Evening and Morning Transition of Katabatic Flows

An experimental investigation of the evening and morning transition phases of katabatic slope flows has been conducted to identify the mechanisms for their development and destruction over an isolated slope. The momentum and energy equations of the flow have been used to describe these mechanisms for the particular topographic features of the studied slope, and to outline the differences from the dynamics of well-developed simple slope flows. In the lowest portion of the slope, frontal characteristics have been identified in early evening periods when the local pre-existing near-surface thermal structure does not impose a katabatic acceleration. The frontal shape is determined by the near-surface thermal stability and ambient wind. The flow initiation is distinctly different when it is linked to the local surface cooling, in which case it develops gradually and produces a slight local warming.The erosion of the katabatic layer at mid-slope precedes that at the foot and is closely linked to dilution of the local surface inversion. The flow erosion at the foot is often delayed, as the warming of air proceeds uniformly at all heights near the ground, so maintaining the inversion due to warming produced by mixing and advective processes linked to the upslope flow development. The latter initiates first at mid-slope and then at the foot, where for a non-negligible time period it flows over the persistent katabatic flow. The prerequisite for the development of this structure is the maintenance of a shallow inversion in the first 2–3 m above the ground surface.The morning dilution of the katabatic flow is apparently different from common experience over simple slopes and may be attributed to the steep upper portion of the slope in association with its easterly orientation, which results in strong non-uniformity of the solar heating along the slope.     

A study of orographic effects on mountain-generated precipitation systems under weak synoptic forcing

Summary ?Mountains profoundly impact precipitation systems in Taiwan, particularly in areas occupying roughly two-thirds of the island’s landmass. This study examines the terrain structures possibly affecting the formation of rainfall systems in northern Taiwan by analyzing radar data, surface rainfall data, and simulation results from MM5 (Fifth-Generation NCAR/Penn State Mesoscale Model) under a weak synoptic influence condition. More specifically, this study analyzes precipitation systems formed in three different days with different ambient wind directions (i.e., southwesterly, southerly and south-southeasterly flows) in a low Froude number regime in Mei-Yu (or Baiu) season. The southwesterly (southerly) predominant wind was blocked by CMR (central mountain range) over southwestern (southern) Taiwan. Consequently, the southwesterly (southerly) winds were diverted around southern Taiwan, traveled northward following the terrain contour of CMR and then converged in northeastern (northern) Taiwan to produce a NE-SW (N-S) orientated convergence area. As anabatic flow and onshore flow intensified in northern Taiwan and thus enhanced the existing convergence in the late morning and early afternoon, the precipitation system appeared over slope first and then moved down the slope following the predominant wind direction. Upwards motion persisted in this convergence region, and initiated a new precipitation system. Consequently, rainfall accumulation was orientated in a NE-SW (N-S) direction in northern Taiwan. On the windward side of CMR in central Taiwan, precipitation was first produced in the slope by anabatic flow and was generated in lower land because of the interaction between down slope and onshore flow in the late afternoon. When the flow was predominantly from the south-southeast, the convergence due to the splitting of the predominant over western Taiwan became weaken after onshore flow over west coast developed since the direction of onshore flow was against the splitting predominant flow. Precipitation only appeared in the sloping areas of northwestern and central Taiwan in the relatively dry environment resulting from the anabatic flow. Several sensitivity tests indicated that the lee-side convergence in a low Froude number regime superimposed by anabatic flow and onshore flow is important for producing rainfall in northern Taiwan. The prevailing wind direction determined the orientation of the rainfall accumulation in northern Taiwan. The high relative humidity is important for precipitation to form in lower elevations. Received February 9, 2001; Revised November 23, 2001     

Numerical Modelling of Katabatic Flow Over a Melting Outflow Glacier

A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, ‘a state of the art’ mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 °C while the air temperature warms adiabatically as it descends the slope. This provides a ‘self enhanced’ negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large ‘curvature term’ in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of non-local processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum.     

Temperature And Wind Velocity Oscillations Along a Gentle Slope During Sea-Breeze Events

The flow structure on a gentle slope at Vallon dOl in the northern suburbs of Marseille in southern France has been documented by means of surface wind and temperature measurements collected from 7 June to 14 July 2001 during the ESCOMPTE experiment. The analysis of the time series reveals temperature and wind speed oscillations during several nights (about 60--90 min oscillation period) and several days (about 120–180 min oscillation period) during the whole observing period. Oscillating katabatic winds have been reported in the literature from theoretical, experimental and numerical studies. In the present study, the dynamics of the observed oscillating katabatic winds are in good agreement with the theory.In contrast to katabatic winds, no daytime observations of oscillating anabatic upslope flows have ever been published to our knowledge, probably because of temperature inversion break-up that inhibits upslope winds. The present paper shows that cold air advection by a sea breeze generates a mesoscale horizontal temperature gradient, and hence baroclinicity in the atmosphere, which then allows low-frequency oscillations, similar to a katabatic flow. An expression for the oscillation period is derived that accounts for the contribution of the sea-breeze induced mesoscale horizontal temperature gradient. The theoretical prediction of the oscillation period is compared to the measurements, and good agreement is found. The statistical analysis of the wind flow at Vallon dOl shows a dominant north-easterly to easterly flow pattern for nighttime oscillations and a dominant south-westerly flow pattern for daytime oscillations. These results are consistent with published numerical simulation results that show that the air drains off the mountain along the maximum slope direction, which in the studied case is oriented south-west to north-east.     

Numerical simulations of katabatic jumps in coats land,Antartica

A non-hydrostatic numerical model, the Regional Atmospheric Modeling System (RAMS), has been used to investigate the development of katabatic jumps in Coats Land, Antarctica. In the control run with a 5 m s^-1downslope directed initial wind, a katabatic jump develops near the foot of the idealized slope. The jump is manifested as a rapid deceleration of the downslope flow and a change from supercritical to subcritical flow, in a hydraulic sense, i.e., the Froude number (Fr) of the flow changes from Fr > 1 to Fr> 1. Results from sensitivity experiments show that an increase in the upstream flow rate strengthens the jump, while an increase in the downstream inversion-layer depth results in a retreat of the jump. Hydraulic theory and Bernoulli's theorem have been used to explain the surface pressure change across the jump. It is found that hydraulic theory always underestimates the surface pressure change, while Bernoulli's theorem provides a satisfactory estimation. An analysis of the downs balance for the katabatic jump indicates that the important forces are those related to the pressure gradient, advection and, to a lesser extent, the turbulent momentum divergence. The development of katabatic jumps can be divided into two phases. In phase I, the t gradient force is nearly balanced by advection, while in phase II, the pressure gradient force is counterbalanced by turbulent momentum divergence. The upslope pressure gradient force associated with a pool of cold air over the ice shelf facilitates the formation of the katabatic jump.     

Modification of the local winds due to hypothetical urbanization of the Zagreb surroundings

Summary The aim of this study was to investigate possible effects of two hypothetical scenarios of the urbanization of Zagreb’s surroundings on the local winds, which are established under summertime anticyclonic conditions. For this purpose, the nonhydrostatic mesoscale meteorological model MEMO was applied to the greater Zagreb area. Three simulations were performed. One employed the current land-use distribution, while the other two corresponded to an increase of the densely urbanized area by 12.5% (test 1) and 37.5% (test 2), respectively. Apart from the hypothetically urbanized areas, where average surface wind speed reductions of 8% and 18% were obtained for test 1 and test 2, respectively, the rest of the domain was not significantly affected by hypothetical urbanization. The differences between the wind vectors for the predicted current state and the hypothetical state were more pronounced and found at higher altitudes during the night compared to daytime values. For all three simulations the same diurnal variation of the depth of anabatic/katabatic wind flow generated on south-facing slopes of 1 km high mountain Medvednica was obtained. During the night the depth of well-developed katabatic flow was about 370 m, while during the day the depth of anabatic flow grew from about 550 m in the late morning up to about 1140 m in the late afternoon. Received October 27, 2000 Revised August 4, 2001     

干、湿环境下中尺度对流系统发生的环流背景和地面特征分析

对2007～2010 年暖季(6～9 月)发生在江淮和黄淮流域46 个对流天气过程的环流背景和地面特征进行了统计研究。根据整层可降水量小于或大于等于50 mm 将这些个例发生的环境分成干环境(10 个个例)和湿环境(36 个个例)。干环境下发生强对流的天气形势可以分为槽后型和副高边缘型,湿环境下的天气形势可分为槽前型、副高边缘型和槽后型,湿环境下有明显的暖湿区配合。湿环境下槽前型发生的概率最高,地面系统较为复杂,有静止锋、倒槽、冷锋和暖锋,而干环境下在本研究的个例中无槽前型发生。干、湿环境下副高边缘型的对流,从地面到500 hPa 都发生在副高后部的“S”流型的拐弯处,但部分湿环境个例低层有切变线。干环境下槽后型的发生概率较高,而湿环境下发生概率则相对较少。由这些研究表明,干、湿环境下强对流系统的触发和维持机制存在明显的差异。     

Structure and Dynamics of Katabatic Flow Jumps: Idealised Simulations

For the first time, results from a high-resolution numerical simulation (with horizontal grid spacing of 35m) were used to reveal the detailed structure near an atmospheric katabatic jump over an idealized slope. The simulation represents flow over the slopes of Coats Land, Antarctica for austral winter conditions. The katabatic jump is characterised by an updraft with vertical velocities of order 1ms⁻¹ and serves as a possible forcing mechanism for the gravity waves frequently observed over the ice shelves around the Antarctic. Results also indicate that strong turbulence is generally confined within a mixing zone near the top of the katabatic layer upstream of the jump and extends downstream through the top of the strong updraft associated with the jump. Detailed analyses of momentum and heat budgets across the katabatic jump indicate that, upstream of the jump, turbulent mixing is important in decelerating the upper part of the katabatic layer, while within the jump the upslope pressure gradient force associated with the pool of cold air plays a role in decelerating the flow near the surface. The heat budget near the jump reveals a simple two-term balance: the turbulent heat flux divergence is balanced by the advection. A comparison of model results with available theories indicates that mixing between layers of different potential temperature structure indeed plays some role in the development of katabatic flow jumps, especially for strong jumps. Theories used to study katabatic jumps should include this mixing process, of which the amount depends on the intensity of the jump. A conceptual model of a katabatic jump, including the main dynamical processes, is constructed from these detailed analyses.     

Mean Characteristics of the Katabatic Flow of a 1024 m High Knife Edge Mountain

Summary In this study an attempt is made to examine and analyse the mean characteristics of the katabatic flows at the western slope foot of a 1024 m high knife edge mountain using a meteorological tower and three surface meteorological stations. In addition, the frequency distribution of the occurrence of the katabatic flow over one year period is studied along the characteristics of the flow arriving in the neighbouring urban area at a distance of 1.5 km. It was found that the katabatic flow occurs mainly in autumn and spring with the highest frequency in April. The flow is generally characterised by small depth as it is affected substantially by the background flow. The expected direction of the katabatic wind dominates mainly at the level of 7 m, where the influence of the background flow is minimised. At the level of 18 m the wind direction shifts, due to the interaction of the katabatic wind with the background flow. The katabatic flow can penetrate at a distance of 1.5 km being substantially weakened. Received September 18, 1996 Revised August 4, 1997     

Aircraft-Based Measurements Of Turbulence Structures In The Katabatic Flow Over Greenland

Turbulence structures in the katabatic flow in the stable boundary layer (SBL) over the ice sheet are studied for two case studies with high wind speeds during the aircraft-based experiment KABEG (Katabatic wind and boundary layer front experiment around Greenland) in the area of southern Greenland. The aircraft data allow the direct determination of turbulence structures in the katabatic flow. For the first time, this allows the study of the turbulence structure in the katabatic wind system over the whole boundary layer and over a horizontal scale of 80 km.The katabatic flow is associated with a low-level jet (LLJ), with maximum wind speeds up to 25 m s^-1. Turbulent kinetic energy (TKE) and the magnitude of the turbulent fluxes show a strong decrease below the LLJ. Sensible heat fluxes at the lowest level have values down to -25 W m^-2. Latent heat fluxes are small in general, but evaporation values of up to +13 W m^-2 are also measured. Turbulence spectra show a well-defined inertial subrange and a clear spectral gap around 250-m wavelength. While turbulence intensity decreases monotonously with height above the LLJ for the upper part of the slope, high spectral intensities are also present at upper levels close to the ice edge. Normalized fluxes and variances generally follow power-law profiles in the SBL.Terms of the TKE budget are computed from the aircraft data. The TKE destruction by the negative buoyancy is found to be very small, and the dissipation rate exceeds the dynamical production.     

Influence of canyon-induced flows on flow and dispersion over adjacent plains

Summary The study investigates two effects that a valley or canyon opening onto a plain can have on flow and contaminant dispersion over the downwind plain. The first effect is the channeling of strong ambient flow by the canyon when the wind is nearly aligned with the canyon axis. Two cases showed that these conditions produced a region of focused flow downwind of the canyon mouth. The second effect is the formation of canyon exit jets on nights with weaker ambient flow. In two case studies under these conditions strong exit jets formed that were several hundred meters deep. The jets remained narrow and strong at least 10 km onto the plains, and in one of the cases the jet extended more than 20 km over the plains. These deep jets only lasted 2–3 h, and they had a small but significant effect on surface-released tracer transport as indicated by surface sampling. We hypothesize that the near-surface advection of tracer was accomplished by a thin katabatic layer of flow, and that an elevated release or elevated sampling would have indicated a greater effect of the exit jet on tracer transport.With 18 Figures     

Influence of Topography and Large-scale Forcing on the Occurrence of Katabatic Flow Jumps in Antarctica： Idealized Simulations

The Regional Atmospheric Modeling System （RAMS）, which is a non-hydrostatic numerical model, has been used to investigate the impact of terrain shape and large-scale forcing on the Antarctic surface-wind regime, focusing on their roles in establishing favorable flow conditions for the formation of katabatic flow jumps. A series of quasi-2D numerical simulations were conducted over idealized slopes representing the slopes of Antarctica during austral winter conditions. Results indicate that the steepness and variations of the underlying slope play a role in the evolution of near-surface flows and thus the formation of katabatic flow jumps. However, large-scale forcing has a more noticeable effect on the occurrence of this small-scale phenomenon by establishing essential upstream and downstream flow conditions, including the upstream supercritical flow, the less stably stratified or unstable layer above the cold katabatic layer, as well as the cold-air pool located near the foot of the slope through an interaction with the underlying topography. Thus, the areas with steep and abrupt change in slopes, e.g. near the coastal areas of the eastern Antarctic, are preferred locations for the occurrence of katabatic flow jumps, especially under supporting synoptic conditions.     

Aspects of the Fine-Scale Climatology Over Lake Tanganyika as Resolved by a Mesoscale Model

Summary  An hourly averaged climatology at 0.05 ° horizontal resolution over the Lake Tanganyika region was created by making simulations with a mesoscale model (HIRLAM) using a high resolution physiography to represent the surface. Initial and boundary values were interpolated from ECMWF analyses. Climatologies for a typical dry season month (July 1994) and wet season month (March 1994) were created by 7-day segmenting. Model results were validated by utilizing a special coastal observation network. A number of experiments were made with changes to the physiography (mountains/no mountains, lake/no lake). The results reveal local channelling and blocking effects of the near-surface southeasterly trade winds by the high mountain chains in the region of the East African rift. Furthermore, surface winds display regular diurnal cycles in many places, due to slope winds over hills and lake-land-type breezes near the coast. The diurnal coastal winds (defined by the observation network) are reasonably well simulated. Precipitation patterns display the semi-annual march of the ITCZ across the area, plus considerable topographic effects. There is high evaporation from lakes and wetlands during the windy dry season, while evaporation from the moist land surface dominates the rainy season. Received October 15, 1998/Revised September 2, 1999     

Numerical Simulation of Wind and Temperature Fields over Beijing Area in Summer

1. IntroductionThe basic role of urban-rural boundary layer re-search is to study all kinds of physical process changesin the atmosphere boundary layer over urban and itssurrounding areas. Urban heat island (UHI) is a well-known feature of urban-rural climate. Attempts toincrease the understanding of the causes of the UHIand other urban-rural boundary layer phenomena haveused observational, theoretical and modelling methodssince long before. Seaman (1989) used a hydrostaticmodel, with real …     

Analytical and Numerical Investigation of Two-Dimensional Katabatic Flow Resulting from Local Surface Cooling

The analysis of katabatic flows is often complicated by heterogeneity in surface characteristics. This study focuses on an idealized type of katabatic flow driven by a simple form of inhomogeneous surface forcing: a buoyancy or buoyancy flux that varies down the slope as a top-hat profile (cold strip). We consider the two-dimensional Boussinesq system of governing flow equations with the slope angle, Brunt–Väisälä frequency, and coefficients of eddy viscosity and diffusivity treated as constants. The steady-state problem is solved analytically in a linearized boundary-layer framework. Key flow structures are a primary katabatic jet (essentially the classical one-dimensional Prandtl jet), a rotor-like feature straddling the upslope end of the strip, and two nearly horizontal jets: an inward jet of environmental air feeding into the primary jet on the upslope end of the strip and an outward jet resulting from the intrusion of the primary katabatic jet into the environment on the downslope end of the strip. Next, the corresponding nonlinear initial value problem is solved numerically until a steady state is reached at low levels. The main features of the linear solution are seen in the numerical results, but with some notable differences: (i) the primary jet in the numerical simulation requires a longer distance to attain a one-dimensional boundary-layer structure and extends further downslope off the strip before intruding into the environment; (ii) the numerically simulated outward environmental jet is narrower and more intense than the inward jet, and has a pronounced wave-like structure.     

Numerical Study of Boundary Layer Structure and Rainfall after Landfall of Typhoon Fitow(2013): Sensitivity to Planetary Boundary Layer Parameterization

The boundary layer structure and related heavy rainfall of Typhoon Fitow(2013), which made landfall in Zhejiang Province, China, are studied using the Advanced Research version of the Weather Research and Forecasting model, with a focus on the sensitivity of the simulation to the planetary boundary layer parameterization. Two groups of experiments—one with the same surface layer scheme and including the Yonsei University(YSU), Mellor–Yamada–Nakanishi–Niino Level 2.5,and Bougeault and Lacarrere schemes; and the other with different surface layer schemes and including the Mellor–Yamada–Janjic′ and Quasi-Normal Scale Elimination schemes—are investigated. For the convenience of comparative analysis, the simulation with the YSU scheme is chosen as the control run because this scheme successfully reproduces the track, intensity and rainfall as a whole. The maximum deviations in the peak tangential and peak radial winds may account for 11% and 33%of those produced in the control run, respectively. Further diagnosis indicates that the vertical diffusivity is much larger in the first group, resulting in weaker vertical shear of the tangential and radial winds in the boundary layer and a deeper inflow layer therein. The precipitation discrepancies are related to the simulated track deflection and the differences in the simulated low-level convergent flow among all tests. Furthermore, the first group more efficiently transfers moisture and energy and produces a stronger ascending motion than the second, contributing to a deeper moist layer, stronger convection and greater precipitation.     

Determining the Oxygen Isotope Composition of Evapotranspiration Using Eddy Covariance

Impacts of different terrain configurations on the general behaviour of idealised katabatic flows are investigated in a numerical model study. Various simplified terrain models are applied to unveil modifications of the dynamics of nocturnal cold drainage of air as a result of predefined topographical structures. The generated idealised terrain models encompass all major topographical elements of an area in the tropical eastern Andes of southern Ecuador and northern Peru, and the adjacent Amazon. The idealised simulations corroborate that (i) katabatic flows develop over topographical elements (slopes and valleys), that (ii) confluence of katabatic flows in a lowland basin with a concave terrainline occur, and (iii) a complex drainage flow system regime directed into such a basin can sustain the confluence despite varying slope angles and slope distances.     

Random-walk model studies of the transport and diffusion of pollutants in katabatic flows

The flow and turbulence quantities governing dispersion in katabatic flows vary with both height and downslope distance. This variation cannot be accounted for in conventional plume dispersion models. In this study, three random-walk models of varying complexity are formulated to simulate dispersion in katabatic flows, and their strengths and weaknesses are discussed. The flow and turbulence parameters required by these models are determined from a high-resolution two-dimensional katabatic flow model based on a turbulent kinetic energy closure. Random-walk model calculations have been performed for several values of source height and slope angle to examine the influence of these parameters on dispersion. Finally, we simulated the perfluorocarbon and heavy methane tracer releases for Night 4 of the 1980 ASCOT field study over a nearly two-dimensional slope in Anderson Creek Valley, California. The observed peak concentrations are generally well-predicted. The effects of the pooling of the drainage air could not be taken into account in our katabatic flow model and, consequently, the predicted concentrations decay much more rapidly with time than the observed values.     

Theoretical determination of the surface energy balance and thermal regimes of bare soils

An analytical theory that determines the thermal regimes in the soil and the thermal and moisture regimes in the atmosphere for bare surfaces is derived. Both soil and atmospheric thermal properties are assumed to be power functions of depth and height, respectively. Evaporation is determined using a surface resistance to vapour flow. Fourier superposition is used to represent nonsinusoidal variations in time due to effects such as variable cloud cover. The theory is in acceptable agreement with micrometeorological measurements made at two bare soil sites of contrasting surface bulk density. It is concluded that the surface resistance model for evaporation is applicable to bare soils which remain wet at depth, particularly if their surface is loosened. The theory is used to predict the diurnal thermal regimes of saturated and dry sand, loam, and peat soils.     

The thermoinsulation effect of snow cover within a climate model

We use a state of the art climate model (CAM3–CLM3) to investigate the sensitivity of surface climate and land surface processes to treatments of snow thermal conductivity. In the first set of experiments, the thermal conductivity of snow at each grid cell is set to that of the underlying soil (SC-SOIL), effectively eliminating any insulation effect. This scenario is compared against a control run (CTRL), where snow thermal conductivity is determined as a prognostic function of snow density. In the second set of experiments, high (SC-HI) and low (SC-LO) thermal conductivity values for snow are prescribed, based on upper and lower observed limits. These two scenarios are used to envelop model sensitivity to the range of realistic observed thermal conductivities. In both sets of experiments, the high conductivity/low insulation cases show increased heat exchange, with anomalous heat fluxes from the soil to the atmosphere during the winter and from the atmosphere to the soil during the summer. The increase in surface heat exchange leads to soil cooling of up to 20 K in the winter, anomalies that persist (though damped) into the summer season. The heat exchange also drives an asymmetric seasonal response in near-surface air temperatures, with boreal winter anomalies of +6 K and boreal summer anomalies of −2 K. On an annual basis there is a net loss of heat from the soil and increases in ground ice, leading to reductions in infiltration, evapotranspiration, and photosynthesis. Our results show land surface processes and the surface climate within CAM3–CLM3 are sensitive to the treatment of snow thermal conductivity.