Meso-γ Scale Research Articles

Relative Humidity as an Indicator for Cloud Formation over Heterogeneous Land Surfaces

The influence of land surface heterogeneity on potential cloud formation is investigated using relative humidity as an indicator. This is done by performing numerical experiments using a large-eddy simulation model (LES). The land surface in the model was divided into two patches that had the same sum of latent and sensible heat flux but different Bowen ratios to simulate heterogeneous land surfaces. For heterogeneity in the meso-γ scale (2–20 km), sensitivity analyses were carried out on the heterogeneity amplitude (Bowen ratio difference between contrasting areas) and the inversion strength of potential temperature and specific humidity. The competition between absolute temperature decrease by ABL growth and dry air entrainment in heterogeneous conditions is analyzed using the LES results. First, it is shown that entrainment is located and enhanced over patches with higher Bowen ratios (warm patches) than their surroundings (cold patches). The heterogeneity-induced strong thermals can further penetrate the inversion at the ABL top, thereby reaching lower absolute temperatures than in homogeneous conditions. Second, because of the heterogeneity-induced circulations the moisture is located over the warm patch, and higher time-averaged RH values at the ABL top (RHzi) than over the cold patches are found here, even for dry atmospheres. These RHzi exceed values found over homogeneous land surfaces and are an indication that surface heterogeneity may facilitate cloud formation. In vertical profiles of RH, few differences are found between the homogeneous and heterogeneous cases, but the essential heterogeneity-induced modifications are within the domain variability.

Wind Channeling by the Dead-Sea Wadis

Wind channeling by small wadis of few kilometers width at the Dead-Sea area is demonstrated with highresolution modeling. Previous works have not shown such channeling in very small rivers due to computation difficulties on the meso-γ and the lack of suitable observations for verification. The wind channeling is demonstrated in three wadis, and was verified by meteorological model simulations on the meso-γ scale, as well as by surface and upper layers observations. There are two dominant mechanisms for the wind channeling, the geometrical forcing and the thermal katabatic effect as shown by the model negative divergence zone inside the wadis. The unique features of the Dead-Sea i.e., the absence of vegetation cover and urban areas can serve as a proposed natural model for the wind-wadi interaction.

Revisiting the latent heat nudging scheme for the rainfall assimilation of a simulated convective storm

Next-generation, operational, high-resolution numerical weather prediction models require economical assimilation schemes for radar data. In the present study we evaluate and characterise the latent heat nudging (LHN) rainfall assimilation scheme within a meso-γ scale NWP model in the framework of identical twin simulations of an idealised supercell storm. Consideration is given to the model’s dynamical response to the forcing as well as to the sensitivity of the LHN scheme to uncertainty in the observations and the environment.

The effect of mountainous topography on moisture exchange between the “surface” and the free atmosphere

Typical numerical weather and climate prediction models apply parameterizations to describe the subgrid-scale exchange of moisture, heat and momentum between the surface and the free atmosphere. To a large degree, the underlying assumptions are based on empirical knowledge obtained from measurements in the atmospheric boundary layer over flat and homogeneous topography. It is, however, still unclear what happens if the topography is complex and steep. Not only is the applicability of classical turbulence schemes questionable in principle over such terrain, but mountains additionally induce vertical fluxes on the meso-γ scale. Examples are thermally or mechanically driven valley winds, which are neither resolved nor parameterized by climate models but nevertheless contribute to vertical exchange. Attempts to quantify these processes and to evaluate their impact on climate simulations have so far been scarce. Here, results from a case study in the Riviera Valley in southern Switzerland are presented. In previous work, measurements from the MAP-Riviera field campaign have been used to evaluate and configure a high-resolution large-eddy simulation code (ARPS). This model is here applied with a horizontal grid spacing of 350 m to detect and quantify the relevant exchange processes between the valley atmosphere (i.e. the ground “surface” in a coarse model) and the free atmosphere aloft. As an example, vertical export of moisture is evaluated for three fair-weather summer days. The simulations show that moisture exchange with the free atmosphere is indeed no longer governed by turbulent motions alone. Other mechanisms become important, such as mass export due to topographic narrowing or the interaction of thermally driven cross-valley circulations. Under certain atmospheric conditions, these topographical-related mechanisms exceed the “classical” turbulent contributions a coarse model would see by several times. The study shows that conventional subgrid-scale parameterizations can indeed be far off from reality if applied over complex topography, and that large-eddy simulations could provide a helpful tool for their improvement.

Two possible mechanisms for vortex self-organization

The vortex self-organization is investigated in this paper by four groups of numerical experiments within the framework of quasi-geographic model, and based on the experimental results two types of possible mechanisms for vortex self-organization are suggested. The meso-scale topography may enable separated vortices to merge into a larger scale vortex; and the interaction of meso-γ and meso-β scale systems may make separated vortices to self organize a typhoon-like vortex circulation.

Radar‐duct and boundary‐layer characteristics over the area of The Gulf

AbstractRadar‐duct characteristics and the associated atmospheric boundary layer over the The Gulf (50 ° N, 28 ° E) on 28 April 1996 were simulated with the NCAR–Penn State MM5V3 numerical model. The model was validated against meteorological observations and duct data gathered in a special observational programme. A benchmark study with horizontal grid‐length of 5 km revealed complete coverage of The Gulf by ducts throughout the whole twenty‐four hours. Over land, ducts existed only at night. Over The Gulf, meso‐β scale variation occurred: shallow (250 m) surface ducts existed within about 200 km of the north coast; elevated ducts, over 500 m deep, near the southern coast, and S‐shaped ducts (about 400 m deep) over the majority of the area. Duct strength also increased from north to south. These features were related to the marine internal boundary layer (MIBL) that formed over The Gulf in the ambient north‐west wind. Meso‐γ scale variations in duct‐top height occurred within 200 km of the north coast, immediately downwind of Qatar and off the central east and west coasts. Duct characteristics were influenced at the meso‐γ scale by coastal configuration and at the meso‐β scale by sea surface temperature (SST), land–sea temperature difference, orography and ambient wind. An increase in grid length to 25 km lost much of the meso‐γ scale structure but retained the meso‐β scale features. At the meso‐β scale, duct‐top height and strength were consistently lower in the sensitivity runs than those in the control run because of changes in the depth of the MIBL. Elimination of the north–south SST gradient resulted in a shallower and more stable MIBL. Maximizing the land–sea temperature difference increased the subsidence associated with the stronger sea‐breeze circulation in the sensitivity case and therefore decreased the depth of the MIBL. When the orography was much reduced the lower wind‐speeds in the sensitivity run were again associated with a shallower MIBL. The lower wind‐speeds resulted from the lack of the thermal‐wind effect induced by the high orography around much of The Gulf. The explicit reduction of the ambient wind also led to a reduction in depth of the MIBL. Duct type also changed in the sensitivity studies, surface ducts replacing S‐shaped ducts in those areas where the depth of the MIBL decreased. Of the meso‐β scale factors influencing the ducts, ambient wind speed was most important, closely followed by orography. The distribution of SST had the smallest effect. Copyright © 2005 Royal Meteorological Society.

The Meso-.GAMMA. scale Water Vapor Distribution Associated with a Thunderstorm Calculated from a Dense Network of GPS Receivers

The Tsukuba GPS Dense Net campaign that took place in the autumn of 2000 and in the summer of 2001 measured the meso-γ scale distribution of water vapor. As part of the campaign, 75 GPS receivers and 20 automatic meteorological observation systems (AMOS) recorded water vapor variations associated with a thunderstorm on 1 August 2001. The three-dimensional water vapor distribution in the area was estimated from slant water vapor (SWV) data derived from GPS receivers using tomographic methods. SWV is the total amount of water vapor per unit area between a GPS receiver on the ground and a GPS satellite. The SWV data used in this study were obtained with sufficient accuracy by carefully removing multi-path effects, phase center variations of the GPS antenna, and other error sources. SWV was converted to a value that was projected onto the vertical direction (VSWV), so that the influence of the elevation angle on the slant path was removed. VSWV values from adjacent receivers to individual satellites were strongly correlated with each other. Variations in VSWV depended on the GPS receiver positions relative to the developing or moving thunderstorm. Our results indicate that SWV data can provide useful information about the water vapor distribution in the vicinity of thunderstorms. Correlations between the variation of VSWV and the precipitable water vapor (PWV) distribution around the GPS receiver were also calculated. The directions of the large values of VSWV corresponded to regions of high PWV. The three-dimensional water vapor distribution, estimated tomographically, agreed well with Doppler radar-observed reflectivity. Regions of high water vapor near the surface occurred on the northern side of a region of intense reflectivity. The more humid regions above 3 km corresponded to regions where reflectivity increased. The water vapor distribution estimated from the GPS showed an increase of water vapor above a height of 1 km, which preceded the appearance of radar echoes by about 20 minutes during the thunderstorm formation.

Phase-Correcting Data Assimilation and Application to Storm-Scale Numerical Weather Prediction. Part I: Method Description and Simulation Testing

An objective method of determining and correcting phase or position errors in numerical weather prediction is described and tested in a radar data observing system simulation experiment (OSSE) addressing the forecasting of ongoing thunderstorms. Such phase or position errors are common in numerical forecasts at grid resolutions of 2–20 km (meso-γ scale). It is proposed that the process of correcting a numerical forecast field can be simplified if such errors are addressed directly. An objective method of determining the phase error in the forecast by searching for a field of shift vectors that minimizes a squared-error difference from high-resolution observations is described. Three methods of applying a phase error correction to a forecast model are detailed. The first applies the entire correction at the initial time, the second in discrete steps during an assimilation window, and the third applies the correction continuously through the model's horizontal advection process. It is shown that the phase correction method is effective in producing an analysis field that agrees with the data yet preserves the structure developed by the model. The three methods of assimilating the correction in the forecast are successful, and a long-term positive effect on the thunderstorm simulation is achieved in the simulations, even as the modeled storms go through a cycle of decline and regeneration.

Meso-gamma scale forecasts using the nonhydrostatic model LM

¶The nonhydrostatic model LM was developed for small scale operational predictions. Advances in computer development will give the possibility of operational models of a rather fine scale, which will cover the meso-gamma scale. The LM is currently applied at a scale of 7 km and an increase of the operational resolution to 2.5 km is planned for the next few years. Predictions of such high resolution require to abandon the hydrostatic assumption, which is used with most current operational weather prediction models. The LM was designed to cover all resolutions from 50 m to 50 km with an efficiency making it suitable for operational use. It is a fully elastic model, using second order centred finite differences. The time integration is done using the Klemp–Wilhelmson method, treating the slow modes by a larger time step than the fast modes. The vertical propagation of the fast waves is done implicitly. After describing the design of the LM, this paper gives examples of model predictions at the meso-γ scale. Some results of the current operational application at the resolution 7 km are presented. Deficiencies in the localisation of model generated precipitation are investigated using an idealised bell shaped mountain and applying different resolutions. In this way the convergence to the correct solution can be investigated. From these results it is concluded, that orographic filtering is necessary and the effect of such filtering on precipitation forecasts is investigated. Finally, the prediction of a squall line over northern Germany is shown in order to demonstrate the potential of the model in forecasting the meso-γ scale.

Simulation of a Summer Urban Breeze Over Paris

Numerical simulations for an anticyclonic summer episode in the Paris area have been performed at the meso-γ scale for a 48-hour period, and compared to observations from a dense operational observational network. The meteorological stations have been classified, according to the extent of urbanization of their surroundings, into four classes (central Paris, urban, suburban, and rural). The atmospheric model, coupled with an urban surface scheme, correctly reproduces the temperature (within 1 K from the observations) and humidity. The intense urban heat island during the night is also well represented. Following the validation, the model is used to quantify atmospheric effects of Paris on the bound- ary layer, through a comparison with a purely rural simulation. At night, the model simulates a neutral or even slightly unstable boundary layer to a depth of 200 m over the city. In contrast, a very stable layer formed in the countryside. During the day, the boundary layer was more turbulent and 500 m deeper over Paris; vertical velocities of up to 1 m s −1 were created over the city. This leads to an urban breeze with convergence at low levels (with winds around 5 to 7 m s −1 ), and divergence at the boundary-layer top (with similar wind speeds). The horizontal extent of the breeze reaches for more than 50 km from the city centre, and could have an important impact on pollutant diffusion in the area for calm days. Finally, three other spring cases are presented briefly. These show that an urban breeze develops if the synoptic wind is weak enough or disorganized; an urban plume develops otherwise.

The use of dual—Doppler radar data in the study of 1998 meiyu frontal precipitation in huaihe river basin

During the Meiyu period in June and July of 1998, intensified field observations have been carried out for the project “Huaihe River Basin Energy and Water Cycle Experiment (HUBEX)”. For studying Meiyu front and its precipitation in Huaihe River basin, the present paper has performed analysis on the middle and lower level wind fields in the troposphere by using the radar data obtained from the two Doppler radars located at Fengtai district and Shouxian County. From June 29 to July 3 in 1998, the continuous heavy precipitation occurred in Huaihe River basin around Meiyu front. The precipitation process on July 2 occurred within the observation range of the two Doppler radar in Fengtai district and Shouxian County. The maximum rainfall of the Meiyu front was over 100 mm in 24 h, so it can be regarded as a typical mesoscale heavy precipitation process related to Meiyu front. Based on the wind field retrieved from the dual Doppler radar, we find that there are meso-γ scale vertical circulations in the vertical cross-section perpendicular to Meiyu front, the strong upward motion of which corresponds to the position of the heavy rainfall area. Furthermore, other results obtained by this study are identical with the results by analyzing the conventional synoptic data years ago. For example: in the vicinity of 3 km level height ahead of Meiyu front there exists a southwest low-level jet; the rainstorm caused by Meiyu front mainly occurs at the left side of the southwest low-level jet; and the Meiyu front causes the intensification of the low-level convergence in front of it.

Semi-Implicit Scheme for the DWD Lokal-Modell

The fourth generation of numerical weather prediction (NWP) models is currently under development at the Deutscher Wetterdienst (DWD) consisting of a global grid point model (GME) and limited-area Lokal-Modell (LM). The nonhydrostatic fully compressible LM has been designed to meet high-resol ution regional fore-cast requirements at meso-β and meso-γ scales. The initial LM implementation is based on the NCAR/Penn State MM5 with the addition of a novel generalized terrain-following coordinate and rotated lat-lon grid. A fully 3D semi-implicit time-stepping scheme has been implemented by retaining the full buoyancy term instead of the approximate form found in MM5. In contrast with earlier schemes, mass-lumping is not applied to simplify the elliptic operator on an Arakawa-C/Lorenz grid. The resulting variable-coefficient elliptic problem is solved using a minimal residual Krylov iterative method with line relaxation preconditione rs. The new semi-implicit scheme is compared with a variant of the Klemp–Wilhelmson split-explicit scheme (horizontal explicit, vertical implicit) on the basis of computational efficiency and accuracy at resolutions ranging from 7 km to 400 m. Both idealized 3D mountain wave flows and naturally occuring flows are analyzed. Below the tropopause, the 3D semi-implicit scheme can be more efficient for low Mach number M ≪ 1 flows when the number of small time steps Δt s of the split-explicit approach increases with the sound-speed Courant number.

滑らかにうねった地表面上における中立条件時の風速と比湿の鉛直プロファイルの観測

Analyses were made of wind speed and specific humidity profiles under neutral conditions obtained by radiosoundings over the gently rolling terrain of the Little Washita Basin, Oklahoma. Inspection of the wind speed profiles showed that the regional (scale of 1 to 10 km) roughness of this basin was z 0 = 0.45 ± 0.21 in, while the displacement height, do, was found to be 8.9 m. The logarithmic layer of wind speed was observed to occupy the range, (38 ± 43)z 0 ≤ (z - d 0 ) ≤ (297 ± 147)z 0 or (0.15 ± 0.10)h i where h i is the height of the bottom of the capping inversion. This result is comparable with those of previous investigations for the lower limit of the range; however, this result also suggests that over this terrain h i is the relevant length scale for the upper limit. Evaporation values at the regional scale were computed by combining the wind speed profiles with the specific humidity profiles. Comparison with surface flux measurements at Bowen ratio ground stations supports the idea that profiles in the neutral atmospheric surface layer aloft at around 100 m above the ground are conditioned by upwind surface characteristics at the meso-γ scale.

A Dual-Doppler Radar Study of Longitudinal-Mode Snowbands. Part I: A Three-Dimensional Kinematic Structure of Meso-γ-Scale Convective Cloud Systems within a Longitudinal-Mode Snowband

Abstract Observations of snowstorms were carried out around Ishikari Bay, Hokkaido, Japan, from December 1991 to February 1992. On 15 January 1992, a longitudinal-mode snowband was observed by a dual-Doppler radar system. The snowband was composed of meso-γ-scale (∼20 km) convective cloud systems. The three-dimensional kinematic structures and organization processes of the meso-γ-scale systems were studied in detail. Strong band-parallel winds appeared in the upper-north rear of the meso-γ-scale systems. Since their speed was faster than that of the cellular radar echoes, the winds formed rear-to-front currents. Increasing in volume and speed, the rear-to-front currents developed into meso-γ scale and penetrated toward the lower front and lower south of the systems. Consequently, the rear-to-front currents caused strong meso-γ-scale convergence and the enhancement of updrafts at their leading edges. The transport of the band-parallel momentum greatly contributed to the successive development of new convec...

Uncertainty of Boundary Layer Heat Budgets Computed from Wind Profiler—RASS Networks

Uncertainties in the evaluation of the atmospheric heat budget, in which the turbulent heat flux divergence term is calculated as a residual, are investigated for a triangular array of 915-MHz wind profilers—radio acoustic sounding systems (RASS) using a surface-integral method. A scaling analysis of the residual error heat budget equation reveals the basic characteristics and magnitudes of the uncertainties. These values are verified with a Monte Carlo simulation technique for synthetic datasets in which the triangle size is of the order of 30 km (meso-γ scale). The uncertainties depend on measurement errors, atmospheric stability, mean wind speed, triangle size, and averaging time. In addition, we estimate the effects of baroclinity and mean wind divergence on the accuracy of the calculation of the heat budget. Idealized, barotropic, and divergence-free conditions are studied to investigate the influence of various instrument accuracies on profiles of the turbulent virtual potential temperature flux divergence term. Results show that this term can be computed as a residual of the other terms with an uncertainty that varies from approximately 0.4 to 1.6 K h−1 for typical ranges of mean wind speed and stability, given current accuracies for 1-h averages of wind profiler—RASS. Uncertainties of the remaining terms in the equation are smaller. Although the uncertainties found are of about the same magnitude as typical maximum daytime boundary layer turbulent sensible heat flux divergences, 1.2 K h−1, it is found that under favorable conditions meaningful turbulent heat flux divergences can be obtained. The computations, however, become very uncertain under conditions of strong baroclinity or wind divergence.

Dynamical Characteristics of Cirrus Clouds from Aircraft and Radar Observations in Micro and Meso-γ Scales

Abstract Cirrus clouds that formed on 26 November and 6 December 1991 during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE) II, which took place over the Kansas region. are studied because of significant dynamic activity in the micro (<1 km) and meso γ (<25 km) scales within the cloud. Observations are obtained from the NCAR King Air, NOAA Doppler, and PSU conventional radar. For this reason coherent structures (e.g., cells, vortex) that transfer significant heat, moisture, and turbulence are analyzed using aircraft and radar observations. Aircraft data is collected at 20 Hz, and calculations are made at two different scales. Scale separation is chosen at about 1 km. A coherence analysis technique is used to specify the correlation between temperature and vertical velocity w fluctuations. A swirling coefficient, indicating spirality, is calculated to better understand cloud dynamics. Sensible heat, latent heat, and radiative fluxes are compared with each other in tw...

An explicit three-dimensional nonhydrostatic numerical simulation of a tropical cyclone

A nonhydrostatic numerical simulation of a tropical cyclone is performed with explicit representation of cumulus on a meso-β scale grid and for a brief period on a meso-γ scale grid. Individual cumulus plumes are represented by a combination of explicit resolution and a 1.5 level closure predicting turbulent kinetic energy (TKE).

Organization and Structure of Clouds and Precipitation on the Mid-Atlantic Coast of the United States. Part IV: Retrieval of the Thermodynamic and Cloud Microphysical Structures of a Frontal Rainband from Doppler Radar Data

Abstract The thermodynamic and cloud structure of a relatively weak wintertime frontal rainband are derived from dual-Doppler radar measurements, soundings and high resolution surface observations, and with a cloud model. These derivations are simplified by the high degree of two-dimensionality and steadiness of the rainband. Water vapor, cloud water, cloud ice, rain, and snow are parameterized in the cloud model, subject to a temperature distribution that is constrained to a dynamical balance. Air temperature is derived from buoyancy, which is retrieved from the airflow assuming momentum and heat balance. The results of the thermodynamic and cloud microphysical retrieval are compared with airborne measurements in the rainband. The analysis indicates that the rainband was driven by a weak cold front aloft (CFA), which made the prefrontal air conditionally symmetrically unstable. The CFA appeared as a midlevel intrusion of cold, dry air on the mesoγ scale. The CFA interacted dynamically with the planetary ...

Role of detailed wind-topography interaction in orographic rainfall

A meso γ-scale (Δx = 1 km) model for orographic rainfall was used to investigate the dependence of orographic enhancement on wind speed and direction and on detailed topography. Applying the model to cases reported in south Wales, high sensitivity to wind direction and the corresponding upwind topographical cross-sections—factors neglected in some previous studies—was found. This suggests that studies of high-resolution orographical rainfall characteristics (such as orographic enhancement as function of wind speed) or model verification should account for detailed interaction of the wind vector with the high-resolution topography. Comparison with microphysical models illustrates that the effect of detailed wind-topography interaction is at least of similar magnitude to that due to microphysical processes.

Role of detailed wind‐topography interaction in orographic rainfall

AbstractA meso γ‐scale (Δx = 1 km) model for orographic rainfall was used to investigate the dependence of orographic enhancement on wind speed and direction and on detailed topography. Applying the model to cases reported in south Wales, high sensitivity to wind direction and the corresponding upwind topographical cross‐sections—factors neglected in some previous studies—was found. This suggests that studies of high‐resolution orographical rainfall characteristics (such as orographic enhancement as function of wind speed) or model verification should account for detailed interaction of the wind vector with the high‐resolution topography. Comparison with microphysical models illustrates that the effect of detailed wind‐topography interaction is at least of similar magnitude to that due to microphysical processes.