YSU : computation of the wind tendency

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YSU : computation of the wind tendency

Postby volod » Thu Jun 04, 2015 12:35 pm

Hello everybody,

I am looking into module_bl_ysu.F to understand the momentum balance in the first layer. In the computation, the surface wind stress seems to be multiplied by a factor (wspd1(i)/wspd(i))**2 (wspd1 being the horizontal wind speed in the first layer) :
if(present(ctopo)) then
ad(i,1) = 1.+ctopo(i)*ust(i)**2/wspd1(i)*rhox(i)*g/del(i,1)*dt2 &
*(wspd1(i)/wspd(i))**2
else
ad(i,1) = 1.+ust(i)**2/wspd1(i)*rhox(i)*g/del(i,1)*dt2 &
*(wspd1(i)/wspd(i))**2
endif

(lines 1303 to 1309 in the version 3.7)

I do not understand the physical origin for this factor (wspd1(i)/wspd(i))**2. Does anyone knew why this factor is present?

Thank you very much !
volod
 
Posts: 3
Joined: Wed Feb 25, 2015 3:08 pm

Re: YSU : computation of the wind tendency

Postby volod » Fri Jun 12, 2015 2:32 pm

Hello,

I have my answer. If somebody wonder, in the surface scheme (sfclay=1), ustar is computed using a velocity wspd that also contain a parameterized convective velocity (vconv). ustar^2=Cm.wspd^2
However, the surface stress used in the PBL momentum balance should not be computed using wspd but just using the horizontal velocity at the first layer (wspd1). So, the surface stress used is Cm.wspd1^2, i.e. ustar^2(wspd1/wspd)^2.
volod
 
Posts: 3
Joined: Wed Feb 25, 2015 3:08 pm


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