Equivalent Neutral Wind Research Articles

Development of X-Band Geophysical Model Function for Sea Surface Wind Speed Retrieval with ASNARO-2

In the present study, a new geophysical model function (GMF) is developed for the X-band synthetic aperture radar (SAR) on board the Advanced Satellite with New System Architecture for Observation-2 (ASNARO-2) to retrieve accurate offshore wind speeds. Equivalent neutral wind speeds based on the local forecast model (LFM) are employed as reference wind vectors, and 12,259 matching points from 502 SAR images obtained with horizontal transmitting, horizontal receiving polarization around Japan are collected. To ensure convergence of the calculation, 8129 points are selected from the matching points to determine the basic formula for the GMF and 23 coefficients based on the relationships among the normalized radar cross section, wind speed, incidence angle, and relative wind direction. Compared with the reference wind speeds, the GMF wind speeds showed reproducibility with a bias of −0.10 m/s and an RMSD of 1.37 m/s. Additionally, it can be confirmed that the retrieved wind speed has the bias of 0.03 and the RMSD of 1.68 m/s when compared to the in situ wind speed from the Kuroshio Extension Observatory (KEO) buoy. The accuracy of these retrieved wind speeds is comparable to previous studies, and it is indicated that the developed GMF can be used to retrieve offshore winds from ASNARO-2 images.

MODELING OF SPATIAL-TEMPORAL VARIATIONS OF PHYSICAL PROCESS PARAMETERS IN THE IONOSPHERIC PLASMA OVER UKRAINE DURING THE MAXIMUM PHASE OF THE 24-TH SOLAR ACTIVITY CYCLE (2012—2015)

The object of research is physical processes occurring in the ionospheric plasma. The subject of research: spatial-temporal variations of the main parameters of the ionospheric plasma, which were obtained using incoherent scatter radar. Research methods include the ground-based radiophysical method of incoherent scatter of radio waves, statistical analysis of observation results, and semi-empirical modeling of parameters of dynamic and thermal processes. Modeling and analysis of spatial-temporal variations of the dynamic and thermal processes parameters in the ionosphere during the maximum phase of the 24th solar activity cycle (2012—2015) have been performed using experimental data obtained by the Kharkiv incoherent scatter radar. For the periods of the equinoxes and solstices, diurnal dependences of process parameters in the ionospheric plasma at altitudes from 210 to 450 km were constructed. The values of plasma transfer velocity due to ambipolar diffusion, the density of the full plasma flux, and the flux of charged particles due to ambipolar diffusion were determined. The values of the energy supplied to the electron gas, the density of the heat flux transferred by electrons from the plasmasphere to the ionosphere, as well as the velocity of the equivalent neutral wind and the meridional component of the neutral wind velocity, were calculated. It was established that for the considered periods, the effects of space weather variations and geomagnetic activity were significantly manifested in variations of the plasma flux density due to ambipolar diffusion, the density of the full plasma flux, as well as the energy supplied to electrons per unit time. Quantitative and qualitative characteristics of these parameters were typical but changed significantly (by 2…3 times) in some cases, even with a slight increase in geomagnetic activity.

MODELING OF SPATIAL-TEMPORAL VARIATIONS OF DYNAMIC AND THERMAL PROCESS PARAMETERS IN GEOSPACE OVER UKRAINE DURING THE MINIMUM OF 24-TH CYCLE OF SOLAR ACTIVITY (2009, 2019)

We have performed the modelling of spatiotemporal variations of parameters of dynamic and thermal processes in ionospheric plasma at the phases of the minimum of the 24-th cycle of solar activity according to the Kharkiv radar of incoherent scattering. The diurnal dependences of parameters of the processes in the ionospheric plasma at altitudes from 210 to 450 km are constructed for typical geophysical periods (vernal and autumn equinoxes, summer and winter solstices). In the paper, the analysis of spatial and temporal variations of parameters of dynamic and thermal processes in the ionosphere is presented. We determined the value of the plasma transfer velocity due to ambipolar diffusion, the density of the full plasma flux, and the flux of charged particles due to ambipolar diffusion, the value of the energy supplied to the electron gas, the density of the heat flux transferred by electrons from the plasmasphere to the ionosphere, as well as the velocity of the equivalent neutral wind, and the meridional component of the neutral wind velocity. We found that weak variations in space weather do not lead to significant changes in spatiotemporal variations of the parameters of dynamic and thermal processes in the ionosphere for most of the studied periods. Quantitative and qualitative characteristics of most of these parameters and their diurnal variations were typical for the considered seasons. On the contrary, the velocity of the equivalent neutral wind changed significantly (up to 2—2.5 times) even with a weak increase in geomagnetic activity. The reasons for such changes may be the strengthening of horizontal thermospheric winds and the penetration of zonal magnetospheric electric fields into midlatitudes during the equinoxes. The obtained results of calculations can be used in basic studies of solar-terrestrial relations and geospace, for the solution of applied problems related to the ability to predict the state of space weather, as well as for further development of the regional ionosphere model CERIM IION. The object of research: physical processes in the ionospheric plasma. The subject of research: spatiotemporal dependences of the main parameters of ionospheric plasma, which were obtained using incoherent scattering radar. Research methods — terrestrial radiophysical method of incoherent scatter of radio waves, statistical analysis of observation results, semi-empirical modelling of parameters of dynamic and thermal processes.

The impact of wind corrections and ocean-current influence on wind stress forcing on the modeling of Pacific North Equatorial Countercurrent

Bias correction of reanalysis-based wind stress using scatterometer derived equivalent neutral wind has been a common practice in producing the forcing datasets used in recent global ocean model intercomparisons (OMIPs). Here we systematically evaluate the effect of this wind correction procedure on the simulation of the Pacific North Equatorial Countercurrent (NECC) with multiple sets of model experiments. The weak NECC evident in earlier OMIPs employing the Coordinated Ocean-ice Reference Experiments (COREs) forcing dataset persists with the new JRA55-do (Japanese 55-year Reanalysis) forcing dataset. Two factors appear to significantly affect the Pacific NECC in forced ocean simulations: i) the bias correction procedure using QuikSCAT derived winds and ii) whether or not the ocean current is considered in the bulk formula. In the forced ocean simulations, the QuikSCAT correction weakens the averaged NECC transports by about 60%. Taking the ocean currents into account in the bulk formula may weaken the averaged NECC transports by about 26%–30%. Under the current OMIP protocol the above two procedures are used together to force the ocean model resulting in a double-counting of ocean surface current feedback on wind stress because the QuikSCAT estimates the equivalent 10-m neutral winds relative to surface current. We further systematically verify and investigate the impacts of this double-counting of the ocean surface currents on the modeled Pacific NECC using offline linear Sverdrup transport analysis, in which the observational data of vector wind and surface current are used to calculate the surface wind stress. It shows that including the ocean current in the bulk formula may reduce the zonal Sverdrup transport (ZST) by about 6.6 Sv (33%) and the further double-counting of the ocean current leads to an additional reduction of 6.4 Sv (48%). Next, using “perfect model” experiments with output from a coupled ocean–atmosphere model we further identify that the double-counting of current feedback in the bulk formula results in approximately 21% weakened volume transport. The built-in nonlinear processes in the model, such as the advection and friction terms, may partly damp the reduction due to the double-counting of the ocean current. However, the double-counting bias can only explain 26%–30% of the Pacific NECC simulation bias and the other part of the bias, around 30%, caused by the correction with QuikSCAT has not been explained. We speculate that this part may be explained by the retrieval biases in QuikSCAT wind data and the use of annual mean climatological wind adjustment factors.

Accuracy of Wind Observations from Open-Ocean Buoys: Correction for Flow Distortion

AbstractThe comparison of equivalent neutral winds obtained from (i) four WHOI buoys in the subtropics and (ii) scatterometer estimates at those locations reveals a root-mean-square (RMS) difference of 0.56–0.76 m s−1. To investigate this RMS difference, different buoy wind error sources were examined. These buoys are particularly well suited to examine two important sources of buoy wind errors because 1) redundant anemometers and a comparison with numerical flow simulations allow us to quantitatively assess flow distortion errors, and 2) 1-min sampling at the buoys allows us to examine the sensitivity of buoy temporal sampling/averaging in the buoy–scatterometer comparisons. The interanemometer difference varies as a function of wind direction relative to the buoy wind vane and is consistent with the effects of flow distortion expected based on numerical flow simulations. Comparison between the anemometers and scatterometer winds supports the interpretation that the interanemometer disagreement, which can be up to 5% of the wind speed, is due to flow distortion. These insights motivate an empirical correction to the individual anemometer records and subsequent comparison with scatterometer estimates show good agreement.

Surface Stress and Atmospheric Boundary Layer Response to Mesoscale SST Structure in Coupled Simulations of the Northern California Current System

Abstract The influence of mesoscale sea surface temperature (SST) variations on wind stress and boundary layer winds is examined from coupled ocean–atmosphere numerical simulations and satellite observations of the northern California Current System. Model coupling coefficients relating the divergence and curl of wind stress and wind to downwind and crosswind SST gradients are generally smaller than observed values and vary by a factor of 2 depending on planetary boundary layer (PBL) scheme, with values larger for smoothed fields on the 0.25° observational grid than for unsmoothed fields on the 12-km model grid. Divergence coefficients are larger than curl coefficients on the 0.25° grid but not on the model grid, consistent with stronger scale dependence for the divergence response than for curl in a spatial cross-spectral analysis. Coupling coefficients for 10-m equivalent neutral stability winds are 30%–50% larger than those for 10-m wind, implying a correlated effect of surface-layer stability variations. Crosswind surface air temperature and SST gradients are more strongly coupled than downwind gradients, while the opposite is true for downwind and crosswind heat flux and SST gradients. Midlevel boundary layer wind coupling coefficients show a reversed response relative to the surface that is predicted by an analytical model; a predicted second reversal with height is not seen in the simulations. The relative values of coupling coefficients are consistent with previous results for the same PBL schemes in the Agulhas Return Current region, but their magnitudes are smaller, likely because of the effect of mean wind on perturbation heat fluxes.

Coupling Ocean Currents and Waves with Wind Stress over the Gulf Stream

This study provides the first detailed analysis of oceanic and atmospheric responses to the current-stress, wave-stress, and wave-current-stress interactions around the Gulf Stream using a high-resolution three-way coupled regional modeling system. In general, our results highlight the substantial impact of coupling currents and/or waves with wind stress on the air–sea fluxes over the Gulf Stream. The stress and the curl of the stress are crucial to mixed-layer energy budgets and sea surface temperature. In the wave-current-stress coupled experiment, wind stress increased by 15% over the Gulf Stream. Alternating positive and negative bands of changes of Ekman-related vertical velocity appeared in response to the changes of the wind stress curl along the Gulf Stream, with magnitudes exceeding 0.3 m/day (the 95th percentile). The response of wind stress and its curl to the wave-current-stress coupling was not a linear combination of responses to the wave-stress coupling and the current-stress coupling because the ocean and wave induced changes in the atmosphere showed substantial feedback on the ocean. Changes of a latent heat flux in excess of 20 W/m2 and a sensible heat flux in excess of 5 W/m2 were found over the Gulf Stream in all coupled experiments. Sensitivity tests show that sea surface temperature (SST) induced difference of air–sea humidity is a major contributor to latent heat flux (LHF) change. Validation is challenging because most satellite observations lack the spatial resolution to resolve the current-induced changes in wind stress curls and heat fluxes. Scatterometer observations can be used to examine the changes in wind stress across the Gulf Stream. The conversion of model data to equivalent neutral winds is highly dependent on the physics considered in the air–sea turbulent fluxes, as well as air–sea temperature differences. This sensitivity is shown to be large enough that satellite observations of winds can be used to test the flux parameterizations in coupled models.

Disentangling the Mesoscale Ocean‐Atmosphere Interactions

AbstractIn the decades, the use of scatterometer data allowed to demonstrate the global ubiquity of the ocean mesoscale thermal feedback (TFB) and current feedback (CFB) effects on surface winds and stress. Understanding these air‐sea interactions is of uttermost importance as the induced atmospheric anomalies partly control the ocean circulation and thus can influence the Earth climate. Whether the TFB and CFB effects can be disentangled, and whether satellite scatterometers can properly reveal them, remain rather unclear. Here, using satellite observations and ocean‐atmosphere coupled mesoscale simulations over 45°S to 45°N, we show that the CFB effect can be properly characterized and unraveled from that due to the TFB. We demonstrate that the TFB can be unambiguously characterized by its effect on the stress (and wind) divergence and magnitude. However, its effect on the wind and stress curl is contaminated by the CFB and thus cannot be estimated from scatterometer data. Finally, because scatterometers provide equivalent neutral stability winds relative to the oceanic currents, they cannot characterize adequately the CFB wind response and overestimate the TFB wind response by ≈25%. Surface stress appears to be the more appropriate variable to consider from scatterometer data.

MENTAT: A New Wind Model for Earth's Thermosphere

AbstractA global database of ionosonde hmF2 observations was used to develop a global database of equivalent neutral winds, which was then used to develop a new empirical horizontal neutral wind model called the Magnetic mEridional NeuTrAl Thermospheric model. In this paper, we (1) developed and optimized a technique for deriving magnetic meridional thermospheric neutral winds using the altitude of the peak ionosphere (hmF2) observations from bottomside ionospheric sounders, (2) validated these neutral winds with observations from Fabry‐Pérot interferometers (FPIs), (3) developed a new model of horizontal, magnetic meridional, equivalent neutral winds in the midlatitude regions from a global database of ionosonde observations spanning the years 1961 to 1990. Our new method for generating the winds compares well with FPI wind observations and illuminates nonphysical behavior in the horizontal thermospheric neutral wind model. Magnetic mEridional NeuTrAl Thermospheric neutral winds compare well with FPI wind observations over both short‐ and long‐time periods, and the model produces winds as a function of the year, day of year, universal time, solar flux, geographic latitude, and geographic longitude. Two major findings are (1) a distinct solar cycle wind variation that was not captured in other empirical models using other data sets and (2) global measurements of ionospheric hmF2, which provide constraints on the global wind field that is particularly valuable considering the persistent lack of space‐based F region winds and the inability of FPIs to operate during the day, when cloudy, or under moon‐up conditions.

Two decades [1992–2012] of surface wind analyses based on satellite scatterometer observations

Surface winds (equivalent neutral wind velocities at 10m) from scatterometer missions since 1992 have been used to build up a 20-year climate series. Optimal interpolation and kriging methods have been applied to continuously provide surface wind speed and direction estimates over the global ocean on a regular grid in space and time. The use of other data sources such as radiometer data (SSM/I) and atmospheric wind reanalyses (ERA-Interim) has allowed building a blended product available at 1/4° spatial resolution and every 6h from 1992 to 2012. Sampling issues throughout the different missions (ERS-1, ERS-2, QuikSCAT, and ASCAT) and their possible impact on the homogeneity of the gridded product are discussed. In addition, we assess carefully the quality of the blended product in the absence of scatterometer data (1992 to 1999). Data selection experiments show that the description of the surface wind is significantly improved by including the scatterometer winds. The blended winds compare well with buoy winds (1992–2012) and they resolve finer spatial scales than atmospheric reanalyses, which make them suitable for studying air-sea interactions at mesoscale. The seasonal cycle and interannual variability of the product compare well with other long-term wind analyses. The product is used to calculate 20-year trends in wind speed, as well as in zonal and meridional wind components. These trends show an important asymmetry between the southern and northern hemispheres, which may be an important issue for climate studies.

Daytime twin-peak structures observed at southern African and European middle latitudes on 8–13 April 2012

Abstract. Daytime twin-peak structures, also known as bite-out or diurnal double-maxima structures, are ionospheric phenomena in which the diurnal ionospheric trend shows two peaks (instead of the normal one) during the daytime. This study reports on first simultaneous observations of these structures in the Global Positioning System and ionosonde measurements from the southern African and European middle-latitude stations during a mostly quiet geomagnetic condition period of 8–13 April 2012, which indicates that their occurrence and therefore driving mechanism(s) may not be localised. It is found that the daytime twin-peak structures generally appear later in the Northern Hemisphere with a 1–3 h latency although they propagate mostly equatorward in both hemispheres. Proxies of meridional neutral winds were calculated from available manually scaled ionosonde measurements and used to explore their potential as drivers of the structures. Bite-out events were linked to downward drifts of the vertical component of equivalent neutral winds causing plasma depletions. In addition, evidence of sporadic E layers at the same time as enhancements of daytime twin-peak structures suggests that the tides had influence via the meridional wind shear in generating these structures through the dynamo electric field which resulted in upward E × B drifts.

Homogenization of scatterometer wind retrievals

ABSTRACTSurface winds (10 m equivalent neutral wind velocity) from scatterometer missions since 1992 to present require homogenization to meet the requirements for oceanic and atmospheric climate data records. Sources of differences between winds retrieved from different scatterometer measurements mainly arise from calibration/validation procedures used for each scatterometer and differences in measurement physics. In this study, we focus on the calibration/validation component of the European Remote Sensing Satellite (ERS)‐1 and ERS‐2 wind speed biases. ERS‐1 and ERS‐2 data, named as WNF products, are from the Institut Français de Recherche pour l'Exploitation de la MER (IFREMER). In addition to WNF data, the newly calibrated ERS‐2 products provided by the European Space Agency (ESA), indicated as ASPS2.0 products, are also used. Our approach utilizes collocated satellite‐buoy data. Expected values of the normalized radar cross section (NRCS) are calculated from buoy winds for each antenna beam using the Cmod5.n geophysical model function. The comparisons between expected and measured NRCS examine differences along with variables such as backscatter coefficient and incidence angle ranges. The difference between the expected and measured NRCS is then used to set up empirical models aiming at the correction for biases in ERS‐1 and ERS‐2 WNF NRCS calibrations. Finally, ERS‐1 and ERS‐2 wind retrievals are reprocessed using the corrected NRCS and Cmod5.n. These earlier corrected ERS‐1/2 winds are analysed along with later scatterometer data (QuikSCAT and ASCAT‐A) for their deviations from in situ buoy winds during 1992–2011 period. The scatterometer data homogeneity is also investigated at global scales based on the use of collocated scatterometer retrievals and atmospheric re‐analyses winds derived from ERA Interim and CFSR models.

In Situ and Satellite Evaluation of Air–Sea Flux Variation near Ocean Temperature Gradients

Abstract Observations of ocean–atmosphere coupling across persistent mesoscale sea surface temperature (SST) gradients are used to examine the controls of atmospheric stability, pressure gradient force, and heat flux that are considered central to oft-observed coupling between wind and SST. Moored air–sea flux measurements near the Gulf Stream are combined with QuikSCAT satellite scatterometer equivalent neutral wind (ENW) data to assess correlations between SST, air–sea fluxes, pressure, and wind perturbations at scales of 10–100 days. The net effect of ocean fronts meandering past the site enabled buoy observation of SST impacts on wind, with coupling coefficients of 0.3–0.5 similar to past studies. Wind stress–SST and ENW–SST correlation coefficients are slightly higher, and roughly 20% of the ENW perturbation is attributed to stratification impacts predicted by Monin–Obukhov (MO) similarity theory. Significantly higher correlation is observed when relating wind or stress perturbations to buoyant heat flux variation. Atmospheric pressure perturbation with SST of order 0.5 hPa °C−1 is observed, as well as high negative correlation between wind and pressure variations. Length and time scales associated with the coupling indicate that peak correlations occur at 50–70 days and 300–500 km, consistent with mesoscale meander scales. Coupling coefficient values vary significantly depending on analysis time scale and exhibit a range near to recently observed interbasin variability. This variability is attributed to the extent of oceanic length scales permitted in the analysis. Together, results affirm the central role of SST-induced turbulent heat flux in controlling pressure field adjustments and thereby the wind perturbations over SST fronts.

The collapse of the midnight ionosphere and behavior of meridional neutral winds at Townsville over a full solar cycle

AbstractThis paper investigates the causes of the sudden descent (midnight collapse) of the ionosphere at Townsville, Australia, during the equinox periods of years between 1970 and 1980. The collapse of hmF2 at midnight is found to occur on 89% of the 330 equinox nights that are investigated, and the mean magnitude of the midnight collapse is 84 km in the March equinox periods and 99 km in the September equinox periods. Observations of hmF2 are used to determine equivalent meridional neutral winds using a first principles physics model. Harmonic analysis of these derived winds reveals the existence of significant diurnal (24 h), semidiurnal (12 h), and terdiurnal (8 h) tidal components. The contribution of wind harmonics to the midnight collapse is determined by band‐pass filtering the winds to only allow certain tides and then modeling their effect on hmF2 near midnight. The results indicate that the diurnal, semidiurnal, and terdiurnal components of the meridional neutral wind all play a significant role at various times, but the effect of the 6 h wind component is minimal. The spectral analysis also reveals that the terdiurnal wind component becomes dominant during solar maximum. Electric fields do not appear to be responsible for the midnight hmF2 collapse because it is seldom seen at the near‐conjugate station of Akita, Japan.

Comparison of Oceansat-2 scatterometer- to buoy-recorded winds and spatial distribution over the Arabian Sea during the monsoon period

For this wind resource assessment (WRA) study, wind speed and direction are the fundamental inputs. Also, these studies are data driven and require large historical wind speed data sets available on the site. This work explores the application of space-based scatterometer winds for assimilation into WRA studies towards the development of offshore wind energy. This article focuses on estimating the performance of Oceansat-2 scatterometer (OSCAT)-derived wind vector using in situ data from buoys at different locations in the Arabian Sea. A comparative study between three methods for estimating the equivalent neutral winds (ENW) for buoys is carried out. OSCAT winds were closest to ENW estimated by the Liu–Katsaros–Businger (LKB) method. The spatial and temporal windows for comparison were 0.5° and ±60 minutes, respectively. The monsoon months (June–September) of 2011 were selected for study. The root mean square deviation for wind speed is less than 2.5 m s−1 and wind direction is less than 20°, and a small positive bias is observed in the OSCAT wind values. From the analysis, the OSCAT wind values are consistent with in situ-observed values. Furthermore, wind atlas maps were developed with OSCAT winds, representing the spatial distribution of winds at a height of 10 m over the Arabian Sea.

Satellite winds as a tool for offshore wind resource assessment: The Great Lakes Wind Atlas

Abstract This work presents a new observational wind atlas for the Great Lakes, and proposes a methodology to combine in situ and satellite wind observations for offshore wind resource assessment. Efficient wind energy projects rely on accurate wind resource estimates, which are complex to obtain offshore due to the temporal and spatial sparseness of observations, and the potential for temporal data gaps introduced by the formation of ice during winter months, especially in freshwater lakes. For this study, in situ observations from 70 coastal stations and 20 buoys provide diurnal, seasonal, and interannual wind variability information, with time series that range from 3 to 11 years in duration. Remotely-sensed equivalent neutral winds provide spatial information on the wind climate. NASA QuikSCAT winds are temporally consistent at a 25 km resolution. ESA Synthetic Aperture Radar winds are temporally sparse but at a resolution of 500 m. As an initial step, each data set is processed independently to create a map of 90 m wind speeds. Buoy data are corrected for ice season gaps using ratios of the mean and mean cubed of the Weibull distribution, and reference temporally-complete time series from the North American Regional Reanalysis. Generalized wind climates are obtained for each buoy and coastal site with the wind model WAsP, and combined into a single wind speed estimate for the Great Lakes region. The method of classes is used to account for the temporal sparseness in the SAR data set and combine all scenes into one wind speed map. QuikSCAT winds undergo a seasonal correction due to lack of data during the cold season that is based on its ratio relative to buoy time series. All processing steps reduce the biases of the individual maps relative to the buoy observed wind climates. The remote sensing maps are combined by using QuikSCAT to scale the magnitude of the SAR map. Finally, the in situ predicted wind speeds are incorporated. The mean spatial bias of the final map when compared to buoy time series is 0.1 ms− 1 and the RMSE 0.3 ms− 1, which represents an uncertainty reduction of 50% relative to using only SAR, and of 40% to using only SAR and QuikSCAT without in situ observations.

Comparing in situ and satellite‐based parameterizations of oceanic whitecaps

AbstractThe majority of the parameterizations developed to estimate whitecap fraction uses a stability‐dependent 10 m wind (U10) measured in situ, but recent efforts to use satellite‐reported equivalent neutral winds (U10EN) to estimate whitecap fraction with the same parameterizations introduce additional error. This study identifies and quantifies the differences in whitecap parameterizations caused by U10 and U10EN for the active and total whitecap fractions. New power law coefficients are presented for both U10 and U10EN parameterizations based on available in situ whitecap observations. One‐way analysis of variance (ANOVA) tests are performed on the residuals of the whitecap parameterizations and the whitecap observations and identify that parameterizations in terms of U10 and U10EN perform similarly. The parameterizations are also tested against the satellite‐based WindSat Whitecap Database to assess differences. The improved understanding aids in estimating whitecap fraction globally using satellite products and in determining the global effects of whitecaps on air‐sea processes and remote sensing of the surface.

The Effects of Gap-Wind-Induced Vorticity, the Monsoon Trough, and the ITCZ on East Pacific Tropical Cyclogenesis

Abstract Tropical cyclogenesis in the eastern North Pacific (EPAC) basin is related to gap-wind-induced surface relative vorticity, the monsoon trough, and the intertropical convergence zone (ITCZ). There are several gaps in the Central American mountains, on the eastern edge of the EPAC basin, through which wind can be funneled to generate surface wind jets (gap winds). This study focuses on gap winds that occur over the Gulf of Papagayo and Gulf of Tehuantepec. Quick Scatterometer (QuikSCAT) 10-m equivalent neutral winds are used to identify gap wind events that occur during May through November of 2002–08. Dvorak fix locations, Gridded Satellite (GridSat) infrared (IR) data, and National Hurricane Center (NHC) tropical cyclone (TC) reports are used to track the disturbances during the study period. Surface vorticity is tracked using the QuikSCAT winds and the contribution of surface vorticity from the gap winds to the development of each disturbance is categorized as small, medium, or large. Cross-calibrated multiplatform surface wind data are used to verify the tracking of QuikSCAT-computed surface vorticity and to identify when the monsoon trough and the ITCZ are present. It is found that gap winds are present over the Gulf of Papagayo and Gulf of Tehuantepec for about 50% of the QuikSCAT coverage days and that these gap winds appear to contribute to the development of disturbances in the EPAC. Considerably more TCs form when the monsoon trough is present versus the ITCZ and the majority of the contributions from the gap winds also occur when the monsoon trough is present.

Effects of Atmospheric Stability and Wind Fetch on Microwave Sea Echoes

The influences of atmospheric stability and wind fetch on microwave scattering from sea surfaces are presented. The equivalent neutral wind speed (ENWS) and the friction velocity (FV) above sea surface are evaluated by the similarity theory of Monin and Obukhov in combination with the Charnock relation. The numerical simulations show that atmospheric stability and wind fetch would make effects on ENWS and, therefore, further on the scattering coefficients (NRCS). And the NRCS decreases with increasing air-sea temperature difference (ΔT=Ta-Ts) because the ENWS is lower than actual wind speed in stable atmospheric condition, and vice versa. Moreover, the ENWS/FV and the NRCS decrease with increasing wind fetch under unstable atmospheric condition. However, in stable atmospheres, the ENWS/FV and the NRCS are almost insensitive to wind fetch. In this paper, the response of NRCS to ΔT is also discussed. Compared with FV, the variations of the angle spread function with ΔT can be neglected because of its minor contribution.

First six months quality assessment of HY-2A SCAT wind products using in situ measurements

The first Chinese microwave ocean environment satellite HY-2A, carrying a Ku-band scatteromenter (SCAT), was successfully launched in August 2011. The first quality assessment of HY-2A SCAT wind products is presented through the comparison of the first 6 months operationally released SCAT products with in situ data. The in situ winds from the National Data Buoy Center (NDBC) buoys, R/V Polarstern, Aurora Australis, Roger Revelle and PY30-1 oil platform, were converted to the 10 m equivalent neutral winds. The temporal and spatial differences between the HY-2A SCAT and the in situ observations were limited to less than 5 min and 12.5 km. For HY-2A SCAT wind speed products, the comparison and analysis using the NDBC buoys yield a bias of −0.49 m/s, a root mean square error (RMSE) of 1.3 m/s and an increase negative bias with increasing wind speed observation above 3m/s. Although less accurate of HY-2A SCAT wind direction at low winds, the RMSE of 19.19° with a bias of 0.92° is found for wind speeds higher than 3 m/s. These results are found consistent with those from R/Vs and oil platform comparisons. Moreover, the NDBC buoy comparison results also suggest that the accuracy of HY-2A SCAT winds is consistent over the first half year of 2012. The encouraging assessment results over the first 6 months show that wind products from HY-2A SCAT will be useful for scientific community.