scholarly journals A Cloud-Resolving Simulation Study on the Merging Processes and Effects of Topography and Environmental Winds

2012 ◽  
Vol 69 (4) ◽  
pp. 1232-1249 ◽  
Author(s):  
Danhong Fu ◽  
Xueliang Guo
Keyword(s):  
Water Vapor ◽  
Pressure Gradient ◽  
Large Scale ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Low Level ◽  
Level Pressure ◽  
Merging Process ◽  
Upper Level ◽  
Water Vapor Convergence

Abstract The cloud-resolving fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) was used to study the cloud interactions and merging processes in the real case that generated a mesoscale convective system (MCS) on 23 August 2001 in the Beijing region. The merging processes can be grouped into three classes for the studied case: isolated nonprecipitating and precipitating cell merging, cloud cluster merging, and echo core or updraft core merging within cloud systems. The mechanisms responsible for the multiscale merging processes were investigated. The merging process between nonprecipitating cells and precipitating cells and that between clusters is initiated by forming an upper-level cloud bridge between two adjacent clouds due to upper-level radial outflows in one vigorous cloud. The cloud bridge is further enhanced by a favorable middle- and upper-level pressure gradient force directed from one cloud to its adjacent cloud by accelerating cloud particles being horizontally transported from the cloud to its adjacent cloud and induce the redistribution of condensational heating, which destabilizes the air at and below the cloud bridge and forms a favorable low-level pressure structure for low-level water vapor convergence and merging process. The merging of echo cores within the mesoscale cloud happens because of the interactions between low-level cold outflows associated with the downdrafts formed by these cores. Further sensitivity studies on the effects of topography and large-scale environmental winds suggest that the favorable pressure gradient force from one cloud to its adjacent cloud and stronger low-level water vapor convergence produced by the topographic lifting of large-scale low-level airflow determine further cloud merging processes over the mountain region.

scholarly journals On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica

2019 ◽  
Author(s):  
Étienne Vignon ◽  
Olivier Traullé ◽  
Alexis Berne
Keyword(s):  
Pressure Gradient ◽  
Vertical Structure ◽  
East Antarctica ◽  
Radiosonde Data ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Near Surface ◽  
Ice Shelves ◽  
Low Level ◽  
Humidity Profiles

Abstract. Eight years of high-resolution radiosonde data at nine Antarctic stations are analysed to provide the first large scale characterization of the fine scale vertical structure of the low troposphere up to 3 km of altitude over the coastal margins of East Antarctica. Radiosonde data show a large spatial variability of wind, temperature and humidity profiles, with different features between stations in katabatic regions (e.g., Dumont d'Urville and Mawson stations), stations over two ice shelves (Neumayer and Halley stations) and regions with complex orography (e.g., Mc Murdo). At Dumont d'Urville, Mawson and Davis stations, the yearly median wind speed profiles exhibit a clear low-level katabatic jet. During precipitation events, the low-level flow generally remains of continental origin and its speed is even reinforced due to the increase in the continent- ocean pressure gradient. Meanwhile, the relative humidity profiles show a dry low troposphere, suggesting the occurence of low-level sublimation of precipitation in katabatic regions but such a phenomenon does not appreciably occur over the ice-shelves near Halley and Neumayer. Although ERA-Interim and ERA5 reanalyses assimilate radiosoundings at most stations considered here, substantial – and sometimes large – low-level wind and humidity biases are revealed but ERA5 shows overall better performances. A free simulation with the regional model Polar WRF (at a 35-km resolution) over the entire continent shows too strong and too shallow near-surface jets in katabatic regions especially in winter. This may be a consequence of an understimated coastal cold air bump and associated sea-continent pressure gradient force due to the coarse 35 km resolution of the Polar WRF simulation. Beyond documenting the vertical structure of the low troposphere over coastal East-Antarctica, this study gives insights into the reliability and accuracy of two major reanalysis products in this region on the Earth and it raises the difficulty of modeling the low-level flow over the margins of the ice sheet with a state-of-the-art climate model.

scholarly journals The Deflection of the China Coastal Current over the Taiwan Bank in Winter

2018 ◽  
Vol 48 (7) ◽  
pp. 1433-1450 ◽  
Author(s):  
Enhui Liao ◽  
Lie Yauw Oey ◽  
Xiao-Hai Yan ◽  
Li Li ◽  
Yuwu Jiang
Keyword(s):  
Pressure Gradient ◽  
Wind Stress ◽  
Large Scale ◽  
Numerical Models ◽  
Gradient Force ◽  
Coastal Current ◽  
Pressure Gradient Force ◽  
Bottom Stress ◽  
In Situ Data ◽  
Offshore Flow

AbstractIn winter, an offshore flow of the coastal current can be inferred from satellite and in situ data over the western Taiwan Bank. The dynamics related to this offshore flow are examined here using observations as well as analytical and numerical models. The currents can be classified into three regimes. The downwind (i.e., southward) cold coastal current remains attached to the coast when the northeasterly wind stress is stronger than a critical value depending on the upwind (i.e., northward) large-scale pressure gradient force. By contrast, an upwind warm current appears over the Taiwan Bank when the wind stress is less than the critical pressure gradient force. The downwind coastal current and upwind current converge and the coastal current deflects offshore onto the bank during a moderate wind. Analysis of the vorticity balance shows that the offshore transport is a result of negative bottom stress curl that is triggered by the positive vorticity of the two opposite flows. The negative bottom stress curl is reinforced by the gentle slope over the bank, which enhances the offshore current. Composite analyses using satellite observations show cool waters with high chlorophyll in the offshore current under moderate wind. The results of composite analyses support the model findings and may explain the high productivity over the western bank in winter.

scholarly journals On the Orographically Generated Low-Level Easterly Jet and Severe Downslope Storms of March 2006 over the Tacheng Basin of Northwest China

2018 ◽  
Vol 146 (6) ◽  
pp. 1667-1683 ◽  
Author(s):  
Guangxing Zhang ◽  
Da-Lin Zhang ◽  
Shufang Sun
Keyword(s):  
Large Scale ◽  
Inversion Layer ◽  
Dust Storms ◽  
Frozen Soil ◽  
Gradient Force ◽  
Mountain Range ◽  
Pressure Gradient Force ◽  
Low Level ◽  
Downslope Winds ◽  
The Impact

A high-latitude low-level easterly jet (LLEJ) and downslope winds, causing severe dust storms over the Tacheng basin of northwestern China in March 2006 when the dust source regions were previously covered by snow with frozen soil, are studied in order to understand the associated meteorological conditions and the impact of complex topography on the generation of the LLEJ. Observational analyses show the development of a large-scale, geostrophically balanced, easterly flow associated with a northeastern high pressure and a southeastern low pressure system, accompanied by a westward-moving cold front with an intense inversion layer near the altitudes of mountain ridges. A high-resolution model simulation shows the generation of an LLEJ of near-typhoon strength, which peaked at about 500 m above the ground, as well as downslope windstorms with marked wave breakings and subsidence warming in the leeside surface layer, as the large-scale cold easterly flow moves through a constricting saddle pass and across a higher mountain ridge followed by a lower parallel ridge, respectively. The two different airstreams are merged to form an intense LLEJ of cold air, driven mostly by zonal pressure gradient force, and then the LLEJ moves along a zonally oriented mountain range to the north. Results indicate the importance of the lower ridge in enhancing the downslope winds associated with the higher ridge and the importance of the saddle pass in generating the LLEJ. We conclude that the intense downslope winds account for melting snow, warming and drying soils, and raising dust into the air that is then transported by the LLEJ, generated mostly through the saddle pass, into the far west of the basin.

scholarly journals Low-Level Easterly Winds Blowing through the Tsugaru Strait, Japan. Part II: Numerical Simulation of the Event on 5–10 June 2003

2012 ◽  
Vol 140 (6) ◽  
pp. 1779-1793 ◽  
Author(s):  
Teruhisa Shimada ◽  
Masahiro Sawada ◽  
Weiming Sha ◽  
Hiroshi Kawamura
Keyword(s):  
Pressure Gradient ◽  
Air Temperature ◽  
Temperature Difference ◽  
Diurnal Variations ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
The West ◽  
Low Level ◽  
Tsugaru Strait ◽  
Mutsu Bay

Abstract This paper investigates the structures of and diurnal variations in low-level easterly winds blowing through the Tsugaru Strait and Mutsu Bay on 5–10 June 2003 using a numerical weather prediction model. Cool air that accompanies prevailing easterly winds owing to the persistence of the Okhotsk high intrudes into the strait and the bay below 500 m during the nighttime and retreats during the daytime. This cool-air intrusion and retreat induce diurnal variations in the winds in the east inlet of the strait, in Mutsu Bay, and in the west exit of the strait. In the east inlet, a daytime increase in air temperature within the strait produces a large air temperature difference with the inflowing cool air, and the resulting pressure gradient force accelerates the winds. The cool air flowing into Mutsu Bay is heated over land before entering the bay during the daytime. The resulting changes in cool-air depth and in pressure gradient force strengthen the daytime winds. In the west exit, local pressure gradient force perturbations are induced by the air temperature difference between warm air over the Japan Sea and cool air within the strait, and by variations in the depth of low-level cool air. The accelerated winds in the west exit extend southwestward in close to geostrophic balance during the daytime and undergo a slight anticyclonic rotation to westerly during the nighttime owing to the dominance of the Coriolis effect.

scholarly journals On the Formation Dynamics of the North Equatorial Undercurrent

2020 ◽  
Vol 50 (5) ◽  
pp. 1399-1415 ◽  
Author(s):  
Junlu Li ◽  
Jianping Gan
Keyword(s):  
Pressure Gradient ◽  
Large Scale ◽  
Relative Vorticity ◽  
Velocity Shear ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Wind Stress Curl ◽  
Equatorial Undercurrent ◽  
The North ◽  
Southern Branch

AbstractBased on a physics-oriented modeling study, we investigate the underlying forcing processes of the North Equatorial Undercurrent (NEUC). Made up of large-scale (~90%) and mesoscale (~10%) components, the NEUC weakens eastward with a longitude-independent seasonality. The large-scale component reflects the effect of the meridional baroclinic pressure gradient force (PGF_BC). The vertical velocity shear forms the eastward NEUC, when the PGF_BC exceeds the meridional barotropic pressure gradient force (PGF_BT). The mesoscale variability with alternating jets is linked to the wind stress curl in different regions of the tropical North Pacific. Spatially, the NEUC has a northern (NEUC_N) and a southern branch (NEUC_S), which are mainly attributed to the transports from Luzon Undercurrent (LUC) and Mindanao Undercurrent (MUC), respectively. The LUC of ~3 Sv (1 Sv ≡ 106 m3 s−1) feeds the NEUC_N in summer, while the MUC of ~4 Sv fuels the NEUC_S in autumn and the two branches do not coexist. The total NEUC transport peaks in August/September, and there exist three distinct periods in a 1-yr cycle: the non-NEUC period in winter, the LUC-driven period in summer, and the MUC-driven period in autumn. Based on the layer-integrated vorticity equation, we diagnose quantitatively that the variation of the NEUC is dominated by the lateral planetary vorticity influx from the LUC and the MUC. These external influxes interact with the internal dynamics of pressure torques and stress curls in the NEUC layer, to jointly govern the NEUC and its variability. Meanwhile, the nonlinearity due to relative vorticity advection near the coast modulates the strength of the NEUC.

scholarly journals Ocean dynamic equations with the real gravity

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Peter C. Chu
Keyword(s):  
Pressure Gradient ◽  
Large Scale ◽  
Vertical Component ◽  
Ocean Dynamics ◽  
Dynamic Equations ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
The Real ◽  
Ocean Dynamic ◽  
Scale Motion

AbstractTwo different treatments in ocean dynamics are found between the gravity and pressure gradient force. Vertical component is 5–6 orders of magnitude larger than horizontal components for the pressure gradient force in large-scale motion, and for the gravity in any scale motion. The horizontal pressure gradient force is considered as a dominant force in oceanic motion from planetary to small scales. However, the horizontal gravity is omitted in oceanography completely. A non-dimensional C number (ratio between the horizontal gravity and the Coriolis force) is used to identify the importance of horizontal gravity in the ocean dynamics. Unexpectedly large C number with the global mean around 24 is obtained using the community datasets of the marine geoid height and ocean surface currents. New large-scale ocean dynamic equations with the real gravity are presented such as hydrostatic balance, geostrophic equilibrium, thermal wind, equipotential coordinate system, and vorticity equation.

Graphical Determination of the Geostrophic Wind Over a Point or Small Area

1952 ◽  
Vol 33 (8) ◽  
pp. 326-328
Author(s):  
John E. Hovde ◽  
Carl M. Reber
Keyword(s):  
Pressure Gradient ◽  
Sea Level ◽  
Total Pressure ◽  
Small Area ◽  
Geostrophic Wind ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Sea Level Pressure ◽  
Level Pressure

Assuming straight, parallel, and equally-spaced contour heights or isobars over a small area, the total pressure-gradient force (and the geostrophic wind) may be determined graphically using the contour-height or sea-level pressure differences between 3 stations around the area. The method is objective and eliminates personal discrepancies in scaling geostrophic wind from pressure or contour-height analyses.

scholarly journals Extratropical Influence on 200-hPa Easterly Acceleration over the Western Indian Ocean Preceding Madden–Julian Oscillation Convective Onset

2019 ◽  
Vol 76 (1) ◽  
pp. 265-284 ◽  
Author(s):  
Jennifer Gahtan ◽  
Paul Roundy
Keyword(s):  
Indian Ocean ◽  
Pressure Gradient ◽  
Large Scale ◽  
Deep Convection ◽  
Western Indian Ocean ◽  
Eastern Africa ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Madden Julian Oscillation ◽  
Initial Development

Abstract The onset of Madden–Julian oscillation (MJO) deep convection often occurs over the western Indian Ocean and has upper-tropospheric circulation precursors that consist of eastward-circumnavigating tropical easterlies and subtropical cyclonic Rossby gyres near eastern Africa. Moreover, the evolution of the large-scale circulation and its ability to reduce subsidence may be necessary for the initial development of organized deep convection. To better understand the evolution of the circulation precursors and their interaction with convective onset, this paper analyzes the upper-tropospheric zonal momentum budget using a regional index based on the temporal progression of the meridional structure of intraseasonal outgoing longwave radiation anomalies over eastern Africa and the western Indian Ocean. The circumnavigating intraseasonal easterly acceleration produces upper-level divergence when it reaches the western extent of a region of intraseasonal westerlies and may provide a forcing for the in-phase midtropospheric upward vertical motion. For about three-quarters of the identified cases, the easterly acceleration over the western Indian Ocean is a response to the zonal pressure gradient over the region. In the composite, the negative pressure gradient force may be initially induced by the injection of negative geopotential height anomalies from the extratropics of both hemispheres to the tropics over eastern Africa, though tropically circumnavigating and local signals may also contribute to the easterly acceleration, especially in the days following convective onset.

scholarly journals The Role of Surface Drag in Mesocyclone Intensification Leading to Tornadogenesis within an Idealized Supercell Simulation

2017 ◽  
Vol 74 (9) ◽  
pp. 3055-3077 ◽  
Author(s):  
Brett Roberts ◽  
Ming Xue
Keyword(s):  
Pressure Gradient ◽  
Surface Friction ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Surface Drag ◽  
Vertical Pressure ◽  
Low Level ◽  
Physical Mechanisms ◽  
Key Factor

Abstract The idealized supercell simulations in a previous study by Roberts et al. are further analyzed to clarify the physical mechanisms leading to differences in mesocyclone intensification between an experiment with surface friction applied to the full wind (FWFRIC) and an experiment with friction applied to the environmental wind only (EnvFRIC). The low-level mesocyclone intensifies rapidly during the 3 min preceding tornadogenesis in FWFRIC, while the intensification during the same period is much weaker in EnvFRIC, which fails to produce a tornado. To quantify the mechanisms responsible for this discrepancy in mesocyclone evolution, material circuits enclosing the low-level mesocyclone are initialized and traced back in time, and circulation budgets for these circuits are analyzed. The results show that in FWFRIC, surface drag directly generates a substantial proportion of the final circulation around the mesocyclone, especially below 1 km AGL; in EnvFRIC, circulation budgets indicate the mesocyclone circulation is overwhelmingly barotropic. It is proposed that the import of near-ground, frictionally generated vorticity into the low-level mesocyclone in FWFRIC is a key factor causing the intensification and lowering of the mesocyclone toward the ground, creating a large upward vertical pressure gradient force that leads to tornadogenesis. Similar circulation analyses are also performed for circuits enclosing the tornado at its genesis stage. The frictionally generated circulation component is found to contribute more than half of the final circulation for circuits enclosing the tornado vortex below 400 m AGL, and the frictional contribution decreases monotonically with the height of the final circuit.

scholarly journals PADRÕES DE VENTO A NÍVEL DE SUPERFÍCIE PARA REGIÃO DA COSTA NORTE DO BRASIL

2016 ◽  
Vol 38 ◽  
pp. 383
Author(s):  
Luiz Eduardo Medeiros ◽  
Gilberto Fisch ◽  
Paulo Iriart ◽  
Felipe Denardin Costa ◽  
Dionnathan Willian Oliveira ◽  
...  
Keyword(s):  
Pressure Gradient ◽  
Surface Pressure ◽  
Large Scale ◽  
Surface Wind ◽  
Hadley Cell ◽  
Radiosonde Data ◽  
Momentum Balance ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Meridional Component

The atmospheric flow near the surface and in the planetary boundary layer (PBL) are investigated for the coastal part of Maranhão state. Near the coast in the PBL the flow is predominantly from the northeast quadrant with its meridional component increasing during the day and being from north-northeast and decreasing during the course of the night to be from east-northeast at early morning. The result of this is a small counterclockwise rotation but with no flow reversals. Through an analysis of extensive radiosonde data it is found that the flow above the PBL is predominantly southeasterly for the region. It is consequence of the outflow from the descending branch of the large-scale circulation of the Hadley cell. For stations further inland the flow is from approximately northeast during period between morning to noon but rotating clockwise to become from southeast-east (SEE) sector at early evening. The clockwise rotation continues in the afternoon and the wind becomes from south, and later southwest when in the evening it quickly becomes from north. The wind rotation during this period is mainly determined by an oscillating surface pressure gradient-force. During the night the local surface wind tendency is not controlled by the gradient-force probably because the air has to go against higher terrain and negative buoyancy becomes an important force of the momentum balance. The oscillating surface pressure-gradient-force is a response to a sea-breeze circulation. In the coast, we speculate that the flow does not reverse its meridional component because the surface pressure-gradient point south there for most of the time.

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