24 research outputs found
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Subgrid Surface Fluxes in Fair Weather Conditions during TOGA COARE: Observational Estimates and Parameterization
Bulk aerodynamic formulas are applied to meteorological data from low-altitude aircraft flights to observational estimates of the subgrid enhancement of momentum, sensible heat, and latent heat the atmosphericâoceanic boundary in light wind, fair weather conditions during TOGA COARE Global Atmosphere Coupled OceanâAtmosphere Response Experiment). Here, subgrid enhancement the contributions of unresolved disturbances to the grid-box average fluxes at the lower boundary of general circulation model. The observed subgrid fluxes increase with grid-box area, reaching 11%, 24%, and 12% of the total sensible heat, latent heat, scalar wind stress, and vector wind stress respectively, at a grid-box size of 2° X 2° longitude and latitude. Consistent with previous observational and modeling studies over the open ocean, most of the subgrid flux is explained by unresolved directional variability in the near-surface wind field. The authors find that much of the observed variability in the wind field in the presence of fair weather convective bands and patches comes from contributions of curvature and speed variations of simple larger-scale structure across the grid box. Inclusion of a grid-scale-dependent subgrid velocity scale in the bulk aerodynamic formulas effectively parameterizes the subgrid enhancement of the sensible heat flux, latent heat flux, and vector stress magnitude, and to a lesser degree the subgrid enhancement of the scalar wind stress. An observational estimate of the subgrid velocity scale derived from one-dimensional aircraft flight legs is found to be smaller than that derived from a two-dimensional grid-box analysis. The additional enhancement in the two-dimensional case is caused by the nonhomogeneous and nonisotropic characteristics of the subgrid-scale wind variability. Long time series from surface-based platforms in the TOGA COARE region suggest that measures of convective activity, in addition to geometric grid-scale parameters, will be required to more accurately represent the subgrid velocity scales
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The Amplification of East Pacific MaddenâJulian Oscillation Convection and Wind Anomalies during JuneâNovember
MaddenâJulian oscillation (MJO) wind and convection anomalies are locally amplified over the northeast Pacific warm pool during JuneâNovember. Composite analysis using NCEP reanalysis data indicates that perturbation available potential energy (PAPE) production through the positive correlation of intraseasonal temperature and convective diabatic heating anomalies supports the local intensification of MJO-related east Pacific warm pool wind anomalies. PAPE production is maximum during periods of strong MJO convection and low-level westerly wind perturbations. PAPE is converted to perturbation kinetic energy through positive correlations between intraseasonal temperature and vertical velocity. Microwave Sounding Unit (MSU) temperature and NOAA outgoing longwave radiation data support the energy budget results derived from NCEP reanalysis.
The amplified east Pacific circulation enhances surface convergence and latent heat flux anomalies during MJO convective periods. The surface convergence anomalies have a strong frictional component. Intraseasonal surface convergence and latent heat flux anomalies are strongly correlated (greater than 0.7) with the negative outgoing longwave radiation anomalies that is associated with east Pacific MJO convective regions. Surface latent heat and convergence variations may therefore be important in modulating MJO convective anomalies over the east Pacific during JuneâNovember. Enhanced surface flux and convergence anomalies associated with an enhanced surface circulation may intensify MJO convection, thereby creating a feedback loop that leads to the further intensification of local wind and convection anomalies. Work with mesoscale or general circulation models is needed to confirm that surface latent heat and convergence variations are indeed important for modulating east Pacific MJO convection.
Enhanced MJO convection over the boreal summer east Pacific is accompanied by positive water vapor anomalies throughout the troposphere. Column precipitable water anomalies from both NASA Water Vapor Project (NVAP) and NCEP reanalysis are in phase with MJO convection anomalies over the east Pacific. These results support the observations of previous studies that the equatorial troposphere must be sufficiently moistened before significant MJO deep convection can occur. The strongest NCEP reanalysis specific humidity anomalies at lower levels are collocated with positive surface latent heat flux and surface convergence anomalies
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The Effects of SST-Induced Surface Wind Speed and Direction Gradients on Midlatitude Surface Vorticity and Divergence
The effects of surface wind speed and direction gradients on midlatitude surface vorticity and divergence fields associated with mesoscale sea surface temperature (SST) variability having spatial scales of 100â1000 km are investigated using vector wind observations from the SeaWinds scatterometer on the Quick Scatterometer (QuikSCAT) satellite and SST from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) Aqua satellite. The windâSST coupling is analyzed over the period June 2002âAugust 2008, corresponding to the first 6+ years of the AMSR-E mission. Previous studies have shown that strong wind speed gradients develop in response to persistent mesoscale SST features associated with the Kuroshio Extension, Gulf Stream, South Atlantic, and Agulhas Return Current regions. Midlatitude SST fronts also significantly modify surface wind direction; the surface wind speed and direction responses to typical SST differences of about 2°â4°C are, on average, about 1â2 m sâ»Âč and 4°â8°, respectively, over all four regions. Wind speed perturbations are positively correlated and very nearly collocated spatially with the SST perturbations. Wind direction perturbations, however, are displaced meridionally from the SST perturbations, with cyclonic flow poleward of warm SST and anticyclonic flow poleward of cool SST.
Previous observational analyses have shown that small-scale perturbations in the surface vorticity and divergence fields are related linearly to the crosswind and downwind components of the SST gradient, respectively. When the vorticity and divergence fields are analyzed in curvilinear natural coordinates, the wind speed contributions to the SST-induced vorticity and divergence depend equally on the crosswind and downwind SST gradients, respectively. SST-induced wind direction gradients also significantly modify the vorticity and divergence fields, weakening the vorticity response to crosswind SST gradients while enhancing the divergence response to downwind SST gradients.Keywords: Convergence/divergence, Wind stress, Vorticity, Surface observations, Sea surface temperatur
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Satellite Observations of the Wind Jets off the Pacific Coast of Central America. Part I: Case Studies and Statistical Characteristics
Measurements of near-surface winds by the NASA scatterometer (NSCAT) from October 1996 through June 1997 are analyzed to investigate the three major wind jets along the Pacific coast of Central America that blow over the Gulfs of Tehuantepec, Papagayo, and Panama. Each jet is easily identifiable as locally intense offshore winds in the lee of low-elevation gaps through the Sierra Madre mountain range. The jets have relatively narrow cross-stream width but often extend several hundred kilometers or more into the Pacific. The Tehuantepec and Papagayo jets sometimes merge with the northeast trade winds of the Pacific.
The Tehuantepec jet was highly energetic with characteristic timescales of about 2 days. Events were triggered by high pressures associated with cold surges into the Gulf of Mexico that originated over the Great Plains of North America. The Papagayo and Panama jets were much more persistent than the Tehuantepec jets. The winds at both of these lower-latitude locations exhibited a strong seasonal variation with almost exclusively offshore flow from late November 1996 through late May 1997 and periods of onshore flow in October and November during the late stages of the 1996 Central American monsoon season. Superimposed on this low-frequency seasonal variation were events with characteristic timescales of a few days.
Based on NSCAT data, the spatial and temporal evolution of major wind events is described in detail for three representative case studies. In December 1996, the jets developed sequentially from north to south, consistent with the notion that wind events in the two lower-latitude jets are associated with cold-air outbreaks that trigger the Tehuantepec jet a day or so earlier. In November 1996 and March 1997, the Papagayo and Panama jets were strongly influenced by tropical phenomena that had little apparent association with the Tehuantepec jet. These latter two case studies, together with the distinction between the statistical characteristics of the three jets, suggest that the Papagayo and Panama jets are predominantly controlled by a mechanism that is very different from the across-gap pressure gradients associated with high pressure systems of midlatitude origin that control the Tehuantepec jet
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Subseasonal SST Variability in the Tropical Eastern North Pacific during Boreal Summer
Boreal summer intraseasonal (30â90-day time scale) sea surface temperature (SST) variability in the east Pacific warm pool is examined using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) sea surface temperatures during 1998â2005. Intraseasonal SST variance maximizes at two locations in the warm pool: in the vicinity of 9°N, 92°W near the Costa Rica Dome and near the northern edge of the warm pool in the vicinity of 19°N, 108°W. Both locations exhibit a significant spectral peak at 50â60-day periods, time scales characteristic of the MaddenâJulian oscillation (MJO). Complex empirical orthogonal function (CEOF) and spectra coherence analyses are used to show that boreal summer intraseasonal SST anomalies are coherent with precipitation anomalies across the east Pacific warm pool. Spatial variations of phase are modest across the warm pool, although evidence exists for the northward progression of intraseasonal SST and precipitation anomalies. Intraseasonal SSTs at the north edge of the warm pool lag those in the vicinity of the Costa Rica Dome by about 1 week.
The MJO explains 30%â40% of the variance of intraseasonal SST anomalies in the east Pacific warm pool during boreal summer. Peak-to-peak SST variations of 0.8°â1.0°C occur during MJO events. SST is approximately in quadrature with MJO precipitation, with suppressed (enhanced) MJO precipitation anomalies leading positive (negative) SST anomalies by 7â10 days. Consistent with the CEOF and coherence analyses, MJO-related SST and precipitation anomalies near the Costa Rica Dome lead those at the northern edge of the warm pool by about 1 week.Keywords: Warm pool, Tropics, Madden-Julian oscillation, Sea surface temperatur
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Observations of SST-Induced Perturbations of the Wind Stress Field over the Southern Ocean on Seasonal Timescales
The surface wind stress response to sea surface temperature (SST) over the latitude range 30°â60°S in the Southern Ocean is described from the National Aeronautics and Space Administration's QuikSCAT scatterometer observations of wind stress and Reynolds analyses of SST during the 2-yr period August 1999 to July 2001. While oceanâatmosphere coupling at midlatitudes has previously been documented from several case studies, this is the first study to quantify this relation over the entire Southern Ocean. The spatial structures of the surface wind perturbations with wavelengths shorter than 10° latitude by 30° longitude are closely related to persistent spatial variations of the SST field on the same scales. The wind stress curl and divergence are shown to be linearly related, respectively, to the crosswind and downwind components of the SST gradient. The curl response has a magnitude only about half that of the divergence response. This observed coupling is consistent with the hypothesis that SST modification of marine atmospheric boundary layer (MABL) stability affects vertical turbulent mixing of momentum, inducing perturbations in the surface winds. The nonequivalence between the responses of the curl and divergence to the crosswind and downwind SST gradients suggests that secondary circulations in the MABL may also play an important role by producing significant perturbations in the surface wind field near SST fronts that are distinct from the vertical turbulent transfer of momentum. The importance of the wind stress curl in driving Ekman vertical velocity in the open ocean implies that the coupling between winds and SST may have important feedback effects on upper ocean processes near SST front
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Numerical Simulation of Boundary Layer Structure and Cross-Equatorial Flow in the Eastern Pacific
Recent observations from spaceborne microwave sensors have revealed detailed structure of the surface
flow over the equatorial eastern Pacific in the boreal fall season. A marked acceleration of surface wind
across the northern sea surface temperature (SST) front of the cold tongue is a prominent feature of the
regional climate. Previous studies have attributed the acceleration to the effect of enhanced momentum
mixing over the warmer waters. A high-resolution numerical model is used to examine the cross-frontal flow
adjustment. In a comprehensive comparison, the model agrees well with many observed features of crossequatorial
flow and boundary layer structure from satellite, Tropical Atmosphere Ocean (TAO) moorings,
and the recent Eastern Pacific Investigation of Climate Processes (EPIC) campaign. In particular, the model
simulates the acceleration across the SST front, and the change from a stable to unstable boundary layer.
Analysis of the model momentum budget indicates that the hydrostatic pressure gradient, set up in response
to the SST gradient, drives the surface northward acceleration. Because of thermal advection by the mean
southerly flow, the pressure gradient is located downstream of the SST gradient and consequently, divergence
occurs over the SST front, as observed by satellite. Pressure gradients also act to change the vertical
shear of the wind as the front is crossed. However, the model underpredicts the changes in vertical wind
shear across the front, relative to the EPIC observations. It is suggested that the vertical transfer of
momentum by mixing, a mechanism described by Wallace et al. may also act to enhance the change in shear
in the observations, but the model does not simulate this effect. Reasons for this are discussed
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Coupling between Sea Surface Temperature and Low-Level Winds in Mesoscale Numerical Models
This study evaluates the impacts of sea surface temperature (SST) specification and grid resolution on numerical simulations of airâsea coupling near oceanic fronts through analyses of surface winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The 9 May 2001 change of the boundary condition from the Reynolds SST analyses to the NOAA Real-Time Global (RTG) SST in the ECMWF model resulted in an abrupt increase in mesoscale variance of the model surface winds over the ocean. In contrast, the 21 November 2000 change of the grid resolution resulted in an abrupt increase in mesoscale variability of surface winds over mountainous regions on land but had no significant effect on winds over the ocean.
To further investigate model sensitivity to the SST boundary condition and grid resolution, a series of simulations were made with the Weather Research and Forecasting (WRF) model over a domain encompassing the Agulhas return current (ARC: also called âretroflectionâ) region in the south Indian Ocean. Results from three WRF simulations with SST measured by the Advanced Microwave Scanning Radiometer on the Earth Observing System Aqua satellite (AMSR-E) and the Reynolds and RTG SST analyses indicate the vital importance of the resolution of the SST boundary condition for accurate simulation of the airâsea coupling between SST and surface wind speed. WRF simulations with grid spacings of 40 and 25 km show that the latter increased energy only on scales shorter than 250 km. In contrast, improved resolution of SST significantly increased the mesoscale variability for scales up to 1000 km.
Further sensitivity studies with the WRF model conclude that the weak coupling of surface wind speeds from the ECMWF model to SST is likely attributable primarily to the weak response of vertical turbulent mixing to SST-induced stability in the parameterization of boundary layer turbulence, with an overestimation of vertical diffusion by about 60% on average in stable conditions and an underestimation by about 40% in unstable conditionsKeywords: Wind, Mesoscale models, Airâsea interaction, Fronts, Sea surface temperatur