C. The four groups of sensitivity experiments are executed.a.The Brownian coagulation of rain drops is the most important contact nucleation mechanism for seeded cases with great amount of rain drops. When cloud droplets mainly contribute to the liquid water content for seeded cases, the Brownian coagulation of cloud droplets is the primary nucleation mechanism while the inertial impact is the less effective contact nucleation mechanism;b.the mutual interdependence of contact and deposition nucleation mechanisms shows that the contact nucleation is more effective for graupel production than the deposition one for the temperature region considered in this model;c.the imersion freezing is the most important mechanism for all cases with significant amount of rain drops. It is more effective than the contact nucleation mechanism in unseeded cases with insufficient number of rain drops;d.the nucleation mechanisms are more sensitive to temperature changes than to pressure changes.