Surface Wind Direction Research Articles

Evaluations of Mesoscale Models' Simulations of Near-Surface Winds, Temperature Gradients, and Mixing Depths

Mesoscale meteorological models are being used to provide inputs of winds, vertical temperature and stability structure, mixing depths, and other parameters to atmospheric transport and dispersion models. An evaluation methodology is suggested and tested with simulations available from four mesoscale meteorological models (Fifth-Generation Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model, Regional Atmospheric Modeling System, Coupled Ocean‐Atmosphere Mesoscale Prediction System, and Operational Multiscale Environmental Model with Grid Adaptivity). These models have been applied by others to time periods of several days in three areas of the United States (Northeast, Lake Michigan area, and central California) and in Iraq. The authors’ analysis indicates that the typical root-mean-square error (rmse) of hourly averaged surface wind speed is found to be about 2‐3 m s21 for a wide range of wind speeds for the models and for the geographic regions studied. The rmse of surface wind direction is about 508 for wind speeds of about 3 o r4ms 21. It is suggested that these uncertainties in wind speeds and directions are primarily due to random turbulent processes that cannot be simulated by the models and to subgrid variations in terrain and land use, and therefore it is unlikely that the errors can be reduced much further. Model simulations of daytime mixing depths are shown to be often within 20% of observations. However, the models tend to predict weaker inversions than are observed in interfacial layers capping the mixing depth. The models also underestimate the vertical temperature gradients in the lowest 100 m during the nighttime, which implies that the simulated boundary layer stability is not as great as that observed, suggesting that the rate of vertical dispersion may be overestimated. The models would be able to simulate better the structure of shallow inversions if their vertical grid sizes were smaller.

Polarimetric scatterometry: a promising technique for improving ocean surface wind measurements from space

Spaceborne wind scatterometers provide useful measurements of ocean surface winds and are important to climatological studies and operational weather forecasting. Past and currently planned scatterometers use measurements of the copolarized backscatter cross-section at different azimuth angles to infer ocean surface wind speed and direction. Although successful, current scatterometer designs have limitations such as degraded wind performance in the near-nadir and outer regions of the measurement swath and a reliance on external wind information for vector ambiguity removal. Theoretical studies of scattering from the wind-induced ocean surface indicate that polarimetric measurements provide orthogonal and complementary directional information to aid the wind retrieval process. In this paper, potential benefits of making polarimetric backscatter measurements to improve wind retrieval performance are addressed. To investigate the performance of a polarimetric scatterometer, a modified version of the SeaWinds end-to-end simulator at the Jet Propulsion Laboratory (JPL), Pasadena, CA, is employed. To model the effect of realistic measurement errors, expressions for polarimetric measurement variance and bias are derived. It is shown that a polarimetric scatterometer can be realized with straightforward and inexpensive modifications to a current scanning pencil-beam scatterometer system such as SeaWinds. Simulation results show that such a system ran improve wind performance in the nadir region and eliminate the reliance on external wind information.

Mesoscale meteorological conditions in Dronning Maud Land, Antarctica, during summer: the momentum budget of the boundary layer

The momentum budget of the boundary layer flew near the Swedish research station Svea in the mountainous region of western Dronning Maud Land, Antarctica, is evaluated using detailed concurrent observations of surface meteorological variables, high-resolution boundary layer profiles and upper-air profiles taken during the summer of 1997–98. Despite the fact that the irregular topographical situation prohibits easy interpretation of the data, interesting features can still be identified. All terms in the momentum budget (katabatic forcing, synoptic pressure gradient, Coriolis force and friction) can be of the same order of magnitude, which indicates that the influence of each term depends largely on the prevailing conditions. More precisely, the relative importance of the various momentum terms varies strongly with altitude, location, time of day and prevailing synoptic conditions. For example, at the undisturbed snow slopes there is a very regular daily variation of katabatically forced flow during the night and a flow dominated by the synoptic pressure gradient during the day, causing a persistent diurnal cycle in surface wind direction. The interpretation of the flow characteristics in terms of the momentum budget agrees favourably with the conclusions drawn on the basis of a general description of the prevailing meteorological conditions, and can therefore be considered consistent.

Measurements of sea surface temperature and wind vector by nadir airborne microwave instruments in Joint United States/Russia Internal Waves Remote Sensing Experiment JUSREX'92

The results of the Joint United States/Russia Internal Waves Remote Sensing Experiment (JUSREX'92) are presented. A Russian research aircraft, TU-134, equipped with both optical and microwave instruments participated in the experiment. In situ data were obtained from the "Akademik Ioffe;" a research vessel equipped with contact and remote sensing instruments. To determine surface wind speed and direction by means of passive microwave polarimetry, airborne Ka-band radiometer-polarimeter was used. Sea surface temperature was measured by means of the S-band radiometer. Surface temperature and wind vector data were related to true surface data. Special attention was paid to the factors affecting the accuracy of surface parameters retrieval. Wind and temperature fields retrieved from radiometer data were compared with side-looking radar images. The combined analysis of passive and active microwave instrument data has proved important for furthering our understanding of the ocean-atmosphere interaction.

NSCAT normalized radar backscattering coefficient biases using homogenous land targets

The NASA scatterometer (NSCAT) is a spaceborne radar sensor designed to measure the normalized radar backscattering coefficient σ0 of the Earth's surface. Over the ocean, backscatter measurements are used to infer surface wind vectors. Wind retrieval is based on a statistical relationship between short‐ocean wave roughness (that causes the backscatter) and the surface wind speed and direction. For NSCAT geometry, multiple antennas are used to provide backscatter measurements at several azimuth directions to resolve wind direction ambiguities. To achieve the desired wind vector accuracy, these antenna beams must be calibrated within a few tenths of a decibel. A simple relative‐calibration method is applied to the NASA scatterometer backscatter from homogenous, isotropic, large‐area targets. These targets exhibit both azimuth and time invariant radar response. A simple polynomial model for incidence angle dependence of σ0 is used, and the mean radar response from all antenna beams is taken as the reference. Corrections (σ0 biases) are calculated as differences (in log space) between measurements from particular beam and the reference. This simple model is applied to data from the Amazon rain forest and the Siberian plain. These areas are tested for temporal stability within the calibration period (several weeks). High‐resolution masks are applied to extract suitable calibration data sets. Calculated corrections for each antenna beam are added to NSCAT σ0 measurements as a function of incidence angle. The magnitudes of corrections show the necessity of on‐orbit calibration.

Climatological mean and interannual variance of United States surface wind speed, direction and velocity

Means and variances of monthly mean wind speed, direction and velocity (the mean resultant vector) are derived for the period 1961–1990 at 216 stations in the coterminous United States. Direction and velocity means and variances are calculated using a complex-arithmetic extension of the equations for scalar mean and variance. Variance is derived from the 30-year time series of monthly means. While analyses of monthly mean wind fields are common, accompanying analyses of speed, direction, and velocity variance do not generally accompany them. Mean monthly wind direction and velocity fields show a typical seasonal progression from westerly and northwesterly winds in winter, to southerly winds in summer. Scalar and vector wind speeds are highest in winter and spring, and lowest in the summer. Seasonal variation in the mean fields is related to seasonal changes in mean sea level pressure, particularly east of the Rocky Mountains. In the western United States, mean winds often reflect channeling by local topography. The interannual variance of mean monthly wind speed, direction and velocity are related to seasonal variability in synoptic-scale features, such as the frequency of cyclones and anticyclones. Low variance occurs at a number of stations in the west, where topography restricts the range of wind variability. High velocity variance appears when both speed and direction variability are high, but it can occur also when speed variance is high and direction variance is low (or vice versa). Low velocity variance can result from low speed and direction variance, or from low mean wind velocities. The mean and variance characteristics of surface winds provide additional information on the surface climatology of the coterminous United States, and serve as a useful adjunct to other extant land-surface climatologies. Copyright © 1999 Royal Meteorological Society

Climatological mean and interannual variance of United States surface wind speed, direction and velocity

Means and variances of monthly mean wind speed, direction and velocity (the mean resultant vector) are derived for the period 1961–1990 at 216 stations in the coterminous United States. Direction and velocity means and variances are calculated using a complex-arithmetic extension of the equations for scalar mean and variance. Variance is derived from the 30-year time series of monthly means. While analyses of monthly mean wind fields are common, accompanying analyses of speed, direction, and velocity variance do not generally accompany them. Mean monthly wind direction and velocity fields show a typical seasonal progression from westerly and northwesterly winds in winter, to southerly winds in summer. Scalar and vector wind speeds are highest in winter and spring, and lowest in the summer. Seasonal variation in the mean fields is related to seasonal changes in mean sea level pressure, particularly east of the Rocky Mountains. In the western United States, mean winds often reflect channeling by local topography. The interannual variance of mean monthly wind speed, direction and velocity are related to seasonal variability in synoptic-scale features, such as the frequency of cyclones and anticyclones. Low variance occurs at a number of stations in the west, where topography restricts the range of wind variability. High velocity variance appears when both speed and direction variability are high, but it can occur also when speed variance is high and direction variance is low (or vice versa). Low velocity variance can result from low speed and direction variance, or from low mean wind velocities. The mean and variance characteristics of surface winds provide additional information on the surface climatology of the coterminous United States, and serve as a useful adjunct to other extant land-surface climatologies. Copyright © 1999 Royal Meteorological Society

Analysis and modelling of time series of surface wind speed and direction

This work presents a climatological study of winds at a particular site and proposes a simple model to simulate data time series of horizontal surface wind. The model relies on three major hypotheses: (i) speed and direction are treated as independent variables; (ii) wind can be expressed as the sum of a deterministic component (including trend and cyclical variations) plus a probabilistic or stochastic component; and (iii) both components (deterministic and stochastic) are independent. An hourly based 7 year wind series measured at the meteorological station in Almazora, located at the Spanish Mediterranean coast, is analyzed. Only the deterministic component of this series is simulated in accordance with the model proposed. Results indicate that the model is especially suitable for predicting wind at sites in which some cyclic pattern is expected. Copyright © 1999 Royal Meteorological Society

Analysis and modelling of time series of surface wind speed and direction

This work presents a climatological study of winds at a particular site and proposes a simple model to simulate data time series of horizontal surface wind. The model relies on three major hypotheses: (i) speed and direction are treated as independent variables; (ii) wind can be expressed as the sum of a deterministic component (including trend and cyclical variations) plus a probabilistic or stochastic component; and (iii) both components (deterministic and stochastic) are independent. An hourly based 7 year wind series measured at the meteorological station in Almazora, located at the Spanish Mediterranean coast, is analyzed. Only the deterministic component of this series is simulated in accordance with the model proposed. Results indicate that the model is especially suitable for predicting wind at sites in which some cyclic pattern is expected. Copyright © 1999 Royal Meteorological Society

Retrieval of ocean winds from satellite scatterometer by a neural network

This paper presents the reconstruction of a wind field from three-beam scatterometer measurements under the framework of a neural network. A neural network is adopted to implement the inversion of a geophysical model function (GMF) that relates the scatterometer measurements of normalized radar cross section to surface wind speed and direction. To illustrate the functionality and applicability of the neural network, a set of wind fields generated by means of the Monte Carlo simulation are used. At each sample point of the wind field, the speed and direction are simulated. Then, a GMF CMOD4 is used to synthesize the normalized radar cross section at three pointing antennas according to the ERS-1 configuration. In such a case, the neural network is constructed to model the inverse transfer function. For inputs, a pixel-based and area-based scheme are considered. The network training is accomplished by mapping input-output pairs that are randomly selected from the database of simulated wind fields. The effectiveness of the neural network as an inverse transfer function is validated. Four data sets of ERS-1 scatterometer data over the western Pacific were selected for case study. Intercomparison with other methods concludes that the use of neural network has its indispensable advantages and better retrieval accuracy can be obtained.

Contribution of microwave remote sensing from satellites to studies on the Earth energy budget and the hydrological cycle

Microwaves contribute less than marginally to the energy budget of the Earth-atmosphere system. However, the special interaction properties of atmospheric constituents and of the Earth surface in this remote frequency range renders microwaves a valuable tool to obtain information about several components of and important parameters for the Earth-atmosphere energy budget and its hydrological cycle. Similar to the thermal infrared, passive microwave measurements at the wings of strong absorption lines of homogeneously mixed gases and water vapor enable the retrieval of temperature and humidity profiles. Unlike in the infrared and visible wavelengths the cloudy atmosphere is mostly transparent in the microwave spectral range, giving microwave sensors a nearly all-weather sensing capability. Over the oceans the surface longwave radiation balance, total water vapor content, liquid water content of non-precipitating clouds, rain intensity, surface wind speed, wind direction, and wave spectra can be retrieved from passive and active microwave sensors. Over continental areas surface properties, which influence heat and water exchange with the atmosphere, like vegetation and soil moisture, can be estimated with some accuracy in addition to precipitation. In combination with other sensors synergistic effects extend the use of microwaves to ocean surface latent heat fluxes and possibly cloud type information.

Relationship of Synoptic Winds and Complex Terrain Flows during the MISTRAL Field Experiment

Abstract The relationship between the surface and synoptic wind direction is examined climatologically in a complex terrain region. Surface winds were observed over a 1-yr period during the MISTRAL project in the Basel, Switzerland, area. The measurement sites were located in various topographical settings, namely, in broad and narrow valleys, on slopes, at hilltops, on passes, and at mountaintops. The synoptic winds above the MISTRAL area are approximated by upper-level winds from routine rawinsonde observations. The relationship between this synoptic wind and the surface wind at each site was compared to a conceptual model. According to the conceptual model used, there are four mechanisms for the forcing of near-surface winds by synoptic winds. Three of the four forcing mechanisms leading to channeled flow are identified in the MISTRAL area. In this region with its complex terrain, different channeling mechanisms act at different locations as well as different mechanisms may occur at the same location. ...

Identification and Implications of Biases in U.S. Surface Wind Observation, Archival, and Summarization Methods

Observational biases in hourly surface wind speed and direction measurements from 41 northeastern U.S. stations are evaluated. Based on this analysis, a number of consistent wind observation biases are detected which should be considered when using these data in modeling and other climatological applications. These biases include a preference to report wind speed as a multiple of either 2 or 5 kt and a similar bias for reporting even, in terms of tens of degrees, wind directions. Temporal and station-dependent trends in estimating winds less than the starting speed of the anemometer are also evident. Given these biases, potential inaccuracies in analyses using wind data are illustrated. In addition, analyses are conducted to identify biases related to archival and summarization practices. In particular, the comparability of wind climatologies based on varying record lengths is determined. These assessments suggest that at least nine years of data are required to obtain representative wind frequency tabulations. The potential consequences of using too small of a record length are also illustrated. To achieve this minimum record length, climatologies can be constructed using either hourly or 3-hourly observations.

A method for finding sea breeze days under stable synoptic conditions and its application to the Swedish west coast

A method for selecting sea breeze days in an archipelago was developed with special application to the West Coast of Sweden. The primary criterion for identifying a sea breeze was the occurrence of a distinct change in surface wind direction within a 24 h period. The data used in the study were gathered using hourly meteorological records from field stations situated on the West Coast. Six different filters were included in the method; each of them developed from empirical knowledge of the physical processes responsible for the occurrence of a sea breeze system. Using this method it is possible to create a data set of sea breeze days, which does not use distance to the shoreline as a necessary condition for classification. © 1998 Royal Meteorological Society

A method for finding sea breeze days under stable synoptic conditions and its application to the Swedish west coast

A method for selecting sea breeze days in an archipelago was developed with special application to the West Coast of Sweden. The primary criterion for identifying a sea breeze was the occurrence of a distinct change in surface wind direction within a 24 h period. The data used in the study were gathered using hourly meteorological records from field stations situated on the West Coast. Six different filters were included in the method; each of them developed from empirical knowledge of the physical processes responsible for the occurrence of a sea breeze system. Using this method it is possible to create a data set of sea breeze days, which does not use distance to the shoreline as a necessary condition for classification. © 1998 Royal Meteorological Society

Characteristics of spectral optical depths and size distributions of aerosols over tropical oceanic regions

The characteristics of atmospheric aerosols and their dependence on meterological parameters over different marine environments are examined using columnar size distributions retrieved from spectral optical depth estimates over coastal and remote regions of the Arabian sea and equatorial Indian Ocean. In the near coastal regions, the aerosol size distribution is dependent on surface wind direction, showing the influence of proximity to the continent under continental wind conditions. Marine aerosols dominate and the large particle mode becomes well established in the far coastal and remote ocean environments. The optical depths and size parameters depend on surface wind speed. The overall aerosol loading decreases and the relative concentration of coarse particle increases as one moves to oceanic regions farther from the coast. The implications of these findings are discussed.

The European sand belt in eastern Europe ‐ and comparison of Late Glacial dune orientation with GCM simulation results

A compilation is presented of the continuation of the European sand belt, east of Poland. Dune fields encompass most of the aeolian formations in eastern Europe. Supposed sand provenance, dune orientation, and the few available datings suggest initial aeolian activity during cold stages of the Upper‐Pleni (= full)glacial and Late Glacial, similar to northwest and central Europe. Dune fomiation was primarily supply controlled. Comparison of dune orientation with Late Glacial surface wind directions simulated by various GCMs permits reconstruction of dune activation with the positioning of (winter) westerlies over glacial deposits during the Oldest Dryas. Final widespread aeolian activity occurred during the Younger Dryas. As compared to the smaller fields toward the centre of deglaciation, aeolian sand deposits of the intermediate and periglacial zone benefited from distinctly longer intervals of accumulation. In the zone of deglaciation, aeolian activity was restricted to relatively isolated basins.

Effects of the Speed and Direction of Surface Winds on the Radiation in the Atmosphere–Ocean System

Effects of the surface wind speed and direction on the radiation field in the model atmosphere–ocean system is theoretically investigated for the wavelength of 865 nm. The wind direction is considered for 0°, 45°, 90°, 135°, and 180° from the solar direction, where the wind speed is assumed to be 5 m/s and 11 m/s. The computational scheme is the doubling-adding method where polarization effect is included. Computational results show that the effect of the wind direction on the radiance at the top of the atmosphere is generally smaller than the effect of a change in wind speed from 5 m/s to 11 m/s. However, the opposite trend is shown at some viewing angles when wind speed is 11 m/s. For small optical thickness of aerosol (0.052), the effect of the wind direction on the radiance at the top of the atmosphere becomes larger than 20% even if the wind speed is 5 m/s. It is also shown that the degree of polarization is generally less sensitive to the wind speed and direction than the radiance. Therefore, measurements of the polarization from satellites may provide useful additional information for retrieval of the optical characteristics of the atmospheric aerosol.

Chemical characterization of acid snowfall in the coast and inland areas of Akita Prefecture in Japan.

To investigate the characteristics of chemical composition of precipitation, especially, of snowfall, samples were collected on a daily basis in Akita City(39°43'N, 140°08'E, 10m a.s.l)from April 1993 to March 1994 and in Yuzawa City(39°09'N, 140°30'E, 96 m a.s.l.)from January to March 1994. Ions such as H+(as pH), Na+, NH4+, K+, Ca2+, Mg2+, Cl-, NO3- and SO42- were analyzed for 185 samples collected at Akita and for 59 samples at Yuzawa. The pH values at Akita in the winter were lower than those of other seasons, this tendency was the same at other coastal cities of the Japan Sea. It was concluded that the source of acid in the snowfall was non-sea-salt (nss)-SO42-.It is well-known that the sea-salt concentration in the precipitation decreases exponentially with the distance from the coast; however, the sea-salt concentration in the winter precipitation at Yuzawa, which is located about 50 km to the east of the coast, occasionally did not show marked decrease in comparison with the case at Akita which is 6.5 km from the coast. It was likely that when the surface wind direction was nearly NW, Na+ and nss-SO42- were transported from Akita to Yuzawa; high Na+ concentrations at Yuzawa were mainly affected by the surface pressure pattern and the surface wind direction during snowfalls.

Ocean surface wind speed and direction retrievals from the SSM/I

A semiempirical model is developed that retrieves ocean surface wind direction information in addition to improved wind speeds from Special Sensor Microwave/Imager (SSM/I) measurements. Radiative transfer and neural network techniques were combined in the authors' approach. The model was trained and tested using clear sky cases, but atmospheric transmittance is retrieved so that retrieval in other than clear sky conditions is possible. With two SSM/I instruments currently providing operational ocean surface wind speed retrievals, the addition of wind direction information and improved wind speed retrievals will enhance the impact of this data in weather prediction models and marine weather forecasting.