Numerical Simulation of Air–Sea Coupling during Coastal Upwelling

2007 ◽  
Vol 37 (8) ◽  
pp. 2081-2093 ◽  
Author(s):  
Natalie Perlin ◽  
Eric D. Skyllingstad ◽  
Roger M. Samelson ◽  
Philip L. Barbour
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Coastal Upwelling ◽  
Heat Fluxes ◽  
Lower Atmosphere ◽  
Ekman Transport ◽  
Internal Boundary ◽  
Surface Boundary ◽  
Oregon Coast ◽  
Upwelled Water

Abstract Air–sea coupling during coastal upwelling was examined through idealized three-dimensional numerical simulations with a coupled atmosphere–ocean mesoscale model. Geometry, topography, and initial and boundary conditions were chosen to be representative of summertime coastal conditions off the Oregon coast. Over the 72-h simulations, sea surface temperatures were reduced several degrees near the coast by a wind-driven upwelling of cold water that developed within 10–20 km off the coast. In this region, the interaction of the atmospheric boundary layer with the cold upwelled water resulted in the formation of an internal boundary layer below 100-m altitude in the inversion-capped boundary layer and a reduction of the wind stress in the coupled model to half the offshore value. Surface heat fluxes were also modified by the coupling. The simulated modification of the atmospheric boundary layer by ocean upwelling was consistent with recent moored and aircraft observations of the lower atmosphere off the Oregon coast during the upwelling season. For these 72-h simulations, comparisons of coupled and uncoupled model results showed that the coupling caused measurable differences in the upwelling circulation within 20 km off the coast. The coastal Ekman transport divergence was distributed over a wider offshore extent and a thinner ocean surface boundary layer, with consistently smaller offshore and depth-integrated alongshore transport formed in the upwelling region, in the coupled case relative to the uncoupled case. The results indicate that accurate models of coastal upwelling processes can require representations of ocean–atmosphere interactions on short temporal and horizontal scales.

scholarly journals Competition between baroclinic instability and Ekman transport under varying buoyancy forcings in upwelling systems: An idealized analog to the Southern Ocean.

2021 ◽  
Author(s):  
Soeren Thomsen ◽  
Xavier Capet ◽  
Vincent Echevin
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Coastal Upwelling ◽  
Baroclinic Instability ◽  
Heat Fluxes ◽  
Ekman Transport ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Mesoscale Dynamics ◽  
Offshore Transport

AbstractCoastal upwelling rates are classically determined by the intensity of the upper-ocean offshore Ekman transport. But (sub-)mesoscale turbulence modulates offshore transport, hence the net upwelling rate. Eddy effects generally oppose the Ekman circulation, resulting in so-called “eddy cancellation”, a process well studied in the Southern Ocean. Here we investigate how air-sea heat/buoyancy fluxes modulate eddy cancellation in an idealized upwelling model. We run CROCO simulations with constant winds but varying heat fluxes with and without submesoscale-rich turbulence. Eddy cancellation is consistently evaluated with three different methods that all account for the quasi-isopycnal nature of ocean circulation away from the surface. For zero heat fluxes the release of available potential energy by baroclinic instabilities is strongest and leads, near the coast, to nearly full cancellation of the Ekman cross-shore circulation by eddy effects, i.e., zero net mean upwelling flow. With increasing heat fluxes eddy cancellation is reduced and the transverse flow progressively approaches the classical Ekman circulation. Sensitivity of the eddy circulation to synoptic changes in air-sea heat fluxes is felt down to 125 m depth despite short experiments of tens of days. Mesoscale dynamics dominate the cancellation effect in our simulations which might also hold for the real ocean as the relevant processes act below the surface boundary layer. Although the idealized setting overemphasis the role of eddies and thus studies with more realistic settings should follow, our findings have important implications for the overall understanding of upwelling system dynamics.

scholarly journals Study of the air-sea interactions at the mesoscale: the SEMAPHORE experiment

1996 ◽  
Vol 14 (9) ◽  
pp. 986-1015 ◽  
Author(s):  
L. Eymard ◽  
S. Planton ◽  
P. Durand ◽  
C. Le Visage ◽  
P. Y. Le Traon ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Ocean Circulation ◽  
Wave Model ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Upper Ocean ◽  
Test Model ◽  
Model Simulations ◽  
Drifting Buoys

Abstract. The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddies), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 °C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 100 W m-2. This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was investigated using airborne and satellite radars and wave buoys. A wave model, operationally used, was found to get better results compared with radar and wave-buoy measurements, when initialized using an improved wind field, obtained by assimilating satellite and buoy wind data in a meteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underestimated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretation of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales.

scholarly journals Soil Moisture-Boundary Layer Feedbacks on the Loess Plateau in China Using Radiosonde Data with 1-D Atmospheric Boundary Layer Model

Atmosphere ◽  
2021 ◽  
Vol 12 (12) ◽  
pp. 1619
Author(s):  
Yingsai Ma ◽  
Xianhong Meng ◽  
Yinhuan Ao ◽  
Ye Yu ◽  
Guangwei Li ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Soil Moisture ◽  
Atmospheric Boundary Layer ◽  
Loess Plateau ◽  
Atmospheric Stability ◽  
Heat Fluxes ◽  
Radiosonde Data ◽  
Atmospheric Conditions ◽  
The Loess Plateau ◽  
The Impact

The Loess Plateau is one land-atmosphere coupling hotspot. Soil moisture has an influence on atmospheric boundary layer development under specific early-morning atmospheric thermodynamic structures. This paper investigates the sensitivity of atmospheric convection to soil moisture conditions over the Loess Plateau in China by using the convective triggering potential (CTP)—humidity index (HIlow) framework. The CTP indicates atmospheric stability and the HIlow indicates atmospheric humidity in the low-level atmosphere. By comparing the model outcomes with the observations, the one-dimensional model achieves realistic daily behavior of the radiation and surface heat fluxes and the mixed layer properties with appropriate modifications. New CTP-HIlow thresholds for soil moisture-atmosphere feedbacks are found in the Loess Plateau area. By applying the new thresholds with long-time scales sounding data, we conclude that negative feedback is dominant in the north and west portion of the Loess Plateau; positive feedback is predominant in the south and east portion. In general, this framework has predictive significance for the impact of soil moisture on precipitation. By using this new CTP-HIlow framework, we can determine under what atmospheric conditions soil moisture can affect the triggering of precipitation and under what atmospheric conditions soil moisture has no influence on the triggering of precipitation.

scholarly journals An observational study of the evolution of the atmospheric boundary-layer over Cabo Frio, Brazil

2007 ◽  
Vol 25 (8) ◽  
pp. 1735-1744 ◽  
Author(s):  
S. H. Franchito ◽  
V. Brahmananda Rao ◽  
T. O. Oda ◽  
J. C. Conforte
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Mixed Layer ◽  
Coastal Upwelling ◽  
Vertical Mixing ◽  
Austral Summer ◽  
Sea Breeze ◽  
Austral Winter ◽  
Atmospheric Mixed Layer ◽  
Cabo Frio

Abstract. The effect of coastal upwelling on the evolution of the atmospheric boundary layer (ABL) in Cabo Frio (Brazil) is investigated. For this purpose, radiosounding data collected in two experiments made during the austral summer (upwelling case) and austral winter (no upwelling case) are analysed. The results show that during the austral summer, cold waters that crop up near the Cabo Frio coast favour the formation of an atmospheric stable layer, which persists during the upwelling episode. Due to the low SSTs, the descending branch of the sea-breeze circulation is located close to the coast, inhibiting the development of a mixed layer mainly during the day. At night, with the reduction of the land-sea thermal contrast the descending motion is weaker, allowing a vertical mixing. The stable ABL favours the formation of a low level jet, which may also contribute to the development of a nocturnal atmospheric mixed layer. During the austral winter, due to the higher SSTs observed near the coast, the ABL is less stable compared with that in the austral summer. Due to warming, a mixed layer is observed during the day. The observed vertical profiles of the zonal winds show that the easterlies at low levels are stronger in the austral summer, indicating that the upwelling modulates the sea-breeze signal, thus confirming model simulations.

scholarly journals Evaluation of Polar WRF from Modeling the Atmospheric Boundary Layer over Antarctic Sea Ice in Autumn and Winter

2012 ◽  
Vol 140 (12) ◽  
pp. 3919-3935 ◽  
Author(s):  
Esa-Matti Tastula ◽  
Timo Vihma ◽  
Edgar L Andreas
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Surface Temperature ◽  
Atmospheric Boundary Layer ◽  
Inversion Layer ◽  
Temperature Inversion ◽  
Heat Fluxes ◽  
Poor Correlation ◽  
Antarctic Sea Ice ◽  
Autumn And Winter

Abstract Regional simulations of the atmospheric boundary layer over Antarctic sea ice that have been adequately validated are rare. To address this gap, the authors use the doubly nested Polar Weather Research and Forecasting (Polar WRF) mesoscale model to simulate conditions during Ice Station Weddell (ISW) in the austral autumn and winter of 1992. The WRF simulations test two boundary layer schemes: Mellor–Yamada–Janjic and the Asymmetric Convective Model. Validation is against surface-layer and sounding observations from ISW. Simulated latent and sensible heat fluxes for both boundary layer schemes had poor correlation with the observed fluxes. Simulated surface temperature had better correlation with the observations, with a typical bias of 0–2 K and a root-mean-square error of 6–7 K. For surface temperature and wind speed, the Polar WRF yielded better results than the ECMWF Re-Analysis Interim (ERA-Interim). A more challenging test of the simulations is to reproduce features of the low-level jet and the temperature inversion, which were observed, respectively, in 80% and 96% of the ISW radiosoundings. Both boundary layer schemes produce only about half as many jets as were observed. Moreover, the simulated jet coincided with an observed jet only about 30% of the time. The number of temperature inversions and the height at the inversion base were better reproduced, although this was not the case with the depth of the inversion layer. Simulations of the temperature inversion improved when forecasts of cloud fraction agreed to within 0.3 with observations. The modeled inversions were strongest when the incoming longwave radiation was smallest, but this relationship was not observed at ISW.

Turbulence characteristics of a thermally stratified wind turbine array boundary layer via proper orthogonal decomposition

2017 ◽  
Vol 828 ◽  
pp. 175-195 ◽  
Author(s):  
N. Ali ◽  
G. Cortina ◽  
N. Hamilton ◽  
M. Calaf ◽  
R. B. Cal
Keyword(s):  
Boundary Layer ◽  
Turbulent Flow ◽  
Atmospheric Boundary Layer ◽  
Wind Turbine ◽  
Proper Orthogonal Decomposition ◽  
Orthogonal Decomposition ◽  
Commercial Application ◽  
Surface Boundary ◽  
Stable Regime ◽  
Proper Orthogonal

A large eddy simulation framework is used to explore the structure of the turbulent flow in a thermally stratified wind turbine array boundary layer. The flow field is driven by a constant geostrophic wind with time-varying surface boundary conditions obtained from a selected period of the CASES-99 field experiment. Proper orthogonal decomposition is used to extract coherent structures of the turbulent flow under the considered thermal stratification regimes. The flow structure is discussed in the context of three-dimensional representations of key modes, which demonstrate features ranging in size from the wind turbine wakes to the atmospheric boundary layer. Results demonstrate that structures related to the atmospheric boundary layer flow dominate over those introduced by the wind farm for the unstable and neutrally stratified regimes; large structures in atmospheric turbulence are beneficial for the wake recovery, and consequently the presence of the turbulent wind turbine wakes is diminished. Contrarily, the flow in the stably stratified case is fully dominated by the presence of the turbines and highly influenced by the Coriolis force. A comparative analysis of the test cases indicates that during the stable regime, higher-order modes contribute less to the overall character of the flow. Under neutral and unstable stratification, important turbulence dynamics are distributed over a larger range of basis functions. The influence of the wind turbines on the structure of the atmospheric boundary layer is mainly quantified via the turbulence kinetic energy of the first ten modes. Linking the new insights into structure of the wind turbine/atmospheric boundary layer and their interaction addressed here will benefit the formulation of new simplified models for commercial application.

scholarly journals Extensive Marine Heatwaves at the Sea Surface in the Northwestern Pacific Ocean in Summer 2021

2021 ◽  
Vol 13 (19) ◽  
pp. 3989
Author(s):  
Hiroshi Kuroda ◽  
Takashi Setou
Keyword(s):  
Boundary Layer ◽  
Pacific Ocean ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Northwestern Pacific ◽  
Surface Boundary ◽  
Northwestern Pacific Ocean ◽  
Surface Boundary Layer ◽  
Westerly Jet ◽  
Oceanic Surface

In July–August 2021, intense marine heatwaves (MHWs) occurred at the sea surface over extensive areas of the northwestern Pacific Ocean, including the entire Sea of Japan and part of the Sea of Okhotsk. In extent and intensity, these MHWs were the largest since 1982, when satellite measurements of global sea surface temperatures started. The MHWs in summer 2021 were observed at the sea surface and occurred concomitantly with a stable shallow oceanic surface boundary layer. The distribution of the MHWs was strongly related to heat fluxes at the sea surface, indicating that the MHWs were generated mainly by atmospheric forcing. The MHWs started to develop after around 10 July, concurrent with an extreme northward shift of the atmospheric westerly jet. The MHWs developed rapidly under an atmospheric high-pressure system near the sea surface, associated with a northwestward expansion of the North Pacific Subtropical High. The MHWs exhibited peaks around 30 July to 1 August. Subsequently, following the southward displacement of the westerly jet, the MHWs weakened and then shrank abruptly, synchronously with rapid deepening of the oceanic surface boundary layer. By 18 August, the MHWs had disappeared.

scholarly journals Characteristics of the Near-Surface Boundary Layer within a Mountain Valley during Winter

2012 ◽  
Vol 51 (3) ◽  
pp. 583-597 ◽  
Author(s):  
Warren Helgason ◽  
John W. Pomeroy
Keyword(s):  
Boundary Layer ◽  
Heat Fluxes ◽  
Quadrant Analysis ◽  
Turbulent Heat ◽  
Surface Boundary ◽  
Wind Speed Profile ◽  
Surface Boundary Layer ◽  
Near Surface ◽  
Mountainous Regions ◽  
Mass Fluxes

AbstractWithin mountainous regions, estimating the exchange of sensible heat and water vapor between the surface and the atmosphere is an important but inexact endeavor. Measurements of the turbulence characteristics of the near-surface boundary layer in complex mountain terrain are relatively scarce, leading to considerable uncertainty in the application of flux-gradient techniques for estimating the surface turbulent heat and mass fluxes. An investigation of the near-surface boundary layer within a 7-ha snow-covered forest clearing was conducted in the Kananaskis River valley, located within the Canadian Rocky Mountains. The homogeneous measurement site was characterized as being relatively calm and sheltered; the wind exhibited considerable unsteadiness, however. Frequent wind gusts were observed to transport turbulent energy into the clearing, affecting the rate of energy transfer at the snow surface. The resulting boundary layer within the clearing exhibited perturbations introduced by the surrounding topography and land surface discontinuities. The measured momentum flux did not scale with the local aerodynamic roughness and mean wind speed profile, but rather was reflective of the larger-scale topographical disturbances. The intermittent nature of the flux-generating processes was evident in the turbulence spectra and cospectra where the peak energy was shifted to lower frequencies as compared with those observed in more homogeneous flat terrain. The contribution of intermittent events was studied using quadrant analysis, which revealed that 50% of the sensible and latent heat fluxes was contributed from motions that occupied less than 6% of the time. These results highlight the need for caution while estimating the turbulent heat and mass fluxes in mountain regions.

High-resolution temperature and wind field observations in the atmospheric boundary layer

2020 ◽  
Author(s):  
Matthias Zeeman ◽  
Marwan Katurji ◽  
Tirtha Banerjee
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Land Surface ◽  
Wind Field ◽  
Single Point ◽  
Sonic Anemometer ◽  
Atmospheric Stability ◽  
Surface Boundary ◽  
Three Dimensional Model ◽  
Local Flow

<p>Do we get a better picture of the world around us if we simultaneously observe many aspects instead of a few? Dense sensing networks are an elaborate way to validate our representation of land surface boundary layer processes commonly derived from single point monitoring stations or a three-dimensional model world. More samples promise unique insights into interactions that occur at different scales, separated in space and time.</p><p>We present a combination of techniques that purvey a) observations of the temperature and wind field in high detail and b) the extraction of information about dynamic interactions near the surface. A field experiment was conducted in complex terrain, in which landscape features dramatically modulate local flow patterns and the atmospheric stability during summer days rapidly transitions on a diurnal scale and between locations. Wind and temperature were simultaneously observed using a network of Doppler lidar, sonic anemometer, fiber-optic temperature sensing (DTS) and thermal imaging velocimetry (TIV) instrumentation, centered around the TERENO/ICOS preAlpine grassland observatory station Fendt, Germany, during the ScaleX Campaigns (https://scalex.imk-ifu.kit.edu). Data analyses relied on signal decomposition and statistical clustering, aimed at the characterization of (non-)turbulent motions and their feedback on turbulent mixing near the surface. The combination of methods offered multiple levels of detail about the development and impact of organized structures in the atmospheric boundary layer.</p><p>The study shows that the exploration of novel micrometeorological and data sciences techniques helps advance our knowledge of fundamental aspects of atmospheric turbulence, and provides new avenues for theoretical and numerical studies of the atmospheric boundary layer.</p>

A Rarely Witnessed Summertime Upwelling Event northwest off the Hainan Island

2020 ◽  
Author(s):  
Ai-Jun Pan ◽  
Fang-fang Kuang ◽  
Kai Li ◽  
Xu Dong
Keyword(s):  
Coastal Upwelling ◽  
Hainan Island ◽  
Heat Fluxes ◽  
Tidal Mixing ◽  
Coastal Current ◽  
Beibu Gulf ◽  
Surface Heat Fluxes ◽  
Upwelled Water ◽  
Basin Scale ◽  
Upwelling Event

<p>A field survey revealed a rare realization of upwelling event in the northwestern Hainan Island (UNWHI) on July 24, 2015. Model experiments suggest that the UNWHI is not locally generated, but can be treated as northward extension of the upwelling southwest off Hainan Island (USWHI) under favorable wind conditions. Therefore, presence of the USWHI is vital for the UNWHI occurrence. Tidal mixing is testified to be the primary driving force for the USWHI, whilst southerly winds plays an essential role in the induction of the UNWHI. Moreover, it is demonstrated that the UNWHI is not a stable, but intermittent coastal upwelling system. Shallow basin of the Beibu Gulf makes the interior circulation vulnerable to local monsoon changes. Given the favorable southerly winds, a cyclonic gyre northwest off Hainan Island will be induced and which, leads to northward coastal current and consequently, the UNWHI is to be formed due to the northward transport of the USWHI. Conversely, the UNWHI vanishes during northerly winds period, because the basin-scale anticyclonic gyre results in a southward current west off the Hainan Island and which, acts to push the upwelled water of the USWHI offshore and away from the northwestern Hainan Island. In addition, our diagnostics indicates that contributions from surface heat fluxes to the UNWHI occurrence is negligible. Besides, it also reminds us that application of a high-frequency, much closer to reality wind field is necessary for the coastal upwelling simulation. </p>

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