Thermal wind

D. L. Klass, ed., A. Directory of U.S. Renewable Energy Technology Vendors, Biomass, Photovoltaics, Solar Thermal, Wind, Biomass Energy Research Association, Washington, D.C., 1990, for U.S. Agency for International Development, 74 pp. 

At the Warnemiinde coast, this strengthened northwest wind arises preferably if at cyclonic northwest weather conditions the gradient wind is blowing continuously strongly and from northwest for a longer period of time. At the Warnemunde coast it is often particularly strongly pronounced because it receives an additional thermal wind reinforcement there under certain convective weather conditions due to ascending air over the inhabited cities (thermal lift). 

Application of the Thermal Wind Relation The mean temperature in the layer between 65 and 60 kPa decreases in the eastward direction by 4 C per 100 km. If the geoslrophic wind at 65kPa is from the southeast at 20ms 1, what are the geo-strophic windspeed and direction at 60kPa. Assume/ = 10 4s 1. 

Now the thermal wind relation will allow us to calculate the change in geostrophic wind over this layer. The only temperature gradient is in the eastward (x) direction. From (21.22), there is no change ug with altitude. From (21.22), we can express... 

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Thermal wind Vertical shear of geostrophic wind directed along the isotherms with cold air to the left in the Northern Hemisphere and to the right in the Southern Hemisphere. 

Along the coast of California during north-westerly up-welling favorable winds, the marine atmospheric boundary layer is characterized by a low-level jet, with peak wind speeds of as much as 30 m/sec at elevations of a few hundred meters. The vertical structure is marked by an inversion, usually at or near the elevation of the wind speed maximum. Above the inversion, the stratification is stable, and the wind shear is caused primarily by baro-clinicity (thermal wind) generated by the horizontal temperature gradient between the ocean and land. Below the inversion, the flow is turbulent. 

This alternative formulation of geostrophic equilibrium is known as the thermal wind equation it relates the vertical shear of the zonal-mean zonal wind to the horizontal gradient of the zonal-mean temperature. If the temperature decreases toward the poles (as observed in the wintertime stratosphere), then a positive vertical shear will develop so that the zonal-mean zonal wind becomes increasingly eastward with altimde. On the other hand, if the pole-ward temperature gradient is positive (as is the case in the summer stratosphere), then the zonal wind will become more westward with altitude. This is precisely what is observed, as may be ascertained by comparing the mean temperature distributions of Fig. 5 with the zonal-mean zonal wind distributions of Fig. 6. 

The thermal wind equation relates the appearance of the summer westward and winter eastward jets in the stratosphere to the temperatiue distribution, which is in turn a result of the latitudinal variation of heating due to absorption of ultraviolet radiation by ozone and large-scale motion of air parcels. 

The oscillating wind regimes depicted schematically in Fig. 16 (and from observations in Figs. 10 and 11) are accompanied by secondary circulations in the meridional plane, and by temperature anomalies in balance with the adiabatic heating and cooling associated with the secondary circulations. The magnitude of these anomalies can be estimated because, even in the tropics, the zonal-mean zonal wind and temperature are in thermal wind balance. Near the equator, Eq. (13) can be written as follows ... 

Thermal wind Vector difference between the geostro-phic wind at two pressure levels. Its magnitude is proportional to the horizontal gradient of mean temperature in the layer, and its direction is parallel to lines of constant mean temperature (isotherms) with the colder air on the left (in the northern hemisphere). 

Strictly speaking, the thermal wind is a measure of the vertical shear of the geostrophic wind and not a real wind composed of air in motion. Yet it is useful to think of it as a velocity. From Eq. (52) the thermal wind is directed parallel to lines of constant thickness with the cold air (low thickness) on the left and is stronger in regions of greater mean temperature gradient. 

As with the thermal wind, the gradient wind has many apphcations. One application is an explanation of why the pressure gradient is generally weaker near the center of an anticyclone than near the center of a cyclone. This can... 

Barotropic instability. Barotropic instability can result from a basic state with horizontal shear but no vertical shear of the zonal average wind. By the thermal wind relationship, the basic-state horizontal temperature gradient vanishes and temperature advection does not occur. Furthermore, it is assumed that the perturbed flow is purely horizontal, and therefore, by necessity, nondivergentinthe horizontal plane with no vertical shear. In this situation the potential vorticity reduces to the absolute vorticity. H.-L. Kuo showed in 1949 that such a flow is susceptible to instability. A necessary condition for barotropic instability may be derived that states that somewhere in the fluid... 

Microscale 10Vi Thermals, wind gusts, and dust devils... 

Normal—self weight, traffic, thermal, wind load,... 

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We concentrate on the information obtained from infrared spectroscopy and radiometry, both directly and in conjunction with other data sets, such as those from visible imaging. To provide the necessary background for the subjects of this section, we first review the equations of fluid motion and the succession of approximations leading to a tractable set of equations that can be used to describe the motion of a planetary atmosphere. Eor most of the cases considered, geostrophic balance and the associated thermal wind equations play major diagnostic roles in the inference of atmospheric motions from remotely sensed temperatures. For this reason, the derivation of these relations will be discussed in some detail. Other... 

The geostrophic relations and the associated thermal wind equations can provide significant insight into the behavior of rotating atmospheres they are the lowest order approximation in a systematic development of large-scale atmospheric dynamics. In addition, these equations have been used to obtain information on atmospheric winds from remotely sensed measurements for many of the planetary atmospheres considered here. Therefore, we examine the geostrophic... 

We have developed the basic tools necessary for the application of remotely sensed data to problems in the dynamics of planetary atmospheres. The thermal wind equations are used extensively for this purpose, while the complete set of primitive equations, (9.2.20)-(9.2.24), forms the starting point for most of the other relevant approximations and models. We now turn to selected examples of applications to specific planetary atmospheres. 

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