Vorticity Advection

I am computing vorticity advection on a fixed 20-km grid with the script below.
With high resolution 500-hPa wind data, slight noises in the wind lead to moderate
noise in the vorticity and much worse noise when computing the vorticity advection,
after computing the derivate twice with the leap frog method.
You can see what I mean from the attached plot of vorticity advection.

Any suggestions? Should I smooth the wind data first? Or is there a better method
than using leap frog?

Thanks, Michael

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/shea_util.ncl"
begin

a=addfile("/volumes/data/azi/regcm_output/spinup/1996/GL_ATM.1996090100.nc","r")
uu=a->u
vv=a->v
pp=a->ps
u=dim_avg_Wrap(uu(iy|:,jx|:,kz|:,time|48:51))
v=dim_avg_Wrap(vv(iy|:,jx|:,kz|:,time|48:51))
sigma=a->sigma
xlat=a->xlat
xlon=a->xlon
ptop=a->ptop ; hPa
ps=dim_avg_Wrap(pp(iy|:,jx|:,time|48:51)) ; hPa

p=new((/126,150,18/),float)
do i=0,17
  p(:,:,i)=ps*sigma(i)
end do
copy_VarCoords(u,p)

uwnd=int2p(p,u,500.,1)
vwnd=int2p(p,v,500.,1)
uwind=uwnd(:,:,0)
vwind=vwnd(:,:,0)

copy_VarCoords(ps,uwind)
copy_VarCoords(ps,vwind)

dvdx=new((/126,150/),float)
dvdx=0.
dudy=new((/126,150/),float)
dudy=0.
vort=new((/126,150/),float)
vort=0.

do i=1,124
  do j=1,148
    dvdx(i,j)=(vwind(i,j+1)-vwind(i,j-1))/40./1000. ; s-1
    dudy(i,j)=(uwind(i+1,j)-uwind(i-1,j))/40./1000. ; s-1
    vort(i,j)=dvdx(i,j)-dudy(i,j)
  end do
end do

copy_VarCoords(ps,dvdx)
copy_VarCoords(ps,dudy)
copy_VarCoords(ps,vort)

dvortdx=new((/126,150/),float)
dvortdx=0.
dvortdy=new((/126,150/),float)
dvortdy=0.

do i=1,124
  do j=1,148
    dvortdx(i,j)=(vort(i,j+1)-vort(i,j-1))/40./1000.
    dvortdy(i,j)=(vort(i+1,j)-vort(i-1,j))/40./1000.
  end do
end do

adv=new((/126,150/),float)
adv=0.

do i=0,125
  do j=0,149
    adv(i,j)=-1.*uwind(i,j)*dvortdx(i,j)-1.*vwind(i,j)*dvortdy(i,j)
  end do
end do
copy_VarCoords(ps,adv)

adv@grid_mapping="rcm_map"
adv@coordinates="xlat xlon"

vort@grid_mapping="rcm_map"
vort@coordinates="xlat xlon"

end

_______________________________________________
ncl-talk mailing list
List instructions, subscriber options, unsubscribe:
http://mailman.ucar.edu/mailman/listinfo/ncl-talk