NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/sci/geo/meteorol/2791

Internet Message Format | 1992-08-18 | 2.7 KB

Path: sparky!uunet!zaphod.mps.ohio-state.edu!uwm.edu!rutgers!uwvax!meteor!stvjas
From: [e-mail] (Steve Jascourt)
Newsgroups: sci.geo.meteorology
Subject: Re: Green sky and T-storms
Message-ID: <[e-mail]>
Date: 19 Aug 92 00:00:40 GMT
References: <[e-mail]> <[e-mail]> <[e-mail]>
Distribution: na
Organization: University of Wisconsin, Meteorology and Space Science
Lines: 38

In article <[e-mail]> [e-mail] (Harold_Brooks) kindly gives us a quick and dirty look at terminal velocities
>In article <[e-mail]> [e-mail] (Steve Jascourt) writes:
>>at SPS)? What is the terminal velocity of a six inch hailstone (assume it
>>is ellispoidal in shape)?
>
>Moore and Pino,1990: An interactive method for estimating maximum hailstone
>size fromforecast soundings.  _Weather and Forecasting_, 5, 508-525, we can
>get the terminal velocity, Vt, as a function of hailstone diameter, d--
>  ...
>where d is in cm and Vt is in m/s.  For a 6 inch diamater hailstone, then,
>Vt ~ 38.7 m/s ~ 87 miles/hour, which is not all that huge.

Which would correspond to a CAPE of only 1600 J/kg after using the rule of
thumb that you take half of the CAPE when predicting updraft velocities.
1600 J/kg is not all that unusual even after accounting for water-loading in
computing the CAPE, plus, storms with the strong long-lived updrafts tend to
be supercells whose dynamic pressure perturbations will add more vertical
velocity than the same amount of buoyancy would otherwise generate.

>I make no claims to the validity of the assumptions of Pino and Moore (and
>the references therein) or the relevance of the terminal velocity of a
>hailstone to its size.

I'll have to check the article before I can comment on their assumptions. As
for the relevance, in a simplistic sense, *assuming* terminal velocity is
reached quickly, the updraft would have to be at least this large to sustain
such a large hailstone. Of course, we also really need to consider how much
additional growth by riming or reduction by melting the hailstone undergoes as
it falls through a weaker updraft (or even gets caught in the downdraft; both
may happen at different heights if the updraft is tilted). You guys at
Oklahoma could ask Jerry Straka about how much extra growth or melting loss
might be expected (of course, it depends on storm structure and the relative
humidity experienced on the hailstones trip down), but he would know for large
hailstones how much of a factor this is.

>Harold Brooks                    [e-mail]

Stephen Jascourt   [e-mail]