Using Level 2 radar in NEXRADPro to extrapolate Harvey surface wind

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Using Level 2 radar in NEXRADPro to extrapolate Harvey surface wind

Postby Paul » Fri Aug 25, 2017 1:26 pm

The 1:05pm 0.5 degree velocity scan from KCRP indicates a 142mph beam-axis velocity at 2800m above ground and 153km from radar:
Harvey1.png (409.3 KiB) Viewed 148 times

Correcting trigonometrically for the 0.5 tilt and assuming prevailing wind direction is parallel to this beam (a solid assumption, since the maximum beam-relevant velocity from the whole scan was in this beam, and given the storm direction), the horizontal wind speed would be 142.5*cos(0.5deg), which due to the extremely small angle comes to an equivalent 142.5mph due to significant digits, so it is proven that this radar return can suffice for a horizontal wind estimate.

Now applying the wind profile power law (link) the reduced 10m wind would be 142.5*(10/2800)^a, where a is a stability coefficient. One estimate (Hsu, S.A., E.A. Meindl, and D.B. Gilhousen, 1994, Determining the power-law wind-profile exponent under near-neutral stability conditions at sea, J. Appl. Meteor., Vol. 33, pp. 757-765) on open water conditions would yield 142.5*(10/2800)^0.11 = 76.7mph at 10m, however this does not account for the different synoptic conditions of the tropical system. Hsu (2014) (link) states that this exponent is related to the gust factor by G = 1+2a. It is safe to assume that the synoptic conditions at this point are similar to that of a surrounding area in the tropical cyclone; therefore NDBC 42020 can be used to approximate the gust factor:
Harvey2.png (494.25 KiB) Viewed 148 times

G = 69.3/55.9 = 1.2397, therefore a = 0.1198 by this measure. This is very similar to the original Hsu (1994) estimate, and produces 142.5*(10/2800)^0.1198 = 72.55mph.

Now correct for the 17.8-degree horizontal disparity between storm motion (315 NW latest advisory) and beam heading direction (117.2+180):
Corrected = mph/cos(17.8deg) = 80.6mph (first method) or 76.19mph (second method)

Since the NDBC currently has max sustained winds at 110mph (~30mph greater), and since that does not account for higher gusts likely captureable within the distance of a Level 2 bin, it is likely that the concentration of strongest wind is therefore not along this radar beam's path, which is potentially a result of the eye-wall replacement cycle visible on reflectivity (since one would expect it to be here with concentric wind distribution, at the right front quadrant relevant to storm motion).

Can anyone suggest other reasons why the strongest wind is not found here, or potential false assumptions in my analysis?

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Re: Using Level 2 radar in NEXRADPro to extrapolate Harvey surface wind

Postby NorthAlabamaStorm » Sat Aug 26, 2017 3:07 pm

just asking.

Would small microburst in or around this wind field have an affect on wind speed data or maybe the copius amount of precipitation in contrast to radar return give a false wind field?

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