Boundary Layer Scaling

The NCL graphics package.

Boundary Layer Scaling

Postby henryirvine » Sun Oct 11, 2015 2:26 am

Hello all,
I am trying to calculate the Deardorff velocity w*
Image
more fully described http://glossary.ametsoc.org/wiki/Convective_velocity_scale

A few questions have come up. (I am a pilot/engineer trying to create local convective layer forecasts in my area, similar to http://www.drjack.info/BLIP/NAM/INFO/parameters.html#wfpm and am learning the meteorological nitty gritty as I go )

I am planning to use an arbitrary constant Tv of approximately 300 kelvin, but should I be calculating/using a potential temperature instead?

I am using the PBLH parameter from my wrf output as Zi -- as I understand it, PBLH is 'over' terrain, and if I wanted to convert it to an altitude over sea level, I would compute ter+PBLH?

I am struggling to calculate the kinematic vertical turbulent flux of virtual potential temperature.
At present, I have found
HFX -- wrf output field defined as the surface sensible heat flux in W/m^2
LH -- latent heat flux defined as the surface latent heat flux in W/m^2

I found equations that describe surface virtual heat flux (as part of calculating bouyancy production of turbulent kinetic energy), which come down to something nice and simple
HFX + 0.07 * LH

I believe that I should be able to put this into the formula for w*, but when I do, I get numbers that are off by almost an order of magnitude. w* should be on the order of 1 m/s certainly not more than 10, so when I calculate something that gives me w* > 25 m/s I am sure that I am doing something wrong.

My code for calculating and plotting w* is included below. Any thoughts would be appreciated.

Code: Select all

load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"
;load "plotdefaults.ncl"

a = addfile("./wrfout_d01_2015-05-01_00:00:00.nc","r")
type = "x11"


wks = gsn_open_wks(type,"plt_Surface1")
gsn_define_colormap(wks,"WhViBlGrYeOrReWh")

res = True
res@MainTitle = "Real-Time WRF"
pltres = True
mpres = True

Times = wrf_user_getvar(a,"times",-1)
ntimes = dimsizes(Times)

function calcws(q,z)
local  dims, itr_rs, ws, conv2ftmin, cbouy
begin
  ; wstar = ((g/Tavg) * Qsurf * D) ^ (1/3)
  ; g := gravitational field = ~ 9.8062 m/s^2
  ; Tavg := average temperature ~300 k
  ; Qsurf :=  virtual temperature heat flux at surface into air W/m^2
  ; D := boundary layer thickness
  dims = dimsizes(z)
  ws = new (dims,float)
  ws@description="Thermal updraft velocity W*"
  ws@units="m/s"

  ;conv2ftmin = 196.85  ; const conversion factor from m/s to ft/min
 
  cbouy = 9.8062 / 300.0 ; hard code average temp as 300 K
  do i = 0, dims(0) -1
    do j = 0, dims(1)-1
      itr_rs = cbouy*q(i,j)*z(i,j)
      if (itr_rs .lt. 0.05)
        ; don't waste time calculating small values of wstar
        ws(i,j) = 0.0
      else
        ws(i,j)=(itr_rs^0.333)
      end if
    end do
  end do
  return(ws)
end

ter = wrf_user_getvar(a,"HGT_M",0)

do it = 6,ntimes-1,1
print("Working on time: " + Times(it) )

ts = wrf_user_getvar(a,"TSK",it)
hfx = wrf_user_getvar(a,"HFX",it)
tc = wrf_user_getvar(a,"tc",it)
vhf = wrf_user_getvar(a,"LH",it)
qsurf = hfx + 0.07*vhf
qsurf@description="Surface virtual heat flux"
qsurf@units="W/m^2"
pblh = wrf_user_getvar(a,"PBLH",it)
zi = pblh+ter
zi@description="Boundary layer top height"
zi@units="meters"

wstar = calcws(qsurf,pblh)

opts = res
opts@cnFillOn = True
;con_ts=wrf_contour(a,wks,ts,opts)
;con_hfx=wrf_contour(a,wks,hfx,opts)
;con_vhf=wrf_contour(a,wks,qsurf,opts)

con_pblh=wrf_contour(a,wks,pblh,opts)
con_qsurf=wrf_contour(a,wks,qsurf,opts)
con_wstar=wrf_contour(a,wks,wstar,opts)

;plot = wrf_map_overlays(a,wks,(/con_ts/),pltres,mpres)
;plot = wrf_map_overlays(a,wks,(/con_hfx/),pltres,mpres)
plot = wrf_map_overlays(a,wks,(/con_qsurf/),pltres,mpres)
plot = wrf_map_overlays(a,wks,(/con_pblh/),pltres,mpres)
plot = wrf_map_overlays(a,wks,(/con_wstar/),pltres,mpres)

end do
henryirvine
 
Posts: 4
Joined: Thu Oct 01, 2015 5:11 pm

Re: Boundary Layer Scaling

Postby lixx » Thu Jan 21, 2016 11:07 pm

It seems that the dimension is not right. Qsurf should be in m s{-1} K, rather than W m{-2}. There is a rho*Cp to link them. If you take that into account, it may be right.
lixx
 
Posts: 3
Joined: Fri Sep 26, 2008 11:37 pm


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