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# pslec

Computes sea level pressure from CCM/CAM hybrid model variables using the ECMWF formulation.

## Prototype

```	function pslec (
t    [*][*] : numeric,
phis [*][*] : numeric,
ps   [*][*] : numeric,
pres [*][*] : numeric
)

return_val [dimsizes(ps)] :  numeric
```

## Arguments

t

A two-dimensional array containing temperatures (K) at the lowest model level. The two dimensions must be lat and lon.

phis

A two-dimensional array containing surface geopotential (m^2/s^2). Same dimensionality as t.

ps

A two-dimensional array equal to the surface pressure (Pa). Same dimensionality as t.

pres

A two-dimensional array containing pressures (Pa) of the lowest model level. Same dimensionality as t.

## Return value

A multi-dimensional array (nlat,mlon) Double if any of the input arguments is double, float otherwise.

## Description

Computes sea level pressure using the ECMWF formulation. The underlying code was taken directly from an old model post-processor that ran only on NCAR's Cray computers. It was developed for the 'old' CCM (Community Climate Model) hybrid model because the CCM did not output a sea level pressure variable. Also, at that time, the surface geopotential was constant. This function may be used with CAM hybrid model variables. However, generally, the CAM outputs the sea level pressure variable (PSL) and that should be used.

## Examples

Example 1 Derive sea level pressure from CCM/CAM hybrid level variables. The image produced by the following code may be viewed here .

```

diri = "./"
fili = "ccsm35.h0.0021-01.demo.nc"                ; CAM hybrid model

t    = f->T      ; (time,lev,lat,lon), [K]
phis = f->PHIS   ; (time,lat,lon), [m2/s2]
ps   = f->PS     ; (time,lat,lon), [Pa]

; calculate pressures at each grid point and level
hyam = f->hyam   ; read from a file the mid-layer coef
hybm = f->hybm   ; read from a file
p0   = f->P0
pres = pres_hybrid_ccm(ps,p0,hyam,hybm)  ; (ntim,klvl,nlat,lmon), [Pa]

; get dimensions sizes
dimt = dimsizes(t)
ntim = dimt(0)
klvl = dimt(1)
nlat = dimt(2)
mlon = dimt(3)

; preallocate memory
slp  = new( (/ntim,nlat,mlon/), typeof(ps), getFillValue(ps))

; loop over all times. 'klvl-1' is the CCM/CAM lowest model level
do nt=0,ntim-1
slp(nt,:,:) = pslec(t(nt,klvl-1,:,:),phis(nt,:,:),ps(nt,:,:),pres(nt,klvl-1,:,:))
end do

copy_VarMeta(ps,slp)
slp           = slp*0.01                  ; change units to hPa (not necessary)
slp@long_name = "sea level pressure"
slp@units     = "hPa"

printVarSummary(slp)
print("slp: min="+min(slp)+"  max="+max(slp))

;**********************************************t*
; plotting parameters
;************************************************
wks  = gsn_open_wks("png","tst_pslec")       ; specifies a ps plot

res                       = True     ; plot mods desired
res@gsnMaximize           = True     ; affects ps, eps, pdf only
res@mpFillOn              = False    ; no land gray fill
res@cnFillOn              = True     ; turn on color
res@cnLinesOn             = False    ; turn off contour lines
res@gsnPaperOrientation   = "portrait"

res@cnLevelSelectionMode  = "ManualLevels"
res@cnMinLevelValF        =   975.0
res@cnMaxLevelValF        =  1040.0
res@cnLevelSpacingF       =     5.0

res@tiMainString          = "Test PSLEC"

nt    = 0   ; plot slp at 1st time step
plot  = gsn_csm_contour_map_ce(wks,slp(nt,:,:),res) ; create a default plot
```