Idealized Simulations of the Intertropical Convergence Zone and Its Multilevel Flows

2010 ◽  
Vol 67 (12) ◽  
pp. 4028-4053 ◽  
Author(s):  
David S. Nolan ◽  
Scott W. Powell ◽  
Chidong Zhang ◽  
Brian E. Mapes
Keyword(s):  
Surface Temperature ◽  
Dynamic Instability ◽  
Vertical Motion ◽  
Sea Breeze ◽  
Return Flow ◽  
Eddy Fluxes ◽  
Convergence Zones ◽  
Upper Level ◽  
Static Energy ◽  
Regional Budgets

Abstract A mesoscale numerical model with an idealized tropical channel environment is used to study the dynamics of intertropical convergence zones (ITCZs) and the recently identified shallow return flow (SRF) and midlevel inflow (MLI). Four idealized sea surface temperature (SST) distributions are used: a meridionally symmetric SST profile with a sharply peaked SST maximum at the equator, a similar profile with the maximum SST shifted off the equator, a cosine-shaped SST profile with zero gradient at the equator, and an idealized SST profile modeled after the observed SST of the eastern Pacific. The simulations show that both the SRF and the MLI are robust features of the ITCZ. The prior result that the SRF is a sea-breeze-like response to surface temperature gradients is further supported, whereas the MLI is caused by the upper-level maxima in diabatic heating and vertical motion. Simulations with the SST maximum at the equator generate long-lasting, convectively coupled Kelvin waves. When the SST maximum is off the equator, the meridional circulations become highly asymmetric with strong cross-equatorial flow. Tropical cyclones are frequently generated by dynamic instability of the off-equatorial ITCZs. The contributions of the multilevel circulations to regional budgets of mass, water, and moist static energy (MSE) are computed. About 10% of the total water transported into the ITCZ region is transported out by the SRF. The water transport of the MLI is minimal, but its mass and MSE transports are significant, accounting for about ⅓ of the mass and MSE entering the ITCZ region. Eddy fluxes are found to be a large component of the net vertically integrated transport of MSE out of the ITCZ.

scholarly journals Changes of the Boreal Winter Hadley Circulation in the NCEP–NCAR and ECMWF Reanalyses: A Comparative Study

2007 ◽  
Vol 20 (20) ◽  
pp. 5191-5200 ◽  
Author(s):  
Hua Song ◽  
Minghua Zhang
Keyword(s):  
Radiative Forcing ◽  
Atmospheric Transport ◽  
Vertical Motion ◽  
Hadley Circulation ◽  
Western Pacific ◽  
Boreal Winter ◽  
Tropical Western Pacific ◽  
The Difference ◽  
Upper Level ◽  
Static Energy

Abstract Both the ECMWF and the NCEP–NCAR reanalyses show a strengthening of the atmospheric Hadley circulation in boreal winter over the last 50 years, but the intensification is much stronger in the ECMWF than in the NCEP–NCAR reanalysis. This study focuses on the difference of these trends in the two reanalyses. It is shown that trends in the Hadley circulation in the two reanalyses differ mainly over the tropical western Pacific. This difference is found to be consistent with respective trends of the atmospheric transport of moist static energy, longwave cloud radiative forcing, and upper-level clouds in the two reanalyses. Two independent datasets of upper-level cloud cover and sea level pressure from ship-based measurements are then used to evaluate the reanalyses over the tropical western Pacific. They are found to be more consistent with the trends in the NCEP–NCAR reanalysis than those in the ECMWF reanalysis. The results suggest a weakening of the vertical motion associated with the Hadley circulation in the tropical western Pacific.

An Analytical Solution for Three-Dimensional Sea–Land Breeze

2015 ◽  
Vol 73 (1) ◽  
pp. 41-54 ◽  
Author(s):  
YaoKun Li ◽  
JiPing Chao
Keyword(s):  
Temperature Distribution ◽  
Surface Temperature ◽  
Three Dimensional ◽  
Vertical Motion ◽  
Sea Breeze ◽  
Horizontal Distribution ◽  
Land Breeze ◽  
Island Size ◽  
Surface Temperature Distribution ◽  
Vertical Level

Abstract Based on the hydrostatic, incompressible Boussinesq equations in the planetary boundary layer (PBL), the three-dimensional sea–land breeze (SLB) circulation has been elegantly expressed as functions of the surface temperature distribution. The horizontal distribution of the horizontal or vertical motion is determined by the first or second derivative of the surface temperature distribution. For symmetric land–sea and temperature distribution, the full strength of the sea breeze occurs inland but not at the coastline, and the maximum updraft associates with the heating center. Setting the temperature difference between land and sea (TDLS), which varies with the island size, there would exist an optimal island size corresponding to the strongest SLB circulation that weakens with both a larger and smaller island size. Each velocity component approaches a peak at a certain vertical level. Both the peak value and the corresponding vertical level link with the vertical scale of the surface temperature: the more significant the influence of the surface temperature vertically, the stronger the SLB circulation at a higher vertical level it induces. The Weather Research and Forecasting (WRF) Model's ideal simulation for the two-dimensional sea breeze is applied to verify the theory. Two cases, land breeze and sea breeze, further support the theory's results despite a certain slight discrepancy due to the highly simplified theoretical equations.

scholarly journals Diurnal cycle of precipitation and near-surface atmospheric conditions over the maritime continent: land–sea contrast and impacts of ambient winds in cloud-permitting simulations

2021 ◽  
Author(s):  
Yuntao Wei ◽  
Zhaoxia Pu
Keyword(s):  
Diurnal Cycle ◽  
Large Scale ◽  
Deep Convection ◽  
Vertical Motion ◽  
Sea Breeze ◽  
Return Flow ◽  
Atmospheric Conditions ◽  
Maritime Continent ◽  
Regional Variability ◽  
Near Surface

AbstractA set of cloud-permitting-scale numerical simulations during January–February 2018 is used to examine the diurnal cycle (DC) of precipitation and near-surface variables (e.g., 2 m temperature, 10 m wind and convergence) over the Indo-Pacific Maritime Continent under the impacts of shore-orthogonal ambient winds (SOAWs). It is found that the DC of these variables and their variabilities of daily maxima, minima, and diurnal amplitudes vary over land, sea, and coastal regions. Among all variables, the DC of precipitation has the highest linear correlation with near-surface convergence (near-surface temperature) over coastal (noncoastal) regions. The correlations among the DCs of precipitation, wind, and heating are greater over the ocean than over land. Sine curves can model accurately the DCs of most variables over the ocean, but not over land. SOAWs act to influence the DC mainly by affecting the diurnal amplitude of the considered variables, with DC being stronger under more strengthened offshore SOAWs, though variable dependence and regional variability exist. Composite analysis over Sumatra reveals that under weak SOAWs, shallow clouds are dominant and cause a pre-moistening effect, supporting shallow-to-deep convection transition. A sea breeze circulation (SBC) with return flow aloft can develop rapidly. Cold pools are better able to trigger new updrafts and contribute to the upscale growth and inland migration of deep convection. In addition, warm gravity waves can propagate upward throughout the troposphere, thereby supporting a strong DC. In contrast, under strong SOAWs, both shallow and middle-high clouds prevail and persist throughout the day. The evolution of moistening and SBC is reduced, leading to weak variation in vertical motion and rainwater confined to the boundary layer. Large-scale winds, moisture, and convection are discussed to interpret how strong SOAWs affect the DC of Sumatra.

scholarly journals Investigating the Local Atmospheric Response to a Realistic Shift in the Oyashio Sea Surface Temperature Front

2015 ◽  
Vol 28 (3) ◽  
pp. 1126-1147 ◽  
Author(s):  
Dimitry Smirnov ◽  
Matthew Newman ◽  
Michael A. Alexander ◽  
Young-Oh Kwon ◽  
Claude Frankignoul
Keyword(s):  
High Resolution ◽  
Surface Wind ◽  
Lower Troposphere ◽  
Vertical Motion ◽  
Atmospheric Response ◽  
Transient Eddy ◽  
Sst Front ◽  
Moisture Fluxes ◽  
Upper Level ◽  
Heat And Moisture

Abstract The local atmospheric response to a realistic shift of the Oyashio Extension SST front in the western North Pacific is analyzed using a high-resolution (HR; 0.25°) version of the global Community Atmosphere Model, version 5 (CAM5). A northward shift in the SST front causes an atmospheric response consisting of a weak surface wind anomaly but a strong vertical circulation extending throughout the troposphere. In the lower troposphere, most of the SST anomaly–induced diabatic heating is balanced by poleward transient eddy heat and moisture fluxes. Collectively, this response differs from the circulation suggested by linear dynamics, where extratropical SST forcing produces shallow anomalous heating balanced by strong equatorward cold air advection driven by an anomalous, stationary surface low to the east. This latter response, however, is obtained by repeating the same experiment except using a relatively low-resolution (LR; 1°) version of CAM5. Comparison to observations suggests that the HR response is closer to nature than the LR response. Strikingly, HR and LR experiments have almost identical vertical profiles of . However, diagnosis of the diabatic quasigeostrophic vertical pressure velocity (ω) budget reveals that HR has a substantially stronger response, which together with upper-level mean differential thermal advection balances stronger vertical motion. The results herein suggest that changes in transient eddy heat and moisture fluxes are critical to the overall local atmospheric response to Oyashio Front anomalies, which may consequently yield a stronger downstream response. These changes may require the high resolution to be fully reproduced, warranting further experiments of this type with other high-resolution atmosphere-only and fully coupled GCMs.

scholarly journals Theoretical aspects of the onset of Indian summer monsoon from perturbed orography simulations in a GCM

2006 ◽  
Vol 24 (8) ◽  
pp. 2075-2089 ◽  
Author(s):  
A. Chakraborty ◽  
R. S. Nanjundiah ◽  
J. Srinivasan
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
General Circulation ◽  
Large Scale ◽  
Vertical Motion ◽  
Circulation Model ◽  
Threshold Value ◽  
The Earth ◽  
Static Energy ◽  
The Impact

Abstract. A theory is proposed to determine the onset of the Indian Summer Monsoon (ISM) in an Atmospheric General Circulation Model (AGCM). The onset of ISM is delayed substantially in the absence of global orography. The impact of orography over different parts of the Earth on the onset of ISM has also been investigated using five additional perturbed simulations. The large difference in the date of onset of ISM in these simulations has been explained by a new theory based on the Surface Moist Static Energy (SMSE) and vertical velocity at the mid-troposphere. It is found that onset occurs only after SMSE crosses a threshold value and the large-scale vertical motion in the middle troposphere becomes upward. This study shows that both dynamics and thermodynamics play profound roles in the onset of the monsoon.

Diurnal Mountain Winds

2000 ◽  
Author(s):  
C. David Whiteman
Keyword(s):  
Large Scale ◽  
Blow Up ◽  
Thermal Structure ◽  
Temperature Inversion ◽  
Return Flow ◽  
Wind System ◽  
Valley Wind ◽  
Deep Layers ◽  
Horizontal Pressure ◽  
Upper Level

Diurnal mountain winds develop over complex topography of all scales, from small hills to large mountain massifs and are characterized by a reversal of wind direction twice per day. As a rule, winds flow upslope, up-valley, and from the plain to the mountain massif during daytime. During nighttime, they flow downslope, down-valley, and from the mountain massif to the plain. Diurnal mountain winds are strongest when skies are clear and winds aloft are weak. Diurnal mountain winds are produced by horizontal temperature differences that develop daily in complex terrain. The resulting horizontal pressure differences cause winds near the surface of the earth to blow from areas with lower temperatures and higher pressures toward areas with higher temperatures and lower pressures. The circulations are closed by return, or compensatory, flows higher in the atmosphere. Four wind systems comprise the mountain wind system, which carries air into a mountain massif at low levels during daytime and out of a mountain massif during nighttime. The slope wind system (upslope winds and downslope winds) is driven by horizontal temperature contrasts between the air over the valley sidewalls and the air over the center of the valley. The along-valley wind system (up-valley winds and down-valley winds) is driven by horizontal temperature contrasts along a valley’s axis or between the air in a valley and the air over the adjacent plain. The cross-valley wind system results from horizontal temperature differences between the air over one valley sidewall and the air over the opposing sidewall, producing winds that blow perpendicular to the valley axis and toward the more strongly heated sidewall. The mountain-plain wind system results from horizontal temperature differences between the air over a mountain massif and the air over the surrounding plains, producing large-scale winds that blow up or down the outer slopes of a mountain massif. The mountain-plain circulation and its upper level return flow are not confined by the topography but are carried over deep layers of the atmosphere above the mountain slopes. Because diurnal mountain winds are driven by horizontal temperature differences, the regular evolution of the winds in a given valley is closely tied to the thermal structure of the atmospheric boundary layer within the valley, which is characterized by a diurnal cycle of buildup and breakdown of a temperature inversion.

scholarly journals Characteristics of shallow thermally driven flow in the complex topography of the south-eastern Adriatic

2010 ◽  
Vol 28 (10) ◽  
pp. 1905-1922 ◽  
Author(s):  
M. T. Prtenjak ◽  
I. Tomažić ◽  
I. Kavčič ◽  
S. Đivanović
Keyword(s):  
Large Scale ◽  
Sea Breeze ◽  
Small Scale ◽  
The South ◽  
Fine Grid ◽  
Easterly Wind ◽  
Sodar Data ◽  
South Eastern ◽  
The North ◽  
Convergence Zones

Abstract. Characteristics of thermally induced flow, namely the sea breeze, are investigated along the south-eastern Adriatic. The chosen period 24–25 April 2006 favoured sea breeze development and simultaneously allowed a comparison of the large-scale wind influence (north-westerly wind versus south-easterly wind) and the complex terrain on the local circulations. Particular attention is paid to the small-scale formation of the wind field, convergence zones (CZs), channelling flows and small scale eddies, especially in the vicinity of two airports in the central part of south-eastern Adriatic. The results are based on wind measurements (from meteorological surface stations, radiosoundings, satellite data and sodar data) and further supplemented by model data at fine grid spacing. This study shows the formation of numerous irregular daytime and nighttime CZs, which occurred along the coastline in the lee of mountains and over the larger, elongated islands. The results show that the above mentioned airports are surrounded by daytime CZ formations within the lowermost 1000 m and associated updrafts of 1 m s−1, especially if CZs are maintained by the north-westerly large-scale winds. Whereas the daytime CZ was generated due to merged sea breezes, the weaker and shallower nighttime CZs were formed by wind convergence of the seaward breezes, and significantly modified by the large-scale flow of the topography (e.g., accelerated flow in the sea channels and substantial swirled flows around the islands). The passes between the coastal mountain peaks changed the inflow penetration, provoking the increase in wind speed of the channelled flow. The strongest sea breeze channelling was observed above the valley of the Neretva River, where the onshore flow reached 40 km inland with a strength of 8 m s−1, and the highly asymmetric offshore part was confined within the sea channel.

scholarly journals The Effect of Variable Sea Surface Temperature on Forecasting Sea Fog and Sea Breezes: A Case Study

2012 ◽  
Vol 51 (5) ◽  
pp. 986-990 ◽  
Author(s):  
Yongming Tang
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Coastal Region ◽  
Weather Forecast ◽  
Sea Breeze ◽  
Forecast Model ◽  
Sea Surface ◽  
Sea Breezes ◽  
Shelf Sea ◽  
Numerical Weather Forecast

AbstractA preliminary study of the effect of sea surface temperature (SST) temporal and spatial variability on regional coastal weather forecasts is described. A high-resolution numerical weather forecast model from the Met Office is run for the U.K. region with hourly updates of SST data obtained from a shelf sea model. When compared with a control run in which SST is maintained with Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) data, it is found that there are significant differences in the coastal-region forecasts for sea breezes and fog formation. The control run underestimates surface temperature and the strength of the sea breeze when compared with the run with hourly SST updates.

scholarly journals Integration of Lidar Data into a Coupled Mesoscale–Land Surface Model: A Theoretical Assessment of Sensitivity of Urban–Coastal Mesoscale Circulations to Urban Canopy Parameters

2012 ◽  
Vol 29 (3) ◽  
pp. 328-346 ◽  
Author(s):  
Michael Carter ◽  
J. Marshall Shepherd ◽  
Steve Burian ◽  
Indu Jeyachandran
Keyword(s):  
Urban Environment ◽  
Land Surface ◽  
Urban Canopy ◽  
Vertical Motion ◽  
Sea Breeze ◽  
Surface Model ◽  
Lidar Data ◽  
Mesoscale Circulations ◽  
The Impact ◽  
Urban Weather

Abstract Urban–coastal circulations affect urban weather, dispersion and transport of pollutants and contaminants, and climate. Proper characterization and prediction of thermodynamic and dynamic processes in such environments are warranted. A new generation of observation and modeling systems is enabling unprecedented characterization of the three-dimensionality of the urban environment, including morphological parameters. Urban areas of Houston, Texas, are classified according to lidar-measured building heights and assigned typical urban land surface parameters appropriate to each classification. The lidar data were degraded from 1 m to the model resolution (1 km) with the goal of evaluating the impact of degraded resolution urban canopy parameters (UCPs) and three-dimensionality on the coastal–urban mesoscale circulations in comparison to typical two-dimensional urban slab approaches. The study revealed complex interactions between the sea breeze and urban heat island and offers a novel diagnostic tool, the bulk Richardson shear number, for identifying shallow mesoscale circulation. Using the Advanced Research Weather Research and Forecasting model (ARW-WRF) coupled to an atmosphere–land surface–urban canopy model, the authors simulated a theoretical sea-breeze day and confirmed that while coastal morphology can itself lead to complex sea-breeze front structures, including preferred areas of vertical motion, the urban environment can have an impact on the evolution of the sea-breeze mesoscale boundary. The inclusion of lidar-derived UCPs, even at degraded resolution, in the model’s land surface representation can lead to significant differences in patterns of skin surface temperature, convergence, and vertical motion, which have implications for many aspects of urban weather.

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