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Atms 4320 / 7320 – Lab 7 The Thermal Wind: Forecasting Problems and the Analysis of Fronts

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Thermal wind: Bluestein p 181 - 187, and Holton p 68 - 71. The “thermal” wind: The thermal wind links the temperature field to the wind field via hydrostatic balance.

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts We know that in a geostrophically balanced world we can link the wind field with the mass field (pressure or height). Thus, in order to talk about thermal wind balance we must specify two balance conditions to exist simultaneously. Thus, thermal wind balance is considered a “higher order” balance state.

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts The Thermal wind is simply the vertical wind shear vector: VT = V geo (upper level) - V geo (lower level) Thus if we substitute in for Vgeo we get:

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Carrying out the subtraction, put into differential form,

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Then using the equation of state:

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts In component form:

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Note that the thermal wind is simply a measure of the vertical wind shear. Vertical wind shear: Baroclinicity

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Recall that in a barotropic atmosphere, there is no vertical wind shear. The definition of a baroclinic atmosphere is that there is speed and or directional shear in the vertical. Thus we can regard the strength of Thermal Wind as a measure of baroclinicity.

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts The thermal wind “blows” parallel to the isotherms, or the thickness lines with warm air on the right and cold air on the left. Thus, the thermal wind “blows” perpendicular to both gradient quantities.

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts

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The map (Thanks OU)

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Thus, the thickness lines can also be used to deduce areas of warm and cold air advections, and locate fronts in the atmosphere.

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Another view (Dr. Brad Muller)

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Cold advection (winds back with height): In the Northern Hemisphere, cold advection will be associated with thermal wind with a southerly component.

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Geostrophic Wind Shear and Thermal Advection cold warm Case 2: Geostrophic wind backs (i.e., turns counterclockwise) with height. Lower level wind is from N. Upper level wind is from NW. Since colder air must lie to the left of the thermal wind, the layer average wind blows from cold to warm, which implies cold advection.

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Warm advection (winds veer with height). (thanks to Dr. Broccoli, Rutgers) In the northern hemisphere, warm air advection will tend to be associated with a thermal wind with a westerly component.

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Geostrophic Wind Shear and Thermal Advection cold warm Case 1: Geostrophic wind veers (i.e., turns clockwise) with height. Lower level wind is from SW. Upper level wind is from W. Since colder air must lie to the left of the thermal wind, the layer average wind blows from warm to cold, which implies warm advection.

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Areas of low pressure will tend propagate along with the thermal wind where it is strong (along packed thickness or height contours), and at the speed of the thermal wind.

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Also, we can deduce fronts from the thickness field. The warm front is located on the equatorward edge of a zonal thickness gradient as we move poleward (or perpendicular to the thermal wind moving over it from right to left):

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts The cold front is located on the downstream edge of a meridional thickness gradient as we move from west to east (zonally) (or perpendicular to the thermal wind moving over it from right to left)

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts In the Southern Hemisphere, the thermal wind blows with warm air to the left and cold air to the right. Why? Coriolis Force is negative! We must “flip” all the arguments made above.

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts Example (SH): A backing wind represents represents Warm air advection A veering wind represents cold air advection

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The Thermal Wind: Forecasting Problems and the Analysis of Fronts The End!

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