Mesoscale systems and processes
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In Section 4.3, we saw how the near-surface wind field over much of Antarctica could be explained using simple diagnostic models of the katabatic wind. Such models provide realistic simulations of the mean wind at stations where the local topographic slope is reasonably uniform and not too great. However, the neglect of the non-linear inertial terms in these simplified models is not justified in regions where the topographic slope varies significantly. In such regions the advection of momentum and heat by the katabatic wind must be taken into account in order to model the local wind system correctly.Keywords:
Katabatic wind
Momentum (technical analysis)
Two types of mesoscale wind-speed jet and their effects on boundary-layer structure were studied. The first is a coastal jet off the northern California coast, and the second is a katabatic jet ove ...
Katabatic wind
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In the Takano and Oonishi models the finite-difference analog of the nonlinear momentum advection contains the concept of diagonally upward/downward mass and momentum fluxes along the bottom slope, and the generalized Arakawa scheme for the horizontal advection, modified to be fit to arbitrary coastal shape. It has been said to have a good performance, but is not widely used, largely because of its complicated expression. The purpose of this paper is to reevaluate the Takano–Oonishi scheme for the momentum advection to put it to more practical use by using the redefinition of it in a simple, generalized form and the confirmation of its good performance through a comparison with other schemes. Based on the definition of mass continuity for a momentum cell (U cell) in terms of that for tracer cells (T cell), the vertical and horizontal mass and momentum fluxes for the U cell are generalized on arbitrary bottom relief in simple forms. Although the grid spacing of the present model is different from that of the Geophysical Fluid Dynamics Laboratory model, applicability of the present scheme to the latter grid spacing is discussed. Then, the present scheme is tested in an eddy-resolving ocean model and its results are compared with those of a traditional scheme. The present scheme shows good performance in computational efficiency as well as reality of the simulated flow field.
Momentum (technical analysis)
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Katabatic winds dramatically affect the climate of the McMurdo dry valleys, Antarctica. Winter wind events can increase local air temperatures by 30°C. The frequency of katabatic winds largely controls winter (June to August) temperatures, increasing 1°C per 1% increase in katabatic frequency, and it overwhelms the effect of topographic elevation (lapse rate). Summer katabatic winds are important, but their influence on summer temperature is less. The spatial distribution of katabatic winds varies significantly. Winter events increase by 14% for every 10 km up valley toward the ice sheet, and summer events increase by 3%. The spatial distribution of katabatic frequency seems to be partly controlled by inversions. The relatively slow propagation speed of a katabatic front compared to its wind speed suggests a highly turbulent flow. The apparent wind skip (down‐valley stations can be affected before up‐valley ones) may be caused by flow deflection in the complex topography and by flow over inversions, which eventually break down. A strong return flow occurs at down‐valley stations prior to onset of the katabatic winds and after they dissipate. Although the onset and termination of the katabatic winds are typically abrupt, elevated air temperatures remain for days afterward. We estimate that current frequencies of katabatic winds increase annual average temperatures by 0.7° to 2.2°C, depending on location. Seasonally, they increase (decrease) winter average temperatures (relative humidity) by 0.8° to 4.2° (−1.8 to −8.5%) and summer temperatures by 0.1° to 0.4°C (−0.9% to −4.1%). Long‐term changes of dry valley air temperatures cannot be understood without knowledge of changes in katabatic winds.
Katabatic wind
Automatic weather station
Prevailing winds
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It is important to assess the representativeness of mesoscale wind data because most short range pollution models assume that wind velocity will remain constant over distances in the order of 10 km. Previous observational studies have shown that average hourly mesoscale differences in wind directions and speeds might be typically about 25 degrees and 1 m s−1. Initial results of this study using all available data, tended to agree with the above findings. Further analyses, however, were performed for periods to which most pollution models are restricted. These periods are usually characterized by the absence of mesoscale wind phenomena and terrain effects associated with katabatic winds. Hourly wind direction differences for these periods were found to be typically only about 10 degrees regardless of atmospheric stability. Wind speed differences were still typically about 1 m s−1. Differences of both wind speed and direction were normally distributed, suggesting that horizontal mesoscale wind velocity differences occur randomly. For this reason it may be impractical to attempt the development of short-range plume dispersion models that physically account for horizontal inhomogeneities.
Katabatic wind
Atmospheric instability
Log wind profile
Roughness length
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Katabatic wind
Hydraulic jump
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Abstract Observations from a novel autonomous Doppler sodar wind profiling system are described and analysed. These include the first continuous wintertime soundings of katabatic winds over Antarctica—a continent with which they are synonymous. During 2002 and 2003 over 2600 wind profiles were taken during ‘case‐studies’ of high‐resolution sounding lasting hours to days. These case‐studies have been subjectively classified as: synoptically driven, katabatically influenced (28 days); primarily katabatically driven flows (a subset of 16 days); or other flow types. The Doppler sodar observations were augmented by automatic weather station observations at the field site and further up the slope, as well as synoptic and upper‐air observations at Halley Research Station, some 50 km distant on the Brunt Ice Shelf. In primarily katabatic flows there is a systematic change in the shape and depth of the low‐level katabatic jet with wind speed. Relatively strong katabatic flows (maximum winds of typically 8–10 m s −1 ) have a jet maximum between 20 and 60 m above the surface and are relatively deep (up to 200 m); while moderate katabatic flows (4–8 m s −1 ) typically have a jet maximum between 3 and 30 m and are shallower (∼100 m), although they can also be more diffuse in structure with a wind speed maximum at higher altitude. In all katabatic flows there is backing of wind direction with height, consistent with decreasing friction away from the surface. During summertime katabatic flows there is a clear diurnal signature at all heights, although this is less pronounced in the surface layer where there seems to be a persistent 2–4 m s −1 katabatic flow during all case‐studies. Where the diurnal forcing results in an abrupt katabatic flow deceleration, i.e. what may be a katabatic ‘jump’, there is a concurrent vertical acceleration. Wind profiles from a recent numerical weather prediction study of idealized katabatic flows at this site compare favourably with selected mean profiles; the only significant difference is that the model's wind speed is too low over the lowest ∼10 m. Copyright © 2006 Royal Meteorological Society.
Katabatic wind
SODAR
Automatic weather station
Wind profiler
Sea breeze
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Abstract. We collected ground-based and aircraft vertical profile measurements of meteorological parameters during a 2-week intensive campaign over the Valencia basin, in order to understand how mesoscale circulations develop over complex terrain and affect the atmospheric transport of tracers. A high-resolution version of the RAMS model was run to simulate the campaign and characterize the diurnal patterns of the flow regime: night-time katabatic drainage, morning sea-breeze development and its subsequent coupling with mountain up-slopes, and evening flow-veering under larger-scale interactions. An application of this mesoscale model to the transport of CO2 is given in a companion paper. A careful evaluation of the model performances against diverse meteorological observations is carried out. Despite the complexity of the processes interacting with each other, and the uncertainties on modeled soil moisture boundary conditions and turbulence parameterizations, we show that it is possible to simulate faithfully the contrasted flow regimes during the course of one day, especially the inland progression and organization of the sea breeze. This gives confidence with respect to future applicability of mesoscale models to establish a reliable link between surface sources of tracers and their atmospheric concentration signals over complex terrain.
Sea breeze
Katabatic wind
Diurnal cycle
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Abstract The role of advection of heat and momentum on the evolution of near-surface temperature and wind is evaluated in urban-aware simulations over Houston, Texas, under dry conditions on a light-wind day. Two sets of experiments, each consisting of four simulations using different planetary boundary layer (PBL) schemes, were conducted over 48 h using the default urban scheme (BULK) and the single-layer urban canopy model (SLUCM) available within the Weather Research and Forecasting Model. We focus on understanding and quantifying the role played by temperature and momentum advection, particularly on the windward and leeward sides of the city. Previous studies have largely ignored any quantitative analysis of impacts from the advection of momentum over an urban area. The horizontal advection of temperature was found to be more important in the BULK because of the larger surface temperature gradient caused by warmer surface temperatures over urban areas than in the SLUCM. An analysis of the momentum budget shows that horizontal advection of zonal and meridional momentum plays a prominent role during the period of peak near-surface winds and that this effect is more pronounced in the windward side of the city. The local tendency in peak winds in the leeward side lags that in the windward side by about 1–2 h, similar to the lag found in horizontal momentum advection. The sensitivity of the results to different urban and PBL schemes was explored. The results imply that representation and influence of land-use patterns via sophisticated urban parameterizations generate locally driven winds that best resemble observations.
Momentum (technical analysis)
Urban climatology
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Katabatic winds are a regular feature during night and early morning hours over valleys and low lying areas adjacent to high terrains. In Antarctica, the mass of the earth in the form of plateau is covered with full of ice and has a downslope from the South Pole everyside upto the coast. Due to cooling of air near the ground as well as due to pressure gradient, the coastal belt often experiences strong winds blowing down the slope of the plateau. In this study “katabatic wind” at Maitri over Schirmacher Oasis has been examined using the data collected during the 9th Indian Antarctic Expedition 1990-91. The primary results show that katabatic winds are observed in all the months with higher frequency and strength in winter compared to other seasons. A distinction between pure katabatic wind and extraordinary katabatic wind is done based on the wind direction and associated changes in the surface temperature and weather conditions.
Katabatic wind
Prevailing winds
Automatic weather station
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Momentum (technical analysis)
Eddy
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