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The paper summarises the major scientific achievements of Sergej Zilitinkevich on the occasion of his 70th birthday. [ABSTRACT FROM AUTHOR]

This paper presents a new triple-moment blowing snow model PIEKTUK-T by including predictive equations for three moments of the gamma size distribution. Specifically, predictive equations for the total number concentration, total mass mixing ratio, and total radar reflectivity for blowing snow are included. Tests in the context of idealized experiments and observed case studies demonstrate that the triple-moment model performs better than the double-moment model PIEKTUK-D in predicting the evolution of the number concentration, mixing ratio, shape parameter, and visibility in blowing snow, provided that the fall velocities for the total number concentration, mass mixing ratio, and radar reflectivity are weighted by the same order of the respective moments in both models. The power law relationship between the radar reflectivity factor and particle extinction coefficient found in PIEKTUK-T is consistent with one observed in snow storms. Coupling of the triple-moment blowing snow model to an atmospheric model would allow realistic studies of the effect of blowing snow on weather and climate. [ABSTRACT FROM AUTHOR]

This paper presents the capabilities and limitations of using state-of-the-art kites for atmospheric research. A brief historical review of the subject is first presented, followed by an outline of the current status of kite-borne measurement technology. The utility of the technique is then illustrated by presenting a series of recent measurements made using kite-borne technology. A summary of the advantages and limitations of kite-based measurements relative to other technologies is provided for reference. [ABSTRACT FROM AUTHOR]

During the last two decades, several land surface schemes for use in climate, regional and/or mesoscale, hydrological and ecological models have been designed. For all of them, of great importance is a correct parameterization of the energy and momentum transport from the surface. Mainly, the land-surface schemes have been developed in order to parameterize land-surface variations both on daily and annual time scales. In this paper we describe the Land Air Parameterization Surface Scheme (LAPS) designed for use in atmospheric models and mainly for use in simulations of thermal circulations. It has two modules for parameterizing the processes at the boundary-layer interface: one for a bare surface and one for a vegetated one. Incorrect parameterization of land-surface processes and prescription of the surface parameters in atmospheric modeling, can result in artificial changes of horizontal gradient of the sensible heat flux. Thus, an error in horizontal temperature gradient within the lower atmosphere may be introduced. Of course, the reliability of the model depends on the quality of boundary-layer scheme implemented and its sensitivity to the bare soil and vegetation parameters. In this study, the sensitivity of the LAPS scheme to parameterization of bare soil processes, ground roughness length, vegetation parameters (stomatal resistance, leaf area index and canopy height) and albedo, has been examined. Tests based on time integrations using real data were performed. For sensitivity tests with the non-plant module the data sets for June 3, 1981; June 4, 1982; June 11, 1982 and June 24, 1982 from the experimental site at Rimski Šančevi (Yugoslavia) were used. The performance of the vegetation module was tested by employing the data set for September 8, 1988 measured at the experimental site of De Sinderhoeve (The Netherlands). The computed latent heat flux and the ground temperature outputs were compared with the observations. Finally, the sensitivity of the LAPS scheme to the prescribed parameters was examined by using a simple statistical analysis. [ABSTRACT FROM AUTHOR]

Heard Island (73°30' E, 53°05' S) is an isolated island in the sub-Antarctic located in a region of strong westerly winds. The dominant topographic feature is a large glaciated volcano, which peaks at 2,745 m in altitude. Only limited meteorological information exists for the island, with incomplete records from a station at Atlas Cove covering the period 1948 to 1954, and from a station at The Spit from 1992 onwards. We present the results of wind observations that were conducted on Dovers Moraine at the eastern end of Heard Island over the 2000/2001 summer, with the aim of characterising local winds at this location on the island. Wind was measured using a Woelfle type mechanical wind recorder. Wind speed was comparable, and on occasions stronger, at this location compared to simultaneous records at the western end of the island. Winds were predominantly from the south-south-west, or north-north-west to north. A number of periods of relatively low wind speed coincided with repeated wind direction patterns, which we associate with the surface signature of von Karman vortices. Results from this study suggest there are significant topographically-generated differences in wind characteristics between the eastern and western ends of the island. [ABSTRACT FROM AUTHOR]

The goal of this work is to summarize synoptic meteorological conditions during the Coastal Fog (C-FOG) field project that took place onshore and offshore of the Avalon Peninsula, Newfoundland, from 25 August until 8 October 2018. Visibility was measured at three locations at the Ferryland supersite that are about 1 km from each other, and at two additional sites 66 and 76 km to the north. Supporting meteorological measurements included surface winds, air temperature, humidity, pressure, radiation, cloud-base height, and atmospheric thermodynamic profiles from radiosonde soundings. Statistics are presented for surface measurements during fog events including turbulence kinetic energy, net longwave radiation, visibility, and precipitation. Eleven fog events are observed at Ferryland. Each significant fog event is related to a large-scale cyclonic system. The longest fog event is due to interaction of a northern deep low and a tropical cyclone. Fog occurrence is also examined across Atlantic Canada by including Sable Island, Yarmouth, Halifax, and Sydney. It is concluded that at Ferryland, all significant fog events occur under a cyclonic system while at Sable Island all significant fog events occur under both cyclonic and anticyclonic systems. The fog-formation mechanism involves cloud lowering and stratus broadening or only stratus broadening for the cyclonic systems while for the anticyclonic systems it is stratus broadening or radiation. Although widely cited as the main cause of fog in Atlantic Canada, advection fog is not found to be the primary or sole fog type in the events examined. [ABSTRACT FROM AUTHOR]

Lower tropospheric thermal structure greatly affects atmospheric boundary-layer (ABL) stability and mixing processes with the free troposphere. In particular, in polluted urban zones, ABL stratification becomes a key variable in air quality research. This study focuses on generating a climatology (1990–2017) of the seasonal variability of ABL thermal structure in Mexico City by way of radiosonde analysis. Thermal inversion intensity and frequency are shown to be greater during winter and spring, a behaviour which coincides with greater pollutant concentrations. Higher concentrations are found during the dry season (November to May) than during the rainy months. In addition, significantly higher than normal surface pollutant concentrations are found on days with simple thermal inversion layers as well as during multilayer inversion days. Furthermore, stable layers, determined by potential temperature, are found throughout the year but more frequently during winter, whereas stable layers based on the virtual potential temperature prevail all year. In regions of complex terrain, such multiple stable layers have also been identified by previous authors. Additionally, the most unstable surface layers (in which the bulk Richardson number ( Ri B ) is small) develop during the rainy season, whereas during winter there are more levels in the vertical column with higher Ri B values. Although the Mexico City ABL and pollution episodes have been widely studied, this represents the first long-term investigation to consider the thermal stability of the ABL. Therefore, the present study provides a baseline for further research employing different observational techniques and high-resolution numerical models. [ABSTRACT FROM AUTHOR]

A comparison between various parameterizations for the bulk transfer coefficient for heat and momentum is carried out for a wide range of atmospheric stability and values of the roughness lengths for momentum and heat, z_{0m} and z_{0h} respectively. It is confirmed that the parameterization of Launiainen compares well to a numerical iterative solution for the Obukhov length L as function of the gradients of wind speed and temperature over a limited range of z_{0m}/z_{0h} and stability conditions. For z_{0m}/z_{0h} > 500, an alternative interpolation formulation of Holtslag and Ek, in combination with the formulation of Launiainen, provides a better approximation. [ABSTRACT FROM AUTHOR]

Accurate measurements of the depth of the convective boundary layer (CBL) are fundamental for understanding and forecasting weather, air quality, and climate. However, the CBL depth (BLD) shows significant spatial and temporal variability, which is challenging to measure and model. Ceilometer instruments, which estimate the CBL depth from aerosol layers, are relatively cheap, have high temporal resolution, and have the potential for increased spatial coverage. Nevertheless, their performance in subtropical environments with low aerosol concentrations is unknown. Furthermore, climatological studies of the variability in the urban CBL depth are scarce, especially in the Southern Hemisphere, and very few studies have examined the relationship between BLD variability and synoptic conditions. Using three years of continuous ceilometer data, we present a novel climatology of the temporal variability in CBL depth over the Southern Hemisphere subtropical, coastal city of Auckland, New Zealand. The results indicate that the median daytime maximum CBL depth (BLDMAX) over the Auckland CBD is 1100 m a.g.l. (above ground level) during summer and 700 m a.g.l. during winter. Strong relationships are found between the maximum CBL depth and both solar radiation and soil moisture; however, no relationship is found between the maximum CBL depth and the synoptic conditions as determined by the New Zealand Kidson weather types synoptic classification. The absence of a relationship between the maximum CBL depth and Kidson weather types may explain the poor correlation previously observed between synoptic class and air pollution events in Auckland. [ABSTRACT FROM AUTHOR]

We develop an urban canopy scheme coupled to a mesoscale atmospheric numerical model and evaluate the simulated climate of an Australian city. The urban canopy scheme is based on the Town Energy Budget approach, but is modified to efficiently represent the predominately suburban component of Australian cities in regional climate simulations. Energy conservation is improved by adding a simple model of air-conditioning to prevent the urban parametrization acting as an energy sink during the Australian summer. In-canyon vegetation for suburban areas is represented by a big-leaf model, but with a largely reduced set of prognostic variables compared to previous approaches. Although we have used a recirculation/venting based parametrization of in-canyon turbulent heat fluxes that employs two canyon wall energy budgets, we avoid using a fixed canyon orientation by averaging the canyon fluxes after integrating over 180° of possible canyon orientations. The urban canopy scheme is evaluated by simulating the climate for Melbourne, Australia after coupling it to The Air Pollution Model. The combined system was found to predict a realistic climatology of air temperatures and winds when compared with observations from Environmental Protection Authority monitoring stations. The model also produced a plausible partitioning of the urban energy budget when compared to urban flux-tower studies. Overall, the urban canyon parametrization appears to have reasonable potential for studying present and predicting changes in future Australian urban climates in regional climate simulations. [ABSTRACT FROM AUTHOR]

Flow and turbulence above urban terrain is more complex than above rural terrain, due to the different momentum and heat transfer characteristics that are affected by the presence of buildings (e.g. pressure variations around buildings). The applicability of similarity theory (as developed over rural terrain) is tested using observations of flow from a sonic anemometer located at 190.3 m height in London, U.K. using about 6500 h of data. Turbulence statistics—dimensionless wind speed and temperature, standard deviations and correlation coefficients for momentum and heat transfer—were analysed in three ways. First, turbulence statistics were plotted as a function only of a local stability parameter z/Λ (where Λ is the local Obukhov length and z is the height above ground); the σ/ u values ( i = u, v, w) for neutral conditions are 2.3, 1.85 and 1.35 respectively, similar to canonical values. Second, analysis of urban mixed-layer formulations during daytime convective conditions over London was undertaken, showing that atmospheric turbulence at high altitude over large cities might not behave dissimilarly from that over rural terrain. Third, correlation coefficients for heat and momentum were analyzed with respect to local stability. The results give confidence in using the framework of local similarity for turbulence measured over London, and perhaps other cities. However, the following caveats for our data are worth noting: (i) the terrain is reasonably flat, (ii) building heights vary little over a large area, and (iii) the sensor height is above the mean roughness sublayer depth. [ABSTRACT FROM AUTHOR]

Numerical simulations of tracer transport in an idealised, east-west aligned valley are performed with the Regional Atmospheric Modeling System (RAMS), both two-dimensional and three-dimensional. The results are qualitatively consistent with wintertime observations in the Austrian Inn Valley. The simulations show an asymmetry in wind circulation and tracer distribution between the valley sidewalls according to the orientation of the slope with respect to the sun. Two-dimensional sensitivity experiments are run to investigate the influence of vertical inhomogeneities in thermal stratification and vegetation coverage on the slope-wind circulation and therewith the tracer transport. It is shown that a transition to a layer of higher stability or to a region with higher surface albedo causes a reduction of the mass flux in the upslope-wind layer and due to mass continuity a quasi-horizontal transport out of the slope-wind layer. [ABSTRACT FROM AUTHOR]

Six commonly used models for calculating daily net radiation were tested against measured net radiation. Meteorological data from 32 and 7 consecutive years obtained at two temperate sites were used. The extensive duration of the datasets ensured that all weather conditions and extreme events were captured. A set of statistical procedures was used to evaluate the performance of the models. The mean bias errors ranged from 0.0 W m−2 to 24.8 W m−2 and 0.1–24.7 W m−2 and root mean square error from 11.0 W m−2 to 28.1 W m−2 and 10.0–27.9 W m−2 at the two sites respectively, for days without snow cover on the ground. The best agreement was found when locally calibrated model coefficients were used. Only negligible differences in model performances were found between the two sites and the differences were lower than the inaccuracies of the net radiation instruments used. Including days with snow cover in the analysis lead to a slight increase in the bias and scatter of the predictions. Model performances were in general better during summertime than wintertime. Altered albedo values during winter caused by generally low sun angles were likely the cause of this. Analysis showed that at least 5 years of data were needed to obtain stable calibration coefficients for local calibration of the models. Based on the results from this study, and due to their physical background, two physical based models were recommended for calculating daily values of net radiation under temperate climate regimes. A simple adjustment of the calibration coefficients based on climate regime was suggested for these models. [ABSTRACT FROM AUTHOR]

A primary climate change signal in the central Arctic is the melting of sea ice. This is dependent on the interplay between the atmosphere and the sea ice, which is critically dependent on the exchange of momentum, heat and moisture at the surface. In assessing the realism of climate change scenarios it is vital to know the quality by which these exchanges are modelled in climate simulations. Six state-of-the-art regional-climate models are run for one year in the western Arctic, on a common domain that encompasses the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment ice-drift track. Surface variables, surface fluxes and the vertical structure of the lower troposphere are evaluated using data from the SHEBA experiment. All the models are driven by the same lateral boundary conditions, sea-ice fraction and sea and sea-ice surface temperatures. Surface pressure, near-surface air temperature, specific humidity and wind speed agree well with observations, with a falling degree of accuracy in that order. Wind speeds have systematic biases in some models, by as much as a few metres per second. The surface radiation fluxes are also surprisingly accurate, given the complexity of the problem. The turbulent momentum flux is acceptable, on average, in most models, but the turbulent heat fluxes are, however, mostly unreliable. Their correlation with observed fluxes is, in principle, insignificant, and they accumulate over a year to values an order of magnitude larger than observed. Typical instantaneous errors are easily of the same order of magnitude as the observed net atmospheric heat flux. In the light of the sensitivity of the atmosphere–ice interaction to errors in these fluxes, the ice-melt in climate change scenarios must be viewed with considerable caution. [ABSTRACT FROM AUTHOR]

A simulated climatology of ducts in the Persian Gulf area was produced with the MM3 atmospheric model. From November to January ducts were sporadic, land and surface based, shallow and weak. From February to October ducts of all types occurred. In the duct season, spatial and temporal variations were related to the land/sea distribution and to day and night. Over land at night, widespread, shallow, weak surface ducts occurred well away from the sea; within about 100 km of the south-western coast in the late evening, ducts were S-shaped. Over land in daytime, the dry, convective boundary layer prevented duct formation. Over the Gulf in the season, duct coverage was complete throughout night and day. A spatial sequence of shallow, weak surface ducts, deeper, stronger S-shaped ducts and deep, strong elevated ducts lay from north-west to south-east over the Gulf. This sequence was related to the growth of a marine internal boundary layer (MIBL) and the effects of land- and sea-breeze circulations. Subsidence in the sea-breeze circulation reduced magnitudes of depth and strength and created gradients in a direction normal to the main growth axis of the MIBL. Ducts growing in the MIBL were tilted upward from west to east. The combined effect gave relatively weak surface ducts in the north-west and strong elevated ducts in the south-east. Duct depth and strength increased as the season progressed, owing to increased wind speed within, and increased depth of, the MIBL. [ABSTRACT FROM AUTHOR]

The mean flow within inhomogeneous urban areas is investigated using an urban canopy model. The urban canopy model provides a conceptual and computational tool for representing urban areas in a way suitable for parameterisation within numerical weather prediction and urban air quality models. Average aerodynamic properties of groups of buildings on a neighbourhood scale can be obtained in terms of the geometry and layout of the buildings. These canopy parameters then determine the spatially averaged mean wind speeds within the canopy as a whole. Using morphological data for real cities, computations are performed for representative sections of cities. Simulations are performed to study transitions between different urban neighbourhoods, such as residential areas and city centres. Such transitions are accompanied by changes in mean building density and building height. These are considered first in isolation, then in combination, and the generic effects of each type of change are identified. The simulation of winds through a selection of downtown Los Angeles is considered as an example. An increase in canopy density is usually associated with a decrease in the mean wind speed. The largest difference between mean winds in canopies of different densities occurs near ground level. Winds generally decrease upon encountering a taller canopy of the same density, but this effect may be reversed very near the ground, with possible speed-ups if the canopy is especially tall. In the vicinity of a transition there is an overshoot in the mean wind speed in the bottom part of the canopy. Mechanisms for these effects are discussed. [ABSTRACT FROM AUTHOR]

The temporal evolution and spatial structure of the aerosol layer (AL) height as observed with an airborne downlooking lidar over the Swiss Alps were investigated with a three-dimensional mesoscale numerical model and a particle dispersion model. Convective boundary-layer (CBL) heights were derived from the mesoscale model output, and the behaviour of surface-released particles was investigated with the particle dispersion model. While a previous investigation, using data from the same field study, equated the observed AL height with the CBL height, the results of the current investigation indicate that there is a considerable difference between AL and CBL heights caused by mixing and transport processes between the CBL and the free atmosphere. CBL heights show a more terrain-following behaviour and are lower than AL heights. We argue that processes causing the difference between AL and CBL heights are common over mountainous terrain and that the AL height is a length scale that needs to be considered in air pollution studies in mountainous terrain. [ABSTRACT FROM AUTHOR]

An experimental micrometeorological set-up was established at the CARBOEURO-FLUX site in Tharandt, Germany, to measure all relevant variables for the calculation of the vertical and horizontal advective fluxes of carbon dioxide. The set-up includes two auxiliary towers to measure horizontal and vertical CO2 and H2O gradients through the canopy, and to make ultrasonic wind measurements in the trunk space. In combination with the long-term flux tower an approximately even-sided prism with a typical side-length of 50 m was established. It is shown that under stable (nighttime) conditions the mean advective fluxes have magnitudes on the same order as the daily eddy covariance (EC) flux, which implies that they play a significant, but not yet fully understood, role in the carbon budget equation. The two advective fluxes are opposite and seem to cancel each other at night (at least for these measurements). During the day, vertical advection tends to zero, while horizontal advection is still present implying a flow of CO2 out of the control volume. From our measurements, a mean daily gain of 2.2 gC m-2 d-1 for the horizontal advection and a mean daily loss of 2.5 gC m-2d-1 for the vertical advection is calculated for a period of 20 days. However the large scatter of the advective fluxes has to be further investigated. It is not clear yet whether the large variability is natural or due to measurement errors and conceptual deficiencies of the experiment. Similar results are found in the few comparable studies. [ABSTRACT FROM AUTHOR]

A regional atmospheric climate model is used to examine the effect of changes in the roughness lengths of momentum (z0m) and heat (z0h) on the structure of the lower atmosphere and on the surface energy fluxes over Antarctica. Four experiments were carried out in which z0m and/or z0h were altered with respect to a control experiment. The changes consisted of (1) a lowering of z0m from a field aggregated from a vegetation map with an orographic correction based on the European Centre for Medium-Range Weather Forecasts z0m field, to a constant value of 10-3 m; and (2) a lowering of z0h from a value equal to z0m to a constant value of 10-3 m or a value dependent on the wind speed via a surface renewal model. A reduction of z0m results in the expected increase in near-surface wind speed. It also results in an increase in the depth of the layer in which south-easterly near-surface winds prevail, and in a decrease in the strength of the large-scale flow over the continent, in particular in summer. In the escarpment region a decrease of z0m is found to result in too high wind speeds. Surface temperatures on average decrease while atmospheric temperatures increase, resulting in an increase of near-surface static stability. Changes in roughness lengths do not significantly change the temperature profiles. The surface fluxes, on average found reduced, are modelled best by using the z0h based on the surface renewal method. [ABSTRACT FROM AUTHOR]

Vegetation and other roughness elements distributed across a surface can provide significant protection against wind erosion by extracting momentum from the flow and thereby reducing the shear stress acting at the surface. A theoretical model has previously been presented to specify the partition of drag forces for rough surfaces and to predict required vegetation density to suppress wind erosion. However, the model parameters have not yet been constrained and the predictive capacity of the model has remained uncertain. A wind-tunnel study was conducted to measure the drag partition for a range of roughness densities and to parameterise the model in order to improve its range of potential applicability. The drag forces acting on both an array of roughness elements and the intervening surface were measured independently and simultaneously using new drag balance instrumentation. A detailed measure of the spatial heterogeneity of surface shear stresses was also made using Irwin sensors. The data agreed well with previous results and confirmed the general form of the model. Analysis of the drag partition confirmed the parameter definition β = CR/CS (where CR and CS are roughness element and surface drag coefficients, respectively) and a constant proportional difference between the mean and maximum surface shear stress was found. The results of this experiment suggest that the definition for m, the surface shear stress inhomogeneity parameter, should be revised, although the theoretical and physical reasons for including this parameter in the model appear to be valid. Best-fit values for m ranged from 0.53 to 0.58. [ABSTRACT FROM AUTHOR]

Using data for one year, we examined the vertical wind speed profiles on a mountain slope covered with forest in northern Thailand under a tropical monsoon climate. We defined two profile patterns: higher wind speeds at greater heights (Pattern 1) and lower wind speeds at greater heights (Pattern 2). We classified 9.4% of the data as Pattern 2; this pattern tended to occur during the night, at low wind speeds, and with high outgoing longwave radiation. In addition, stable stratification and decoupling between the canopy surface air and the overlying layers were observed when Pattern 2 occurred frequently. These facts suggested that Pattern 2 was caused by a nocturnal drainage flow. The occurrence of Pattern 2 showed a clear seasonal trend, indicating that there is a seasonal trend in the occurrence of nocturnal drainage flows. Pattern 2 was observed more frequently between August and February and less frequently between March and July. This corresponded to the seasonal trend in wind speed, but did not correspond to the seasonal trend in the outgoing longwave radiation. [ABSTRACT FROM AUTHOR]

A large-aperture scintillometer (LAS) was operated continuously during a period of more than one year over a heterogeneous land surface in Central Europe at the transition between marine and continental climates. The LAS measurements of the refractive index structure parameter, C N2, were used to estimate the sensible heat flux. This was possible for about 60 to 80% of the time under daytime conditions during the summer, with lower values obtained for the cold season (October to March). Using data from a three-week long field experiment, the LAS-based heat flux was compared with a weighed average of local heat flux measurements over the main land use classes (forest, agriculture, water) in the area, resulting in reasonable agreement. LAS-based heat fluxes were then used for comparison with the heat flux values of a numerical weather prediction model. An over-prediction of the model heat flux was found in summer but the modelled values were lower than the LAS derived data during the cold season. [ABSTRACT FROM AUTHOR]

Land surface and climate processes possess dynamics and heterogeneities across a wide range of scales. This study explores the utility of, and procedures for, using local scale measurements to obtain large-scale information. An aggregation scheme is proposed to bridge the scale gap between the scale of measurements including remote sensors and climate and mesoscale models. The proposed scheme derives a set of effective parameters which obeys the energy balance equation exactly and partitions the surface fluxes accurately at different scales. It produces a unique set of aggregated land surface parameters that are easily measurable through remote sensing and have sound mathematical and physical basis. It is shown that aggregated ground heat flux, emissivity and albedo may be obtained by simple areally weighted averaging while temperature, aerodynamic and surface resistances require more involved aggregation operators. The effective surface temperature, although it requires a complicated operator involving subgrid-scale temperature and surface emissivity, is easily measurable through remote sensing. The proposed scheme was compared and contrasted with existing effective parameter approaches. It was shown that several effective parameters of the previous schemes can be easily derived from the proposed scheme by introducing additional assumptions and simplifications. [ABSTRACT FROM AUTHOR]

A series of tracer experiments studying the statistical properties of concentration fluctuations in clouds dispersing in the atmospheric surface layer is described and analyzed. Experiments were conducted at downwind fetches between about 200 and 1200 m, under a wide range of atmospheric conditions ranging from very unstable to moderately stable stratification. The present experiments have addressed basic requirements not met by past field experiments involving instantaneously released clouds; namely, the experiments provided repeat realizations of instantaneously released clouds measured with high-resolution concentration detectors, accompanied by the contemporaneous acquisition of high-quality meteorological and turbulence measurements. Extensive analyses are performed on the cloud concentration data in the framework of relative diffusion. Ensembles of cloud concentration realizations have been constructed. From these ensembles, crosswind and time profiles of the ensemble-mean concentration, concentration variance, ensemble-mean dosage, and dosage variance are obtained. The behaviour of the time profiles of the integral time scale of cloud concentration fluctuations is studied. The use of surface-layer similarity theory for the analysis of the downwind variation of a number of cloud quantities (e.g., cloud size and duration, cloud centre ensemble-mean concentration and dosage, cloud centre concentration and dosage variance, cloud centre integral time scale) is shown to be an effective basis for ordering these quantities. Furthermore, a number of approximate universal relationships describing the behavior of these cloud quantities has been derived. Finally, it is shown that the scaled crosswind and time profiles of ensemble-mean concentration and concentration variance as well as the scaled time profiles of the concentration fluctuation integral time scale exhibit self-similar forms that are independent of atmospheric stratification and downwind fetch. [ABSTRACT FROM AUTHOR]

Recently Frech and Mahrt proposed a closure scheme which includes a large-scale stress term to represent the effects of non-local momentum mixing in the convective boundary layer. Here large-eddy simulation (LES) datasets are used to evaluate the performance of this scheme across a range of stabilities between neutral and highly convective conditions, and as a function of baroclinity. Generally the inclusion of the non-local term in the closure model leads to results in better agreement with LES, although some modifications to the model formulation are suggested. [ABSTRACT FROM AUTHOR]

The distribution of rainfall over small-scale topography of uniform roughness was studied both numerically and from field observations. Field data have shown that small-scale topographic inhomogeneities (hills and valleys of a few tens of metres in height) substantially influence the rainfall distribution over the inhomogeneity itself and also some distance downwind from it. According to comprehensive rainfall measurements carried out in a small watershed, windward slopes received less conventionally-measured rain than lee-sides. Moreover, on the windward slope rainfall decreased uphill, to a pronounced minimum near the crest. In addition, ’directional‘ raingages, situated at the bottom of both slopes revealed a pattern suggesting a reversed surface flow on the lower slopes. This pattern persisted through a variety of background conditions. To explain the observed data, a numerical simulation of both the windfield and of drop trajectories was carried out. The windfield was generated in simulations using the Colorado State University RAMS model with a very high resolution (5 m in the horizontal). Drop trajectories were calculated using an equation for the motion of drops in a flow field that incorporates current experimental results. A strong effect of small-scale topography on drop trajectories was found. The simulation results are encouraging and help to explain many characteristic features of precipitation distributions observed in field experiments. [ABSTRACT FROM AUTHOR]