Wind Speed and Stability Effects on Coupling between Surface Wind Stress and SST Observed from Buoys and Satellite

The surface wind and stress responses to sea surface temperature (SST) are examined using collocated moored buoy and satellite observations in the Gulf Stream and the eastern equatorial Pacific. Using 17 buoy pairs, differences in the wind speed, 10-m equivalent neutral wind speed (ENW), and surface wind stress magnitude between two buoys separated by between 150 and 350 km were all found to be highly correlated to, and satisfy linear relations with, the SST difference on time scales longer than 10 days. This wind–SST coupling is consistent with previous analyses of spatially high-pass-filtered satellite ENW and SST fields. For all buoy pairs, the ENW and wind speed responses to SST differ by only 10%–30%, indicating that the ENW and stress responses to SST are attributable primarily to the response of the actual surface wind speed to SST rather than to stability. This result clarifies the dynamical pathway of the wind–SST coupling on the oceanic mesoscale. This buoy-pair methodology is used further to evaluate the ENW–SST coupling derived from collocated satellite observations of ENW by the Quick Scatterometer (QuikSCAT) and SST by the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) on board the Aqua satellite. Overall, the satellite and buoy ENW responses to SST compare well, with normalized mean differences (satellite minus buoy) of 17% over the Gulf Stream and −31% and 2% over the southern and northern sides of the equatorial Pacific, respectively. Finally, seasonal variability of the large-scale ENW is shown to modulate the wind stress response to SST, whereby stronger winter wind enhances the stress response by a factor of ~2 relative to the ENW response.Keywords: Air-sea interaction, Atmosphere-ocean interaction, Satellite observations, Sea surface temperature, Buoy observations, Surface layerKeywords: Air-sea interaction, Atmosphere-ocean interaction, Satellite observations, Sea surface temperature, Buoy observations, Surface laye

Surface wind modification near mid-latitude ocean fronts : observational and dynamical analysis

Interactions between surface winds and meanders in mid-latitude sea surface temperature (SST) fronts with horizontal length scales of 100-1000 km are investigated from satellite observations and numerical simulations. Observations from the Sea- Winds scatterometer on the QuikSCAT satellite show that the magnitude, direction, curl, and divergence of the surface wind stress and 10-m winds are well correlated with small-scale SST structures associated with large-scale ocean currents. Detailed analysis of the response of the surface winds to SST fronts from these satellite observations exposed shortcomings in previous conceptual hypotheses governing the relationships between surface winds and SST. To gain understanding of the physical mechanisms needed to explain the satellite wind observations, we performed a numerical experiment simulating the atmospheric flow over meandering SST fronts. Based on these results, a new conceptual model is constructed to explain the dynamical response of the surface winds consistent with the satellite observations and numerical simulation analysis. Of particular importance was the finding that the wind stress curl and divergence fields observed from QuikSCAT are linearly related to the crosswind and downwind components of the SST gradient, respectively. This relationship was generally thought to result from modification of the vertical turbulent mixing of momentum within the atmospheric boundary layer (ABL). We show that this mechanism is overly simplistic; nearly all of the terms in the momentum budget are needed to explain these observed statistical relationships, consistent with recent work. SST-induced surface wind changes are a manifestation of more complicated changes to the vertical structure of the dynamic forces within the ABL. Among the most significant of several new findings presented here concerns the influence of SST on the meridional wind field. Since winds are generally westerly at mid-latitudes, SST-induced changes in meridional wind cause changes in the surface wind direction that significantly influence the wind stress curl and divergence fields through modification of streamline curvature and diffluence. From numerical and analytical results, these meridional wind perturbations are shown to result from a baroclinic Ekman adjustment mechanism modified by horizontal advection

Measurements of Air-Sea Interaction from the HY-2A Scatterometer

International Ocean Vector Wind Science Team Meeting (IOVWST), 2-4 June 2014, Brest, France.-- 21 pagesPeer Reviewe

Estimation of Time-Averaged Surface Divergence and Vorticity from Satellite Ocean Vector Winds

Two methods of computing the time-mean divergence and vorticity from satellite vector winds in rain-free (RF) and all-weather (AW) conditions are investigated. Consequences of removing rain-contaminated winds on the mean divergence and vorticity depend strongly on the order in which the time-average and spatial derivative operations are applied. Taking derivatives first and averages second (DFAS_RF) incorporates only those RF winds measured at the same time into the spatial derivatives. While preferable mathematically, this produces mean fields biased relative to their AW counterparts because of the exclusion of convergence and cyclonic vorticity often associated with rain. Conversely, taking averages first and derivatives second (AFDS_RF) incorporates all RF winds into the time-mean spatial derivatives, even those not measured coincidentally. While questionable, the AFDS_RF divergence and vorticity surprisingly appears qualitatively consistent with the AW means, despite using only RF winds. The analysis addresses whether the AFDS_RF method accurately estimates the AW mean divergence and vorticity.Model simulations indicate that the critical distinction between these two methods is the inclusion of typically convergent and cyclonic winds bordering rain patches in the AFDS_RF method. While this additional information removes some of the sampling bias in the DFAS_RF method, it is shown that the AFDS_RF method nonetheless provides only marginal estimates of the mean AW divergence and vorticity given sufficient time averaging and spatial smoothing. Use of the AFDS_RF method is thus not recommended.C-2013 ASCAT data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team. The JPL version 3 QuikSCAT dataset (https://podaac.jpl.nasa.gov/dataset/QSCAT_LEVEL_2B_OWV_COMP_12) was obtained via the PO.DAAC web portal and was processed by the SeaWinds Processing and Analysis Center (SeaPAC).This is the publisher’s final pdf. The published article is copyrighted by American Meteorological Society and can be found at: https://www2.ametsoc.org/ams/index.cfm/publications/journals/journal-of-climate/Keywords: North Atlantic Ocean, Satellite observations, Error analysis, Mesoscale models, Marine boundary laye

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°–60°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 ocean–atmosphere 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

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 100–1000 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 wind–SST coupling is analyzed over the period June 2002–August 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°–4°C are, on average, about 1–2 m s⁻¹ and 4°–8°, 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

GOES-10 microphysical retrievals in marine warm clouds: Multi-instrument validation and daytime cycle over the southeast Pacific

The daytime evolution of warm cloud microphysical properties over the southeast Pacific during October–November 2008 is investigated with optical/infrared retrievals from the Tenth Geostationary Operational Environmental Satellite (GOES-10) imager. GOES-10 retrievals, produced at NASA Langley Research Center, are validated against in situ aircraft observations and with independent satellite observations. Comparisons with in situ observations reveal high linear correlations (r) for cloud effective radius (r[subscript e]) and optical thickness (τ) (r = 0.89 and 0.69 respectively); nevertheless, a GOES-10 positive mean r[subscript e] bias of 2.3 μm is apparent, and consistent with other previously reported satellite biases. Smaller biases are found for liquid water path (LWP) and an adiabatic-based cloud droplet number concentration (N[subscript d]), both variables derived by combining r[subscript e] and τ. In addition, GOES-10 observations are well correlated with their Moderate Resolution Imaging Spectroradiometer (MODIS) counterparts, but with smaller biases and root-mean-square errors for the Aqua satellite passes, arguably associated with a better calibrated MODIS-Aqua instrument relative to MODIS-Terra. Furthermore, the excellent agreement between GOES-10 LWP and microwave-based satellite retrievals, especially at high solar zenith angles (>60°), provide further evidence of the utility of using GOES-10 retrievals to represent the daytime cloud cycle. In terms of the daytime cycle, GOES-10 observations show an afternoon minimum in LWP and an increase thereafter, consistent with satellite microwave climatologies. The τ cycle explains most of the LWP variance with both variables in phase, minima near noon along the coast, and a 13:30–14:00 local solar time (LST) minimum offshore. In contrast, r[subscript e] is not exactly in phase with LWP and τ, having a minimum approximately at 12:30 LST throughout the domain. A unique feature is a striking r[subscript e] maximum along the coast at 16:15 LST, concomitant with a faster τ recovery. An explanation for a coastal r[subscript e] afternoon maximum is lacking although this is consistent with an enhancement of the updraft velocity reported in previous modeling studies. Finally, the GOES-derived N[subscript d] (N[subscript d] ∝ τ1/2 r[subscript e]−5/2) shows a complex daytime cycle with maxima at 7:15 and 13:15 LST. While the first maximum is driven by large τ, the second one is mainly explained by a minimum in r[subscript e]

High-Resolution Satellite Measurements of the Atmospheric Boundary Layer Response to SST Variations along the Agulhas Return Current

The marine atmospheric boundary layer (MABL) response to sea surface temperature (SST) perturbations with wavelengths shorter than 30° longitude by 10° latitude along the Agulhas Return Current (ARC) is described from the first year of SST and cloud liquid water (CLW) measurements from the Advanced Microwave Scanning Radiometer (AMSR) on the Earth Observing System (EOS) Aqua satellite and surface wind stress measurements from the QuikSCAT scatterometer. AMSR measurements of SST at a resolution of 58 km considerably improves upon a previous analysis that used the Reynolds SST analyses, which underestimate the short-scale SST gradient magnitude over the ARC region by more than a factor of 5. The AMSR SST data thus provide the first quantitatively accurate depiction of the SST-induced MABL response along the ARC. Warm (cold) SST perturbations produce positive (negative) wind stress magnitude perturbations, leading to short-scale perturbations in the wind stress curl and divergence fields that are linearly related to the crosswind and downwind components of the SST gradient, respectively. The magnitudes of the curl and divergence responses vary seasonally and spatially with a response nearly twice as strong during the winter than during the summer along a zonal band between 40° and 50°S. These seasonal variations closely correspond to seasonal and spatial variability of large-scale MABL stability and surface sensible heat flux estimated from NCEP reanalysis fields. SST-induced deepening of the MABL over warm water is evident in AMSR measurements of CLW. Typical annual mean differences in cloud thickness between cold and warm SST perturbations are estimated to be about 300

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 air–sea 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 “retroflection”) 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 air–sea 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, Air–sea interaction, Fronts, Sea surface temperatur

Compendium of Single Event Effects Test Results for Commercial Off-The-Shelf and Standard Electronics for Low Earth Orbit and Deep Space Applications

We present the results of Single Event Effects (SEE) testing with high energy protons and with low and high energy heavy ions for electrical components considered for Low Earth Orbit (LEO) and for deep space applications