Showing papers on "Wind shear published in 1994"

A Comparison of Shear- and Buoyancy-Driven Planetary Boundary Layer Flows

Abstract: Planetary boundary layer (PBL) flows are known to exhibit fundamental differences depending on the relative combination of wind shear and buoyancy forces. These differences are not unexpected in that shear instabilities occur locally, while buoyancy force sets up vigorous thermals, which result in nonlocal transport of heat and momentum. At the same time, these two forces can act together to modify the flow field. In this study, four large-eddy simulations (LESs) spanning the shear and buoyancy flow regimes were generated; two correspond to the extreme cases of shear and buoyancy-driven PBLs, while the other two represent intermediate PBLs where both forces are important. The extreme cases are used to highlight and quantify the basic differences between shear and convective PBLs in 1) flow structures, 2) overall statistics, and 3) turbulent kinetic energy (TKE) budget distributions. Results from the two intermediate LES cases are used to develop and verify a velocity scaling and a TKE budget mode...

Determining the power-law wind-profile exponent under near-neutral stability conditions at sea

Abstract: On the basis of 30 samples from near-simultaneous overwater measurements by pairs of anemometers located at different heights in the Gulf of Mexico and off the Chesapeake Bay, Virginia, the mean and standard deviation for the exponent of the power-law wind profile over the ocean under near-neutral atmospheric stability conditions were determined to be 0.11 ± 0.03. Because this mean value is obtained from both deep and shallow water environments, it is recommended for use at sea to adjust the wind speed measurements at different heights to the standard height of 10 m above the mean sea surface. An example to apply this P value to estimate the momentum flux or wind stress is provided.

A Hybrid Vertical Mixing Scheme and Its Application to Tropical Ocean Models

Abstract: A novel hybrid vertical mixing scheme, based jointly on the Kraus–Turner-type mixed layer model and Price's dynamic instability model, is introduced to aid in parameterization of vertical turbulent mixing in numerical ocean models. The scheme is computationally efficient and is capable of simulating the three major mechanisms of vertical turbulent mixing in the upper ocean, that is, wind stirring, shear instability, and convective overturning. The hybrid scheme is first tested in a one-dimensional model against the Kraus–Turner-type bulk mixed layer model and the Mellor–Yamada level 2.5 (MY2.5) turbulence closure model. As compared with those two models, the hybrid model behaves more reasonably in both idealized experiments and realistic simulations. The improved behavior of the hybrid model can be attributed to its more complete physics. For example, the MY2.5 model underpredicts mixed layer depth at high latitudes due to its lack of wind stirring and penetrative convection, while the Kraus–Turn...

The roles of vertical mixing, solar radiation, and wind stress in a model simulation of the sea surface temperature seasonal cycle in the tropical Pacific Ocean

Abstract: The climatological seasonal cycle of sea surface temperature (SST) in the tropical Pacific is simulated using a newly developed upper ocean model. The roles of vertical mixing, solar radiation, and wind stress are investigated in a hierarchy of numerical experiments with various combinations of vertical mixing algorithms and surface-forcing products. It is found that the large SST annual cycle in the eastern equatorial Pacific is, to a large extent, controlled by the annually varying mixed layer depth which, in turn, is mainly determined by the competing effects of solar radiation and wind forcing. With the application of our hybrid vertical mixing scheme the model-simulated SST annual cycle is much improved in both amplitude and phase as compared to the case of a constant mixed layer depth. Beside the strong effects on vertical mixing, solar radiation is the primary heating term in the surface layer heat budget, and wind forcing influences SST by driving oceanic advective processes that redistribute heat in the upper ocean. For example, the SST seasonal cycle in the western Pacific basically follows the semiannual variation of solar heating, and the cycle in the central equatorial region is significantly affected by the zonal advective heat flux associated with the seasonally reversing South Equatorial Current. It has been shown in our experiments that the amount of heat flux modification needed to eliminate the annual mean SST errors in the model is, on average, no larger than the annual mean uncertainties among the various surface flux products used in this study. Whereas a bias correction is needed to account for remaining uncertainties in the annual mean heat flux, this study demonstrates that with proper treatment of mixed layer physics and realistic forcing functions the seasonal variability of SST is capable of being simulated successfully in response to external forcing without relying on a relaxation or damping formulation for the dominant surface heat flux contributions.

Effects of vertical wind shear on the cumulus transport of momentum : observations and parameterization

Abstract: Dynamical effects of organized cumulus convection on its environment with vertical wind shear are studied. Analyses of the wind field and momentum budget residual for mesoscale convective systems observed during SESAME and PRE-STORM reveal systematic differences in the vertical transport of horizontal momentum between mesoscale convective complex (MCC) and squall line cases. In the MCC, a distinct minimum in wind speed appears over the area of intense convection and the momentum budget residual acts to decelerate the environmental wind and to reduce the upper-level vertical shear. Therefore, the inferred vertical transport of momentum in the MCC is downgradient in the upper layer. On the other hand, in the squall line, there is no wind speed minimum and the upper-level vertical shear of the line-normall component of the environmental wind increases as convection develops. Thus, the vertical transport of momentum normal to the squall line is upgradient in the upper layer, although the transport of...

Observed directional characteristics of the wind, wind stress, and surface waves on the open ocean

Abstract: Sonic anemometer data were taken during the Surface Waves Processes Program (SWAPP) in March 1990 in the North Pacific. Measurements of the wind stress vector span several strong wind events. Significant angles between the wind stress vector and the mean wind vector are seen. Simultaneous measurements of the directional wave field were made with a surface scanning Doppler sonar. The data suggest that the wind stress direction is influenced by the direction of the surface waves, especially for stronger winds. Overall, the stress vector lies between the mean wind and the mean wave directions. At the higher wind speeds (over 8.6 m/s), there is a nonzero correlation between the variations in wave directions and stress directions as well. Finally, the stress and wave component directions have similar frequency dependence over the frequency band where wave energy is nonnegligible, suggesting a dynamic link.

Cloud Cover and Its Relationship to Relative Humidity during a Springtime Midlatitude Cyclone

Abstract: Vertical distributions of fractional cloud coverage derived from the U.S. Air Force 3DNEPH satellite, aircraft, and surface-based analysis are compared with related standard meteorological observations over the eastern United States. Cloud cover and related observations are interpolated onto the identical three-dimensional grid consisting of 15 tropospheric levels at various horizontal resolutions ranging from (80 km)2 to (800 km)2 for five local noon periods during a springtime midlatitude cyclone. During the period analyzed, cloud cover maximizes near 900 mb at 35% cloud cover and decreases to near-zero cover at the surface. Above 900 mb, fractional cloudiness gradually decreases to 10%–20% cover at 200 mb. Cloud cover is positively correlated with relative humidity and large-scale vertical velocity, and negatively correlated with wind shear and temperature lapse rate, except in the lowest 100 mb, where cloud cover is weakly correlated with relative humidity, vertical velocity, wind shear, and ...

Understanding the relation between wind- and pressure-driven sea level variability

Abstract: Sea surface adjustment to combined wind and pressure forcing is examined using numerical solutions to the shallow water equations. The experiments use coastal geometry and bottom topography representative of the North Atlantic and are forced by realistic barometric pressure and wind stress fields. The repsonse to pressure is essentially static or close to the inverted barometer solution at periods longer than a few days and dominates the sea level variability, with wind-driven sea level signals being relatively small. With regard to the dynamic signals, wind-driven fluctuations dominate at long periods, as expected from quasi-geostrophic theory. Pressure becomes more important than wind stress as a source of dynamic signals only at periods shorter than approximately three days. Wind- and pressure-driven sea level fluctuations are anticorrelated over most regions. Hence, regressions of sea level on barometric pressure yield coefficients generally smaller than expected for the inverted barometer response known to be the case in the model. In the regions of significant wind-pressure correlation effects, to infer the correct pressure reponse using statistical methods, input fields must include winds as well as pressure. Because of the nonlocal character of the wind response, multivariate statistical models with local wind driving as input are not very successful. Inclusion of nonlocal wind variability over extensive regions is necessary to extract the correct pressure response. Implications of these results to the interpretation of sea level observations are discussed.

Hydrodynamic modulation of short wind-wave spectra by long waves and its measurement using microwave backscatter

Abstract: In this paper we use results of microwave backscattering experiments over the past decade to attempt to present a coherent picture of the ocean wave-radar modulation transfer function (MTF) based on composite surface theory, short-wave modulation, and modulated wind stress. A simplified relaxation model is proposed for the modulation of the gravity-capillary wavenumber spectrum by long waves. The model is based on the relaxation rate and the equilibrium gravity-capillary wavenumber spectrum. It differs from previous models by including all airflow modulation effects in the response of the equilibrium spectrum to changes in the airflow. Thus the explicit modulation of individual source functions such as wind input, short-wave dissipation, and nonlinear interactions need not be known in order to calculate the hydrodynainic MTF. By combining this new model of the hydrodynamic MTF with microwave measurements, we attempt to determine wind shear stress modulation caused by the long waves. In order to extract the hydrodynamic MTF from the microwave data, we remove tilt and range change effects from the measured MTFs using the published analytical forms for these effects. Our results show that the inferred hydrodynamic MTF is higher for II polarization scattering than for V polarization. Since this is impossible if we have obtained the true hydrodynamic MTF, these results strongly indicate a problem with composite scattering theory as it has been traditionally applied. One explanation for this result may be the effects of intermediate-scale waves suggested by Romeiser et al. (1993). Since these effects are much stronger for H polarization than for V polarization, they may explain our observed discrepancy and, if so, imply that V polarization return should yield an acceptable upper limit for the true hydrodynamic MTF. Thus we incorporate our V polarization results into the proposed model to estimate an upper limit for the wind shear stress modulation along the long-wave profile. We infer that the primary source of modulation of Bragg resonant waves depends strongly on Bragg wavenumber and windspeed. For low values of these quantities, straining by long-wave orbital velocities dominates the modulation process, while for higher values modulated wind stress becomes increasingly important. Our calculations indicate that wind stress modulation dominates the process for 3 cm Bragg waves at moderate to high wind speeds.

Flow Distortion by a Solent Sonic Anemometer: Wind Tunnel Calibration and Its Assessment for Flux Measurements over Forest and Field

Abstract: Main flow distortion effects caused by the sonic probe (i.e., deflection and attenuation/amplification of the wind vector) as a function of the azimuth angle of the incoming flow were examined by means of wind tunnel measurements at four different wind speeds and 11 different elevation angles of the flow with respect to the probe. The dependence of the distortion on wind speed and elevation angle turned out to be small compared with the dependence on the azimuth angle. Based on wind tunnel data, a set of calibration coefficients for each azimuth was evaluated. Application of this calibration on wind tunnel data shows an average compensation of the flow distortion of about 61 %. A comparative application of the calibration supplied by the manufacturer leads to an increase of the flow distortion effects, especially at higher wind speeds. Application of the obtained calibration on field data leads to a change of about 20% in momentum flux over forest and grass field. The flux of sensible heat change...

An ERS 1 synthetic aperture radar image of atmospheric lee waves

Abstract: An ERS 1 synthetic aperture radar (SAR) image of the island Hopen shows a distinct 7.6-km wavelength wave phenomenon near the island. This wave phenomenon is interpreted as the surface imprint on open water of atmospheric lee waves. The pattern is visible in the SAR image, since the lee waves modulate the horizontal wind speed near the ocean surface which, in turn, modulates the surface roughness and the radar cross section. The physical setting for the observation is presented and discussed. The lower bound on horizontal wind speed modulation is estimated to range from 3±2 ms−1 (for the wind speed minima) to 12±2 ms−1 (for the wind speed maxima) based upon the observed radar cross-section modulation and the ERS 1 scatterometer wind retrieval model CMOD4. The wavelength and wind speed modulation are consistent with linear lee wave model predications. The model uses an atmosphere with an exponential profile of the Scorer parameter (ratio of buoyancy frequency to wind speed) to represent a shallow, ground-based inversion layer observed at Bear Island and a bell-shaped barrier to represent the forcing effects of Hopen.

Examining the Influence of Wind and Wind Wave Turbulence on Tidal Currents, Using a Three-Dimensional Hydrodynamic Model Including Wave–Current Interaction

Abstract: This paper presents a very brief overview of the development of a three-dimensional hydrodynamic model involving a flow-dependent eddy viscosity and including enhancements of bottom friction due to wave-current interaction in shallow water. The main point of the paper is to examine the physical nature of the process. Consequently, references to published work are given for the background detail. Calculations using both tidal and wind forcing show that tidal elevation amplitude and phase are significantly changed in shallow near-coastal regions due to enhanced frictional effects associated with wind-driven flow and wind wave turbulence. An analysis of tidal current profiles, at the fundamental harmonic and higher harmonies, computed with tidal and wind forcing, shows that significant changes in tidal current profiles can occur due to coupling between the wind-induced current shear and a time-evolving viscosity. The importance of the nonlinearity produced by a surface wind-induced shear and a flow-...

A study of the seasonal cycle of sea surface temperature in the tropical Pacific Ocean using reduced gravity models

Abstract: A simple model of the upper ocean is used to simulate the seasonal cycle of sea surface temperature (SST) in the tropical Pacific Ocean. The model is an extension of the conventional 1½-layer reduced gravity system that includes the physics of the surface mixed layer and allows prediction of the sea surface temperature. Two types of mixed layer models, a constant depth mixed layer model of Cane and Zebiak and a variable depth mixed layer model of Kraus and Turner, are examined. The numerical simulations indicate that the simple models are capable of capturing the essential seasonal SST variability in the tropical Pacific when forced with climatological surface forcing. The sensitivity study of model SST response to changes in the surface forcing demonstrates that although the response to surface heat flux forcing dominates the seasonal cycle of SST in most areas of the tropical Pacific, the dynamical response of the ocean to changes in the wind also makes a significant contribution to the seasonal variation of SST in the eastern equatorial Pacific. The seasonal variation of the winds is particularly important in maintaining the phase structure of the annual SST signal along the equator. Moreover, the sensitivity of the SST response depends on the form of vertical entrainment parameterization in the model. It is shown that the Cane-Zebiak type of model has a bias toward the dynamical response of the ocean to the winds, whereas the Kraus-Turner type of model has bias toward the surface heat flux variations. The simple model results are also compared with those from an ocean general circulation model.

Conditions associated with large-drop regions

Abstract: In light of the significant icing hazard large drops pose to general aviation, two conditions have been previously associated with large-drop formation; these being a warm cloud-top temperature and a low droplet concentration. This paper identifies an additional condition associated with the development of large-drop regions. Wind shear is hypothesized as being a necessary but not sufficient condition for the formation of large drops. Wind shear at cloud top may cause turbulence, Kelvin-Helmholtz waves, and thus the inhomogeneous mixing leading to large drops. This hypothesis was tested in 29 cases where the Wyoming King Air aircraft made a climb or descent through the top of stratiform clouds. The presence of a wind shear layer was defined by the magnitude of the wind shear and the value of the bulk Richardson number across the layer. In 23 of the 29 cases, wind shear was associated with large-drop regions. A χ2 statistical test was applied to the data. The null hypothesis, where wind shear and ...

The Effect of Forest on Mesoscale Rainfall: An Example from HAPEX-MOBILHY

Abstract: A meso-β-scale model is used to model a frontal intrusion in southwest France during HAPEX-MOBILHY. The skill of the model to reproduce the observed variation in temperature, humidity, and wind speed over the domain is reasonable within the limitations of the model parameterizations and initialization procedure, although there were errors in the timing and positioning of the front. A stable boundary layer was both observed and modeled over the forested area. The associated negative sensible heat flux provided the energy to sustain evaporation from the wet forest canopy under conditions of low radiation. A large wind shear over the stably stratified boundary layer provided the required turbulent kinetic energy to maintain the downward transport of sensible heat. Sensitivity experiments showed that local rainfall with a full forest cover changed from 2.9 to 3.8 mm, which represents a 30% increase when compared with a bare-soil domain. Half of this increase is from positive feedback of the intercept...

Implications of the Hydrostatic Assumption on Atmospheric Gravity Waves

Abstract: The validity of the hydrostatic approximation is examined for use in predicting the dynamics of topographically generated atmospheric gravity waves (lee waves) propagating in an atmosphere with realistic wind shear. To isolate nonhydrostatic effects, linear, analytic solutions derived both with and without the hydrostatic assumption are compared. The atmospheric profiles of wind and stability are chosen both to render the governing equations analytically tractable and be representative of typical atmospheric conditions. Two atmospheric models are considered: 1) a troposphere-only model in which the wind increases linearly with height and the stability is constant and 2) a troposphere-stratosphere model, which retains the important effect of the vertical wind shear in the troposphere and adds the essential feature of a stability jump at the tropopause. The nonhydrostatic trapping effect of wind shear on gravity wave modes is clearly illustrated in the troposphere-only atmospheric model. In the tro...

Linear dynamics of wind waves in coupled turbulent air—water flow. Part 1. Theory

Abstract: When air blows over water the wind exerts a stress at the interface thereby inducing in the water a sheared turbulent drift current. We present scaling arguments showing that, if a wind suddenly starts blowing, then the sheared drift current grows in depth on a timescale that is larger than the wave period, but smaller than a timescale for wave growth. This argument suggests that the drift current can influence growth of waves of wavelength i that travel parallel to the wind at speed c. In narrow ‘inner’ regions either side of the interface, turbulence in the air and water flows is close to local equilibrium; whereas above and below, in ‘outer’ regions, the wave alters the turbulence through rapid distortion. The depth scale, l,, of the inner region in the air flow increases with c/u,, (u*, is the unperturbed friction velocity in the wind). And so we classify the flow into different regimes according to the ratio &/A. We show that different turbulence models are appropriate for the different flow regimes. When (u., + c)/UB(A) 4 1 (U,(Z) is the unperturbed wind speed) 1, is much smaller than A. In this limit, asymptotic solutions are constructed for the fully coupled turbulent flows in the air and water, thereby extending previous analyses of flow over irrotational water waves. The solutions show that, as in calculations of flow over irrotational waves, the air flow is asymmetrically displaced around the wave by a non-separated sheltering effect, which tends to make the waves grow. But coupling the air flow perturbations to the turbulent flow in the water reduces the growth rate of the waves by a factor of about two. This reduction is caused by two distinct mechanisms. Firstly, wave growth is inhibited because the turbulent water flow is also asymmetrically displaced around the wave by non-separated sheltering. According to our model, this first effect is numerically small, but much larger erroneous values can be obtained if the rapid-distortion mechanism is not accounted for in the outer region of the water flow. (For example, we show that if the mixing-length model is used in the outer region all waves decay!) Secondly, non-separated sheltering in the air flow (and hence the wave growth rate) is reduced by the additional perturbations needed to satisfy the boundary condition that shear stress is continuous across the interface.

What forces the variability of the southwestern Atlantic boundary currents

Abstract: A marked variability in the location of the front originating at the confluence of the Brazil and Malvinas Currents has been observed from both surface and subsurface observations. Modeling experiments using climatological winds predict a seasonal variability on the latitude of separation of the Brazil Current from the coast. During the Confluence Program (November 1988–February 1990) and from data collected with an array of inverted echo sounders, the location of the confluence front and its variability was established. In this paper, the observed oceanic variability is analyzed simultaneously with the wind product from the European Center for Medium Weather Forecast (ECMWF) obtained for the period of the observations. The ECMWF data is validated against in situ indirect wind magnitude observations obtained from a sub-array of the Confluence deployments. The large-scale anomalies are explored through the comparison with the climatological winds field obtained from Hellerman and Rosenstein (1983), Journal of Physical Oceanography, 13, 1093–1104. From the analysis it is concluded that the main source of variability of the Confluence front is the local wind forcing. There is a variability in the location of the front due to the seasonal cycle of the winds in the South Atlantic. In addition to this seasonal variability, the latitude of separation of the Brazil Current from the coast presents a marked interannual variability that is forced from anomalous wind patterns south of the Confluence. There is no apparent correlation between wind-forced pulses in the Antarctic Circumpolar Current and the observed anomalous northward penetration of the Malvinas Current.

The relationship between the microwave radar cross section and both wind speed and stress: Model function studies using Frontal Air-Sea Interaction Experiment data

Abstract: The Frontal Air-Sea Interaction Experiment (FASINEX) provided a unique data set with coincident airborne scatterometer measurements of the ocean surface radar cross section (RCS)(at Ku band) and near-surface wind and wind stress. These data have been analyzed to study new model functions which relate wind speed and surface friction velocity (square root of the kinematic wind stress) to the radar cross section and to better understand the processes in the boundary layer that have a strong influence on the radar backscatter. Studies of data from FASINEX indicate that the RCS has a different relation to the friction velocity than to the wind speed. The difference between the RCS models using these two variables depends on the polarization and the incidence angle. The radar data have been acquired from the Jet Propulsion Laboratory airborne scatterometer. These data span 10 different flight days. Stress measurements were inferred from shipboard instruments and from aircraft flying at low altitudes, closely following the scatterometer. Wide ranges of radar incidence angles and environmental conditions needed to fully develop algorithms are available from this experiment.

On the wind energy resource of Nigeria

Abstract: An analysis of wind data for the ten-year period 1979–1988 in Nigeria is presented. Surface and upper winds were considered as well as maximum gusts. The need for the provision of new data stations in order to enable a complete and reliable assessment of the overall wind power potential of the country is identified and specific locations suggested. Socioeconomic and political factors affecting the development of wind energy development in Nigeria are also discussed.

Mean zonal momentum balance in the upper and central equatorial Pacific Ocean

Abstract: We examine the mean zonal momentum balance in the tropical mid-Pacific using a year of acoustic Doppler current profiler velocities and conductivity-temperature-depth profiler densities from the Hawaii-to-Tahiti Shuttle Experiment. All significant contributions from the mean, annual cycle, and higher-frequency flow fields are determined with the exception of the vertical stresses. We find that even neglecting vertical stresses, the zonal momentum equation is in rough balance at 90–117-m depth at all latitudes from 4°S to 10°N. While the formal error bars are large, this rough balance is reproducible over four to five independent latitudes and so is probably real. The balance at 90-m depth is geostrophic to within 5° of the equator. Closer to the equator, meridional mean convergence and meridional eddy stresses contribute important forces to balance the mean pressure gradient. Nearer the surface, the zonal momentum equation is dominated by eastward pressure gradients near the equator and eastward Coriolis forces from a strong, northward Ekman flow poleward of 2°N. In the vertical integral these forces roughly balance the surface wind stress; thus vertical stresses suffice to close our momentum budget. We conclude that on average vertical stresses arising from the wind forcing do not penetrate deeper than 90 m into the tropical ocean. This contradicts an earlier study of the equatorial zonal momentum budget but is consistent with turbulent dissipation measurements on the equator. Previous findings of stronger, deeper dissipation on the equator are probably due to the stronger, deeper mean shear there rather than to a locally altered stress profile. Vertical turbulent viscosities derived from our observations agree with previous observations on the equator but contradict the conventional, Richardson number parameterization off the equator.

Effects of atmospheric conditions on wind profiles and aeolian sand transport with an example from white sands national monument

Abstract: Quantifying aeolian sand transport rates relies upon the computation of the near-surface shear velocity (u*) determined from velocity profiles of the wind. While it has been recognized that various conditions, such as saltation, surface roughness, surface slope and atmospheric conditions, have an effect on the velocity profile, it is commonly assumed that measurements made above the surface will be representative of the near-surface shear velocity. Airflow and temperature data collected over a flat substrate at White Sands National Monument in New Mexico, however, show the significant effects that atmospheric conditions have on velocity profiles. During the day, when solar insolation is heating the surface, atmospheric conditions in the lowest several metres become unstable, resulting in enhanced convection and vertical mixing so that the velocity gradient changes little with height. As a result, the shear stress in this region of vertical mixing lessens, while the near-surface shear stress is increased because the higher wind speeds are now nearer the surface. At night, the near-surface atmospheric conditions are stable, thereby reducing convection and vertical mixing, resulting in stratified airflow and increased shear velocity away from the surface. Unless this atmospheric effect is accounted for, estimates of sediment transport rates may be in error by as much as a factor of 15 times when wind speeds are near threshold velocity. At wind speeds approaching 10 ms1, at 5m above the surface, this error in computing sediment transport is reduced to a factor of only two to three times, and may be within the range of measurement error.

The Environmental Influence on Tropical Cyclone Precipitation

Abstract: The intensity, spatial, and temporal changes in precipitation were examined in three North Atlantic hurricanes during 1989 (Dean, Gabrielle, and Hugo) using precipitation estimates made from Special Sensor Microwave/Imager (SSM/I) measurements. In addition, analyses from a barotropic hurricane forecast model and the European Centre for Medium-Range Weather Forecast model were used to examine the relationship between the evolution of the precipitation in these tropical cyclones and external forcing. The external forcing parameters examined were (1) mean climatological sea surface temperatures, (2) vertical wind shear, (3) environmental tropospheric water vapor flux, and (4) upper-tropospheric eddy relative angular momentum flux convergence. The analyses revealed that (1) the SSM/I precipitation estimates were able to delineate and monitor convective ring cycles similar to those observed with land-based and aircraft radar and in situ measurements; (2) tropical cyclone intensification was observed to occur when these convective rings propagated into the inner core of these systems (within 111 km of the center) and when the precipitation rates increased; (3) tropical cyclone weakening was observed to occur when these inner-core convective rings dissipated; (4) the inward propagation of the outer convective rings coincided with the dissipation of the inner convective rings when they came within 55 km of each other; (5) in regions with the combined warm sea surface temperatures (above 26 C) and low vertical wind shear (less than 5 m/s), convective rings outside the region of strong lower-tropospheric inertial stability could be initiated by strong surges of tropospheric moisture, while convective rings inside the region of strong lower-tropospheric inertial stability could be enhanced by upper-tropospheric eddy relative angular momentum flux convergence.

Coherent Doppler lidar signal covariance including wind shear and wind turbulence

Abstract: The performance of coherent Doppler lidar is determined by the statistics of the coherent Doppler signal. The derivation and calculation of the covariance of the Doppler lidar signal is presented for random atmospheric wind fields with wind shear. The random component is described by a Kolmogorov turbulence spectrum. The signal parameters are clarified for a general coherent Doppler lidar system. There are two distinct physical regimes: one where the transmitted pulse determines the signal statistics and the other where the wind field dominates the signal statistics. The Doppler shift of the signal is identified in terms of the wind field and system parameters.

Observed and Modeled Wind and Water-Level Response from Tropical Storm Marco (1990)

Abstract: The Hurricane Research Division (HRD) analyzes surface wind fields in tropical storms and hurricanes using surface wind observations and aircraft flight-level wind measurements in the vicinity of the storms. The analyzed surface wind fields for Tropical Storm Marco (1990) were compared with the wind fields used for input in the National Weather Service's Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model. The HRD wind fields were also used to determine the wind speeds and directions corresponding to the storm surge at tide gauges along Florida's west coast. The observed storm surge at the gauges was compared with the storm surge computed by the SLOSH model. Time series of the SLOSH model winds were compared with time series based on the analyzed wind field at each tide gauge, because in most cases there were no wind observations available at these gauges. The comparisons of the analyzed and modeled winds and the observed and modeled storm surge show that the SLOSH model reasonably repres...

Low altitude wind shear detection using airport surveillance radars

Abstract: This paper describes an enhanced weather processor for the Federal Aviation Administration's airport surveillance radar (ASR-9) that will include Doppler wind estimation for the detection of low altitude wind shear, scan-to-scan tracking to provide estimates of the speed and direction of storm movement and suppression of spurious weather reports currently generated by the ASR-9's six-level weather channel during episodes of anomalous radar energy propagation (AP). This ASR-9 wind shear processor (WSP) will be implemented as a retrofit to the ASR-9 through the addition of interfaces, receiving chain hardware and high-speed digital processing and display equipment. The WSP modification to the ASR-9 will provide the functional capabilities of the terminal Doppler weather radar at airports whose operation levels and/or thunderstorm exposures do not justify the costs of the dedicated radar. Field testing of a prototype version of the ASR-9 WSP has confirmed that the weather information products it generates are accurate and are operationally useful in an air traffic control (ATC) environment. >

Data Assimilation in a Simple Tropical Ocean Model with Wind Stress Errors

Abstract: A major error source in the numerical simulation of tropical oceans is the uncertainty in wind stress forcing. A reduced-gravity shallow-water model has been used to test how assimilated ocean data correct simulation errors caused by erroneous wind stress in the tropics. The geometry of the basin is rectangular and symmetric about the equator, and the long-wave approximation is applied. All experiments are of the identical-twin type: the “observations” are generated by sampling the desired reference solution, and the data are assimilated by optimal interpolation into the same model, with wind stress forcing different from that in the reference case. In this paper, three types of wind stress errors are considered: errors of timing only, as well as persistent errors, systematic or stochastic. The relative usefulness of thermocline depth and current observations, and the effect of data distribution on state estimation are examined. The role of equatorial ocean waves in the process of data assimilati...