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Transport Winds Fire Weather Tutorial

Transport winds are defined as the average wind speed and direction of all winds within the layer bounded by the surface and the mixing height. Knowledge of transport winds is a crucial factor in the effective management of smoke management programs. Transport winds provide land managers with information about the horizontal dispersion (location and distance downwind form the source) of suspended particulates from prescribed fires and slash burns.

The first step in determining transport winds begins with a calculation of the mixing height using a current (or forecast) upper air sounding and a surface temperature forecast. (See the mixing height web-page for instructions.) Once the mixing height has been determined, the transport wind can be calculated by averaging the reported winds (or forecast winds) from the surface to the mixing height. Since wind is a vector, the averaging process begins with the calculation of the zonal (U-component) and the meridional (V-component) of the wind at each level. Instructions and formulas for calculating the individual wind components are included below.

The meridional component of the wind, V, is considered positive when the wind in blowing from south to north. A south wind has a positive meridional component while a north wind has a negative meridional component. The zonal component of the wind, U, is considered positive when the wind is blowing from west to east. Thus, a west wind has a positive zonal component and an east wind a negative zonal component.

For example, a wind that is blowing from the northeast would have a negative meridional component, V, and a negative zonal component, U. Such a wind would have a direction of 45 degrees.

If the speed of the wind is (ff) and the direction in degrees is (dd), then the formulas for obtaining the meridional component, V, and the zonal component, U, are:

V = -ff * cos(dd)

U = -ff * sin(dd)

These formulas are used to calculate the U (U-MAG) and V (V-MAG) wind components in the table below.

The following example illustrates how the morning (12z) sounding and the maximum temperature forecast can be used to estimate the maximum mixing height and transport wind for the today portion of the narrative fire weather forecast.

The chart above shows the upper air sounding for Quillayute, Washington on the morning of March 16, 1998. A surface parcel with a temperature of 62 degrees (the maximum temperature forecast for Quillayute) would rise dry adiabatically until it lost its buoyancy (or intercepted the sounding) at 11,283 feet msl. (Note...In this example, a parcel of air would actually rise much higher after reaching it's lifting condensation level at 5,208 feet. The parcel would undergo a moist ascent from 5,208 feet until intercepting the sounding at the equilibrium level of 23,130 feet.)

In this example, the transport wind would be the average wind speed and direction of the reported winds below 11,283 feet. The chart below shows a portion of the wind data table for Quillayute showing the reported winds from the surface to 12,000 ft msl. When calculating the transport wind, the 12,000 ft wind would be ignored because it would be above the mixing height. The calculated average U-component and V-component are shown on the last line of the table.

WIND LEVEL DATA		3/16/98 @ 12Z

Level	Height-MSL		DIR	Speed		U-MAG	V-MAG
	(THSD-FT)	(METERS)	(DEG)	(KTS)	(M/S)	(M/S)	(M/S)
0	0.2	62	270	05	2.6	2.6	0.0
1	1.0	305	295	23	11.8	10.7	-5.0
2	2.0	610	300	22	11.3	9.8	-5.7
3	3.0	915	300	19	9.8	8.5	-4.9
4	4.0	1220	295	18	9.3	8.4	-3.9
5	6.0	1829	285	18	9.3	8.9	-2.4
6	7.0	2134	285	22	11.3	10.9	-2.9
7	8.0	2439	275	27	13.9	13.8	-1.2
8	9.0	2744	275	28	14.4	14.3	-1.3
9	12.0	3659	270	31	15.9	15.9	0.0

					Avg Speed	9.8	-3.0

Given the U and V components of the average wind speed , the following equation is used to calculate the direction of the transport wind:

In this example, the direction of the transport wind would be :

dd = arctan(9.8/-3.0) + 360 = (-72) + 360

dd = 288 degrees

The wind speed can be calculated using the following formula shown on the left.

In this example the transport wind would be 288 degrees at 19.8 knots. Some NWS fire weather offices define transport winds as the wind at the final height of the plume rise, in which case the transport wind would then be 270-275 degrees at 28-31 knots.

Please refer any questions or comments about this web site to: john.werth@noaa.gov