AGEOSTROPHIC DEVIATIONS AND WIND PROGNOSES
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This paper examines apparent ageostrophic deviations at 300 and 200 mb over the United States, their relationship to upper-air synoptic patterns, and the extent to which ageostrophic deviations affect wind forecasts made from geostrophic maps. For spot winds, ageostrophic RMS vector deviations are 19 kn at 300 mb and 20 kn at 200 mb. For the same set of data, geostrophic wind forecasts (36-hr) result in a 34-kn RMS vector error if verified by the geostrophic winds, and 38-kn RMS error if verified by the rawins. Prognostic errors of route winds decrease as the route length increases: for a route length 1900 n mi, the 36-hr geostrophic wind forecast gives RMS vector error 17 kn if verified by geostrophic wind and 19 kn if verified by the rawins. Instrumental uncertainties inherent in the present upper-wind reports prevent the use of the rawins as an absolute standard for verification. In all, it appears that a medium-period forecast of spot and route winds can be made with tolerable approximation from the geostrophic prognostic charts.Keywords:
Thermal wind
Geostrophic current
Abstract The gradient wind is defined as a horizontal wind having the same direction as the geostrophic wind but with a magnitude consistent with a balance of three forces: the pressure gradient force, the Coriolis force, and the centrifugal force arising from the curvature of a parcel trajectory. This definition is not sufficient to establish a single way of computing the gradient wind. Different results arise depending upon what is taken to be the parcel trajectory and its curvature. To clarify these distinctions, contour and natural gradient winds are defined and subdivided into steady and nonsteady cases. Contour gradient winds are based only on the geostrophic streamfunction. Natural gradient winds are obtained using the actual wind. Even in cases for which the wind field is available along with the geostrophic streamfunction, it may be useful to obtain the gradient wind for comparison to the existing analyzed or forecast wind or as a force-balanced reference state. It is shown that the nonanomalous (normal) solution in the case of nonsteady natural gradient wind serves as an upper bound for the actual wind speed. Otherwise, supergradient wind speeds are possible, meaning that a contour gradient wind or the steady natural gradient wind used as an approximation for an actual wind may not be capable of representing the full range of actual wind magnitude.
Thermal wind
Pressure gradient
Stream function
Pressure-gradient force
Crosswind
Temperature Gradient
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Motion in planetary geostrophic equations (PGEs) is represented by the three‐dimensional geostrophic wind ( u g , v g , w g ) where u g and v g are the standard horizontal components while the vertical component w g can be derived, for example, from the Richardson equation. However, this vertical component appears not to have been evaluated as yet on the basis of data nor compared to the actual vertical component w . Part of this missing information is provided here by an evaluation of w g from observations and by analyzing the role of w g in linear versions of PGEs. The time mean fields in the Northern Hemisphere as well as the standard deviations are compared to the correponding fields of w . It is found that comes reasonably close to in the troposphere but deviates widely in the stratosphere while is smaller than σ w in the troposphere but not in the stratosphere. Linear wave motion is discussed and the linear steady‐state response to the forcing by heat sources and mountains is explored to explain these results.
Forcing (mathematics)
Geostrophic current
Thermal wind
Component (thermodynamics)
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Geostrophic current
Thermal wind
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Geostrophic current
Thermal wind
Momentum (technical analysis)
Trough (economics)
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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.
Thermal wind
Wind Stress
Log wind profile
Prevailing winds
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Objective mapping can remove the equatorial singularity from the problem of estimating geostrophic shear from noisy density measurements. The method uses the complete thermal wind relation, so it is valid uniformly on and off the equator. Errors in the thermal wind balance are due to neglected terms in the momentum balance, which are treated as noise in the inverse problem. The question of whether the geostrophic balance holds near the equator is restated as a need to estimate the size of the ageostrophic noise in the thermal wind equation. Objective mapping formalizes the assumptions about the magnitudes and scales of the geostrophic currents and about the magnitudes and scales of the ageostrophic terms and measurement errors. The uncertainty of the velocity estimates is calculated as part of the mapping and depends on the signal to noise ratio (geostrophic density signal to ageostrophic “noise”) in the data, as well as the station spacing and the scales assumed for the geostrophic velocities. The method is used to map zonal velocity from a mean Hawaii‐Tahiti Shuttle density section. These are compared with previous velocity estimates for the same dataset calculated using other techniques. By choosing appropriate scales, the objective map can duplicate previous results. New temperature data are presented from a repeating, high‐resolution expendable bathythermograph section crossing the equator at about 170° W with four cruises a year between 1987–1991. There appear to be significant differences between this mean temperature and the shuttle mean temperature. Temperature is converted to density with the aid of a mean T‐S relation and geostrophic velocity maps are calculated for the 4‐year mean. The mean geostrophic undercurrent obtained from our sections is weaker than in the shuttle estimate and is centered slightly north of the equator. Enforcing symmetry about the equator removes the offset of the current, giving a stronger, but narrow undercurrent. The density field apparently includes significant ( O (0.5 kg m −3 )) large‐scale ageostrophic variability which makes velocity estimates from single cruises poorly determined near the equator.
Bathythermograph
Thermal wind
Geostrophic current
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Starting with a hypothetical geostrophic zonal current in an unbounded ocean, the investigation points out the response of this simple system to a thermal forcing, applied to the free surface and consistent with the maintenance of the geostrophic balance. The main result is the formation of a meridional component of the current, according to the Sverdrup relation, such that the full velocity vector rotates clockwise for heating and anticlockwise for cooling to adjust eventually in the initial zonal direction for large depths.
Clockwise
Forcing (mathematics)
Zonal flow (plasma)
Geostrophic current
Thermal wind
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Climatelogical aspects of the geostrophic wind calculated from geopotential heights in the planetary boundary layer in Japan have been investigated. The mean geostrophic winds at the 850 mb level nearly agree with the observed 850 mb winds in both magnitude and direction, with some exceptions. The mean geostrophic wind vectors at the 850 mb and 1000 mb levels for each site are significantly different from each other, especially in winter. This suggests that the large thermal wind exists in the 1000–850 mb layer. It is suggested that the mean geostrophic wind shear is nearly constant with height in this layer.
Thermal wind
Geopotential height
Geostrophic current
Geopotential
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Abstract. The variability and evolution of the Northern Current (NC) in the area off Toulon is studied for two weeks in December 2011 using data from a glider, a HF radar network, vessel surveys, a meteo station, and an atmospheric model. The NC variability is dominated by a synoptic response to wind events, even though a seasonal trend is also observed, transitioning from late summer to fall-winter conditions. With weak winds the current is mostly zonal and in geostrophic balance even at the surface, with a zonal transport associated to the NC of ≈ 1 Sv. Strong westerly wind events (longer than 2–3 days) induce an interplay between the direct wind induced ageostrophic response and the geostrophic component: upwelling is observed, with offshore surface transport, surface cooling, flattening of the isopycnals and reduced zonal geostrophic transport (0.5–0.7 Sv). The sea surface response to wind events, as observed by the HF radar, shows total currents rotated at ≈ −55° to −90° to the right of the wind. Performing a decomposition between geostrophic and geostrophic components of the surface currents, the wind driven ageostrophic component is found to rotate of ≈ −25° to −30° to the right of the wind. The ageostrophic component magnitude corresponds to ≈ 2 % of the wind speed.
Thermal wind
Wind Stress
Geostrophic current
Maximum sustained wind
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The ratio between observed surface and geostrophic wind speed has been investigated from observations at the German Bight, taking geostrophic wind and the air-sea temperature difference as parameters. The ratio decreases with increasing geostrophic wind and increasing stability. While stability is an important parameter for light to moderate winds, variation of the ratio with geostrophic wind speed cannot be neglected, taking the full range of geostrophic wind speeds into consideration. From the Navier-Stokes equations, such a variation is to be expected. For light winds, the (local) surface wind may exceed the (mesoscale) geostrophic wind. Both effects together can be described approximately by a linear relation between the surface wind and geostrophic wind, with a slope of 0.56 and a constant term b>0 varying with stability. The residual error was 2 m/s. Variation with latitude is inferred from the Navier-Stokes equations.
Thermal wind
Geostrophic current
Wind Stress
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Citations (54)