## YSU : computation of the wind tendency

Topics related to current and future physics in the WRF as well as any problems you may have.

### YSU : computation of the wind tendency

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
*(wspd1(i)/wspd(i))**2
else
*(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

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