Surface Wind Observations Research Articles

A Numerical Study of Circulations Induced by a Dry Salt Lake

Observations of surface winds in the vicinity of a moderate-sized (about 70 km2) dry salt lake by Tapper suggest that differences in albedo and soil thermal properties between the salt and surrounding sand surfaces may be strong enough to drive a mesoscale thermal circulation. In this paper a numerical mesoscale model is used to investigate disturbances generated by moderate and large-sized (about 7000 km2) lakes. In the latter case, a typical size for the great salt lakes of inland Australia, disturbances with strong horizontal and vertical shear are found at a distance of more than 200 km from the lakeshore by midnight. The separate contribution of albedo and soil property differences to the circulation are assessed, under both dry and moist soil conditions. The possible influence of such circulations on regional climate is discussed and it is suggested that salt lakes may be a source for solitary waves observed in the Australian interior.

The Annual Cycle of Meridional Heat Flux in the Atlantic Ocean at 26.5°N

Abstract Total meridional heat flux through a zonal oceanic section at 26.5°N in the Atlantic Ocean is computed from hydrographic, direct current and surface wind observations. The oceanic current and temperature fields are decomposed into depth-averaged and depth-dependent (including Ekman and geostrophic) components to perform the calculation. The mean annual boat flux is estimated to be 1.21 ± 0.34 PW. Mean monthly values of net heat flux are also computed from the data. The annual cycle of net heat flux determined from these values ranges from a minimum of 0.69 PW in February to a maximum of 1.86 PW in July. Thus, in contrast to an earlier estimate of the annual cycle of oceanic heat flux derived indirectly from surface energy fluxes and upper-layer heat content changes, there is no net southward heat flux during the Call. Results from a simulation of the circulation of the North Atlantic give an annual cycle of heat flux similar to our calculations with a summer maximum and winter minimum. However, t...

Surface Winds from Tropical Pacific Islands—Climatological Statistics

Abstract Multidecadal time series of surface wind observations from tropical Pacific islands have been examined in order to investigate the space and time scales of variability. Climatological monthly means and variances are compared with comparable means and variances derived from ship observations; usually the means agree to within ∼1 m s−1 in speed and ∼10 degrees in direction. Annual and semiannual cycles differ in detail. Island zonal wind variances are often significantly larger, by up to 10 m2 s−2 near the equator between September and December; because of the spatial coherence of the island results, these discrepancies are believed to result from the poor high-frequency sampling typical of ship data. A substantial near-equatorial zonal wind variance maximum is shown to be related to ENSO period variability; excluding ENSO time periods leaves a relatively spatially uniform variance of ∼5 m2 s−2 over a broad region. The frequency distribution of variance, derived from daily-averaged data, exhibits c...

Comparisons of GCM and Observed Surface Wind Fields over the Tropical Indian and Pacific Oceans

Many of the processes that have important effects on both the climatological distribution and interannual variability of sea surface temperatures (SSTs) in the tropical oceans are greatly affected by the surface wind field. For this reason accurate simulation of the surface wind is a key factor governing the success of coupled tropical ocean-atmosphere models. This paper presents the results of two analyses that investigate the quality of wind fields produced by three general circulation models (GCMs) over the tropical Indian and Pacific oceans. The first analysis concerns the annual cycles of the tropical wind fields simulated by versions of the GCM at the Oregon State University (OSU), European Centre for Medium Range Forecasts (ECMWF), and National Center for Atmospheric Research (NCAR). These models have similar horizontal resolutions but vary widely in vertical resolution. The results show that although there are substantial differences in model performance, apparently related to differences in vertical resolution, there are also clear similarities in their behavior. Each GCM did best in major trade wind regions and somewhat poorly in convectively active areas with light winds. This finding suggests that the formulations governing the interactions between persistent convection and the circulation may limit model performance. A second analysis examines the response of the NCAR GCM, in terms of tropical Pacific wind stress, to prescribed SST anomalies over the period 1961–1972. It was found that the model response to SST anomalies associated with the El Niño/Southern Oscillation(ENSO) was distinct and in some respects resembled that of the real atmosphere. However, there were important discrepancies in the spatial configuration of the GCM field and in the amplitude of response of the GCM to the SST anomalies. An analysis of these discrepancies suggests that while the trapped equatorial Kelvin wave response of an ocean model coupled to this GCM would be qualitatively correct, differences in the GCM and observed forcing fields would result in large errors away from the equator. Tests with the Florida State University model of the tropical Pacific, to be reported in a later paper, support this conclusion. Taken together, these findings suggest that while GCMs are capable of reproducing correctly many features of the tropical surface wind fields, discrepancies remain with respect to both the annual cycle and the response to the anomalous SST patterns associated with ENSO. These discrepancies appear to be related, at least in part, to interactions between organized convection and the circulation. To what degree these differences would affect the oceanic component of a coupled model is currently under study.

Moored Surface Wind Observations at Four Sites along the Pacific Equator between 140° and 95°W

Moored surface wind measurements were recorded along the Pacific equator at 140, 124, 110, and 95 deg W during portions of 1980-1985. Minimum record length is one year. The annual mean and monthly mean westward speeds at 110 deg W were about 1.5 m/s higher during the year preceding the 1982-1983 El Nino than in the year following this event. The annual cycle, which moved westward at about 0.8 m/s, consisted of weak westward and northward speeds in February-April and vice versa in September-October. The spectral slope between 5-day and 0.05-day periods was -1.5. The rms amplitude of the 95-percent statistically significant diurnal period oscillation was 0.3 m/s, and the meridional component was nearly twice as large as the zonal component. The diurnal period wave was coherent (at the 95-percent confidence level) between 95 and 124 deg W, with westward phase propagation of about 138 m/s. No statistically significant spectral peak was found in the 40- to 50-day intraseasonal period band. The surface zonal ocean current component, which reached approximately 0.5 and -0.5 m/s in April and October, respectively at 110 deg W, influenced the surface wind stress computed from the quadratic bulk aerodynamic formulation by 10-20 percent.

One-Level Diagnostic Modeling of Mesoscale Surface Winds in Complex Terrain. Part I: Comparison with Three-Dimensional Modeling in Israel

Abstract A one-level sigma-coordinate model originally developed by Danard and modified by Mass and Dempsey and Alpert et al., is applied to the study of surface flow over an averaged summer diurnal cycle in Israel. The detailed flow features are compared to three-dimensional modeling studies and to dense surface wind observations. The winds al a height of 10 m from the one-level model were found comparable to those obtained by three-dimensional simulations, and in some cases the one-level model predicted observed surface flow features that were not simulated by the three-dimensional simulations, probably because of the finer horizontal grid resolution in the one-level model. The two models had similar deficiencies in diagnosing observed flow features in many cases. A severe drawback of the one-level model is the inability to advance the sea-breeze front (SBF) over a ridge crest correctly. Based upon an earlier vertical cross-sectional study by Alpert et al., an explanation for this discrepancy is suggest...

Experimental verification study for system for prediction of environmental emergency dose information; SPEEDI. (I). Three-dimensional interpolation method for surface wind observations in complex terrain to produce gridded wind field.

In order to calculate airflow around a nuclear site, a new weighted interpolation method for sparse wind data at various terrain heights is developed. This method employs weighting functions for vertical distance and the topographic barrier between the station and a grid point, in addition to a weighting function for horizontal distance which is employed in usual methods. The new weighting functions are developed from the analysis of field measurements taken in complex terrain. This method can represent the wind-fields from the bottom to the upper boundary better than usual methods which use only the horizontal weighting factor, even when the upper wind data are not available.

The combined use of three different approaches to obtain the best estimate of meso-? surface winds over complex terrain

Three approaches for estimating meso-β (Δx=5–10 km) surface winds over complex terrain are applied to obtain best estimates of typical summertime surface flow in Israel, based on a detailed 3-D model, a one-level sigma model and a dense network of surface wind observations. A scheme for combining the three approaches is outlined and illustrated through two examples showing how the approaches complement each other. It is suggested that such a man-machine combination is best for estimation of surface winds over complex terrain.

Blocking and Deflection of Airflow by the Alps

A method of computing low-level trajectories from observed sea level pressure is shown to be capable of distinguishing cases of blocked and non-blocked flow around the Alps. A flow-splitting parameter (S), derived from the trajectories, is found to be a reasonable criterion for distinguishing blocked from nonblocked cases, with a value of S = 1.5 serving as a useful threshold for classification. In considering eight cases of postfrontal cold flow against the Alps, only a few can be clearly identified as blocking or noblocking, and even these identifications may not pertain to all levels and all segments of the Alpine chain. The trajectory-based classification scheme compares well with sounding and flight level data from the NOAA P-3 research aircraft. The degree of blocking is shown to be related to the upstream Froude number and the strength of the upstream pressure nose. A remarkable result is that the observed surface wind patterns appear blocked in all cases considered here. We refer to this as “boundary layer blocking” Unsteady effects tend to stretch out streaklines east to west along the northern foothhills of the Alps.

Deriving Surface Winds from Satellite Observations of Low-Level Cloud Motions

Climatological shears between ship winds and satellite-observed, low-level cloud motions are used to derive monthly mean surface winds from satellite observations of low-level cloud drift. The derived surface winds compare well with observed surface winds from ships, moored buoys and the Seasat altimeter.

The influence of the Hawaiian Archipelago upon the wind‐driven subtropical gyre in the western North Pacific

In the middle of the Subtropical Gyre in the western North Pacific is the Subtropical Counter‐current, comprised of at least three narrow bands of eastward geostrophic mean flow near 20°N, 24°N, and 26°N, each extending from the sea surface through the main pycrocline. The origin of these narrow bands of eastward mean flow has not previously been explained. In this study, these bands are shown in the mean to extend zonally from the Philippine Sea (i.e., 140°E) to the vicinity of the Hawaiian Archipelago (i.e., 160°W). East of there, they cannot be found. Therefore a hypothesis is presented wherein the Hawaiian Archipelago (extending from 19°N to 26°N near 165°W) is seen to act as an effective barrier, modifying the response of the western subtropical North Pacific to wind‐driven effects in the eastern subtropical North Pacific and, in so doing, giving rise to these bands of eastward mean flow in the middle of the western Subtropical Gyre. A test of this hypothesis utilizes a Sverdrup [1947] model, driven by the annual, long‐term, mean wind stress curl, constructed on a 2° latitude/longitude grid from synoptic (6 hourly) surface wind observations for the 13‐year period 1968–1980. Two model simulations are conducted, one in the absence of the Hawaiian Archipelago and one in the presence of it. In the absence of the archipelago, none of the bands of eastward mean flow are simulated. In the presence of the archipelago, the narrow bands of eastward mean flow at 20°N, 26°N, and 24°N are simulated, although the latter band is very weak. These narrow bands of eastward flow are shown to occur in response to the deflection northward of the wind‐driven geostrophic streamlines in the central subtropical North Pacific by the Hawaiian Archipelago.

Surface Wind Characteristics along the Icefield Ranges, Yukon Territory, Canada

Surface wind observations were taken during the summers of 1972 and 1973 at two research sites located on the continental interior margin of the Icefield Ranges near Kluane Lake, Yukon Territory, Canada. One site was located in the Slims River valley over the delta environment; the other, 10 km away from the front of the mountain range at Kluane Lake. Surface wind direction and speed patterns for typical summer weather periods show a general pattern expected in large glacierized valleys. Large wind velocity fluctuations and pulsations occur in the main valley, related to the glacier wind of the Kaskawulsh and Vulcan glaciers upslope of the Slims River delta location. Persistent glacier and drainage wind effects are evident at the delta site as indicated by the surface wind speed profiles (0.5 to 12 m layer) over the river delta location. The wind profiles in the shallow 12-m layer display characteristics typical of the lower layers of a classic katabatic wind profile and are a function of the overall strength of the down-valley motion.

Tropical Atlantic wind field variations during sequal: Preliminary results

An evaluation of the U.S. Navy NOGAPS surface wind product is made to establish its utility as an initial descriptor of the surface wind field in the tropical Atlantic during SEQUAL. In this preliminary study, NOGAPS synoptic fields are compared to synoptic ship reports, and to winds measured at three separate stations along the equator during SEQUAL I. At least within 20° of the equator the comparisons suggest that the mean monthly surface winds from NOGAPS almost always differ by less than 2 m/s from averages of surface wind observations, and can be less than 1 m/s. Similar comparisons are also found from daily mean winds derived from NOGAPS, along the equator near 30°W. A comparison of NOGAPS monthly mean winds with 100‐year climatology of merchant ship reports suggests that the major surface wind anomaly during SEQUAL I in the tropical Atlantic occurred in March and April 1983 along the equator west of 30°W.

Two-Dimensional Simulations of Drainage Winds and Diffusion Compared to Observations

A vertically integrated dynamical drainage flow model is developed from conservation equations for momentum and mass in a terrain-following coordinate system. Wind fields from the dynamical model drive a Monte Carlo transport and diffusion model. The model needs only topographic data, an Eulerian or Lagrangian time scale and a surface drag coefficient for input data, and can be started with a motionless atmosphere. Model wind and diffusion predictions are compared to observations from the rugged Geysers, California area. Model winds generally agree with observed surface winds, and in some cases may give better estimates of area-averaged flow than point observations. Tracer gas concentration contours agree qualitatively with observed contours, and point predictions of maximum concentrations were correctly predicted to within factors of 2 to 10. Standard statistical tests of model skill showed that the accuracy of the predictions varied significantly from canyon to canyon in the Geysers area. Model wind predictions are also compared to observations from the South Carolina Savannah River Plant, which has gently rolling terrain. The model correctly simulated the slower development of drainage winds and slower deepening of the drainage layer in the Savannah River Valley, relative to the Geysers, California simulations. The South Carolina simulations and observations suggest that drainage winds are more frequent in the southeastern United States than is generally recognized. They may be responsible for some of the errors in air pollution concentration predictions made by Gaussian models, which assume homogeneous winds and turbulence.

Accuracy of Surface Geostrophic Wind Forecasts in the Central Arctic

Surface geostrophic winds are related simply to observed surface winds (Figure 1) and pack ice motion in the central Arctic (Albright, 1980; Thorndike and Colony, 1982). Their prediction, therefore, assumes special importance for sea ice forecasting, air and marine navigation and offshore resource exploration and development.

Analysis of Nighttime Drainage Winds in Boulder, Colorado during 1980

Characteristics of nighttime drainage winds that occurred along the eastern slope of the Rocky Mountains around Boulder, Colorado during the calendar year 1980 are examined. The data used for this study were acquired from the Boulder Wind Network (BWN) and from the Boulder Atmospheric Observatory (BAO). Data were available almost continuously from BWN and less frequently from BAO. BAO is a 300 m tower, instrumented at eight levels, but only surface wind observations are obtained from BWN. However, the combination of BWM and BAO observations represents a relatively unique set of wind data for the examination of drainage flows. Criteria for the identification of drainage winds are used to isolate events that are relatively free from external influences. Eighteen drainage wind events are identified, and some climatological features of the wind regime are established. In addition, the vertical structure of the flow associated with one event that reached the tower is examined in detail. Descriptions of the features of this flow and physical interpretations are presented. It is concluded, on the basis of this analysis, that observed features may be interpreted on the basis of the physical features contained in the model of Rao and Snodgrass (1981).

SASS Wind Ambiguity Removal by Direct Minimization

Abstract A gridded surface wind analysis is obtained by minimizing an objective function, the magnitude of which measures the error made by the gridded analysis in fitting the SASS wind data, the conventional surface wind observations and the forecast surface wind field. The ambiguity of the SASS winds is then removed by choosing the alias closest to the analyzed wind. Because minimizing the objective function is a problem of nonlinear least-squares, the minimizing gridded analysis is not unique and a good first guess is necessary to assure convergence to a reasonable solution. A good first guess may be generated by performing the analysis in stages. Illustrative results are shown for a limited region in the North Atlantic containing the QE II storm and for a limited amount (∼12 min) of data observed near the synoptic time 1200 GMT 10 September 1978. Within the SASS data swath the resulting gridded analysis is a reasonable representation of the surface wind and is not sensitive to the forecast surface win...

The SEASAT‐A satellite scatterometer: The geophysical evaluation of remotely sensed wind vectors over the ocean

On the SEASAT‐A satellite, a microwave scatterometer was used to determine the vector wind over the world's oceans. The technique is based on the sensitivity of microwave radar backscatter to the centimeter length ocean waves created by the action of the surface wind. This paper describes the algorithm used to convert the scatterometer's measurements of ocean normalized radar cross section, σ°, to the neutral stability vector wind at 19.5 m height and the comparison of these winds with high quality surface observations. The wind vector algorithm used an empirical σ° model function to describe the dependence of the ocean σ0 on the 19.5‐m neutral stability wind vector. Two model functions, developed from a limited base of aircraft and satellite σ° measurements, were evaluated by using an independent set of in situ surface wind observations from the Joint Air Sea Interaction Experiment (JASIN). Although these model functions were found to have some weaknesses, the results of these comparisons produced better results than the SEASAT specifications of wind speed accuracy of ±2 m/s and wind direction accuracy of ±20° over the 0–16 m/s range of winds observed during JASIN. An improved model function was later developed by ’tuning‘ to these JASIN data so that the remaining biases between the observed surface winds and the scatterometer‐derived winds were minimized. Results are presented for this model function compared against other surface wind observations from the Gulf of Alaska SEASAT Experiment and the SEASAT Storms (Hurricane) Experiment.

A Statistical Model for Wind Prediction at a Mountain and Valley Station Near Anderson Creek, California

Abstract Statistical models for surface-wind predictions at a mountain and a valley station near Anderson Creek, California, have been constructed. It is found that the surface wind speed depends primarily on the slope wind, cross-isobaric angle, surface thermal stability and geostrophic wind. The correlations between the calculated and observed surface wind speeds are found to be high for all time periods of the day and night. Because the variability of wind direction, which is greatly affected by topography, geostrophic wind and turbulent motion, is generally larger than that of the surface wind speed, statistical models for wind direction are more complicated than those for the wind speed. It is found that wind direction depends primarily on the geostrophic wind direction, aspect angle of the topography, up-canyon direction and cross-isobaric angle in the boundary layer.

Variationally adjusted surface winds

This paper describes a one-level variational adjustment process for producing mass-consistent surface winds in the Barrow Strait area, N.W.T. To achieve this result, the continuity equation is employed as a physical constraint. The variational technique adjusts mean winds (vertically averaged through the planetary boundary layer); therefore, a relationship is required between surface and mean winds. Use is made of existing velocity profiles, but interpolation may be used between surface and geostrophic winds. The model was found to be strongly dependent upon specification of boundary-layer height. Channeling effects are not readily seen until topography begins to protrude through the boundary layer. The model might thus be better suited to areas where shallow inversion layers are well defined. By application of the variational adjustment, errors in continuity are reduced by six orders of magnitude. Upon comparison of the variational technique with a diagnostic, one-level, primitive equation model, median errors between computed and observed surface winds were found to be comparable.