undef("wind_log_extrap")
function wind_log_extrap(zref:numeric, vref:numeric, rlen:numeric, zext:numeric, opt[1]:logical )
;
; Extrapolate the wind speed at a certain height by using the log law. 
; Appropriate for the change of wind speed with height in the lowest 100m.
;
; Reference: https://websites.pmc.ucsc.edu/~jnoble/wind/extrap/
;
; INPUT:
; zref - height where vref is known
; vref - velocity at height zref
; rlen - roughness length in the current wind direction (rlen>0)
;        same units as zref
; zext - height of extrapolated wind   
; opt  - currently not used
;
; OUTPUT:
; vext - log extrapolated wind at height 'zext'
;
begin
  if (any(rlen.le.0)) then
      print("wind_log_extrap: rlen has inappropriate values")
      exit
  end if
       
  vext = vref*(log(zext/rlen)/log(zref/rlen))

; meta data

  if (isatt(vref,"long_name")) then
      vext@long_name = "wind_log_extrap: "+vref@long_name
  else
      vext@long_name = "wind_log_extrap wind value"
  end if

  if (isatt(vref,"units")) then
      vext@units = vref@units
  end if

  return(xext)
end