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 Oceanic Impetus for Convective Onset of the Madden–Julian Oscillation in the Western Indian Ocean

2017 ◽  
Vol 30 (11) ◽  
pp. 4299-4316 ◽  
Author(s):  
Adam V. Rydbeck ◽  
Tommy G. Jensen
Keyword(s):  
Boundary Layer ◽  
Indian Ocean ◽  
Atmospheric Boundary Layer ◽  
Rossby Waves ◽  
Deep Convection ◽  
Western Indian Ocean ◽  
Convection Boundary Layer ◽  
Madden Julian Oscillation ◽  
Sst Gradient ◽  
Equatorial Rossby Waves

Abstract A theory for intraseasonal atmosphere–ocean–atmosphere feedback is supported whereby oceanic equatorial Rossby waves are partly forced in the eastern Indian Ocean by the Madden–Julian oscillation (MJO), reemerge in the western Indian Ocean ~70 days later, and force large-scale convergence in the atmospheric boundary layer that precedes MJO deep convection. Downwelling equatorial Rossby waves permit high sea surface temperature (SST) and enhance meridional and zonal SST gradients that generate convergent circulations in the atmospheric boundary layer. The magnitude of the SST and SST gradient increases are 0.25°C and 1.5°C Mm−1 (1 megameter is equal to 1000 km), respectively. The atmospheric circulations driven by the SST gradient are estimated to be responsible for up to 45% of the intraseasonal boundary layer convergence observed in the western Indian Ocean. The SST-induced boundary layer convergence maximizes 3–4 days prior to the convective maximum and is hypothesized to serve as a trigger for MJO deep convection. Boundary layer convergence is shown to further augment deep convection by locally increasing boundary layer moisture. Warm SST anomalies facilitated by downwelling equatorial Rossby waves are also associated with increased surface latent heat fluxes that occur after MJO convective onset. Finally, generation of the most robust downwelling equatorial Rossby waves in the western Indian Ocean is shown to have a distinct seasonal distribution.

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 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.

scholarly journals On the Equivalence of Two Schemes for Convective Momentum Transport

2012 ◽  
Vol 69 (12) ◽  
pp. 3491-3500 ◽  
Author(s):  
David M. Romps
Keyword(s):  
Large Eddy Simulation ◽  
Pressure Gradient ◽  
Mass Flux ◽  
Deep Convection ◽  
Momentum Transport ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Convective Momentum Transport

Abstract The Gregory–Kershaw–Inness (GKI) parameterization of convective momentum transport, which has a tunable parameter C, is shown to be identical to a parameterization with no pressure gradient force and a mass flux smaller by a factor of 1 − C. Using cloud-resolving simulations, the transilient matrix for momentum is diagnosed for deep convection in radiative–convective equilibrium. Using this transilient matrix, it is shown that the GKI scheme underestimates the compensating subsidence of momentum by a factor of 1 − C, as predicted. This result is confirmed using a large-eddy simulation.

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 Modulation of South Indian Ocean Tropical Cyclones by the Madden–Julian Oscillation and Convectively Coupled Equatorial Waves

2006 ◽  
Vol 134 (2) ◽  
pp. 638-656 ◽  
Author(s):  
Miloud Bessafi ◽  
Matthew C. Wheeler
Keyword(s):  
Indian Ocean ◽  
Large Scale ◽  
Deep Convection ◽  
Vertical Shear ◽  
Madden Julian Oscillation ◽  
South Indian Ocean ◽  
South Indian ◽  
Ocean Region ◽  
Using Data ◽  
Wave Modes

Abstract The subseasonal modulation of tropical cyclone (TC) genesis by large-scale atmospheric wave modes is studied using data from the south Indian Ocean region. The modes considered are the Madden–Julian oscillation (MJO), and the convectively coupled equatorial Rossby (ER), Kelvin, and mixed Rossby–gravity (MRG) waves. Analysis of all TCs west of 100°E reveals a large and statistically significant modulation by the MJO and ER waves, a small yet significant modulation by Kelvin waves, and a statistically insignificant modulation by MRG waves. Attribution of the observed TC modulation was made through examination of the wave-induced perturbations to the dynamical fields of low-level vorticity, vertical shear, and deep convection. Possible thermodynamic influences on TC genesis were neglected. Different combinations of the three dynamical fields were necessary for successful attribution for each of the large-scale wave modes. For example, for the MJO, the modulation was best attributable to its perturbations to both the vorticity and shear fields, while for the ER wave, it was its perturbations to the convection and vorticity fields that appeared to best be able to explain the modulation. It appears that there is no single factor that can be used for the attribution of all subseasonal TC variability. Finally, it is shown that the modulation of TCs by at least the MJO and ER waves is large enough to warrant further investigation for prediction on the weekly time scale.

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.

scholarly journals A Specific Large-Scale Pressure Gradient Forcing for Computation of Realistic 3D Wind Fields Over a Canopy at Stand Scale

2018 ◽  
Author(s):  
Francois Pimont ◽  
Jean-Luc Dupuy ◽  
Rodman Linn ◽  
Jeremy Sauer
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Boundary Conditions ◽  
Pressure Gradient ◽  
Large Scale ◽  
Interaction Analysis ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Wind Fields ◽  
Macro Scale

Turbulent flows over and within forest canopies have recently been modeled with success using Large Eddy Simulations (LES). Validation exercises against experimental data suggest that models can be applied with a high degree of confidence for many applications, mechanical and physiological plant/atmosphere interaction analysis, seed or pollen dispersal, wildfire spread and firebrand transport, or investigation of causes of eddy-covariance technique bias. Long distances required for shear-induced turbulence to equilibrate, result in the widespread use of cyclic boundary conditions in LES atmospheric boundary layer studies. Vegetation drag dissipates air momentum in the atmosphere, but equilibrium is often achieved through compensatory momentum source, supplied by macro-scale pressure gradient forcing. Unfortunately, both classical Ekman balance or simple spatially-constant pressure gradient techniques for implementing this forcing have major drawbacks in the context of cyclic boundary conditions for the applications listed above. Among them, it is difficult to specify aspects of the mean velocity profile such as a specific desired wind velocity and direction at a reference height. In the present paper, we propose a new technique for capturing the effects of a large-scale pressure gradient force (LSPGF) that can be used at stand scale and enables simulation of realistic and specifiable wind fields. Several variants of this LSPGF are developed and analyzed here and validated against experimental data. Although this LSPGF technique is developed in the context of HIGRAD/FIRETEC wildfire simulations, LSPGF can be used for any LES wind modeling application aimed at generating detailed stand-scale wind fields with resolved turbulence and shear profiles consistent with vegetation structure in the boundary layer.

scholarly journals Some Exact Solutions of the Equations of Magnetohydrodynamics when Both Self-Attraction and Magnetic Fields Are Present

1958 ◽  
Vol 8 ◽  
pp. 1080-1083
Author(s):  
G. C. McVittie
Keyword(s):  
Heat Conduction ◽  
Magnetic Fields ◽  
Pressure Gradient ◽  
Exact Solutions ◽  
Recent Work ◽  
Large Scale ◽  
Gradient Force ◽  
Constant Density ◽  
Pressure Gradient Force ◽  
Compressible Material

This paper describes recent work on magnetohydrodynamics by M. H. Rogers and myself. It has seemed to us that the large-scale dynamics of cosmical gas clouds must take into account the fact that a gas is a compressible material, so that the hypothesis of a constant density must be avoided. Secondly, there must be present a pressure-gradient force. Thirdly, self-gravitation cannot be omitted. Fourthly, the motion should be adiabatic, in the first instance at least. To these conditions we have now also added the effects of the presence of magnetic fields, but have assumed that the conductivity of the material is infinite and that it is of constant permeability. We have neglected the effects of turbulence, of viscosity, and of heat conduction.

Sign in / Sign up

Export Citation Format

Share Document