! coads_demo.jnl *jd* 11/91

! Description: introduces and briefly explores the COADS climatological data 

! COADS is the comprehensive ocean-atmosphere data set compiled at NCAR from
! ship reports over the global ocean.  The monthly climatology introduced here 
! represents a simple average of all data available for each month of the year
! from 1946-1989.

! Advanced use of FERRET commands in the demo will give you helpful examples
! of their use.  See the User's Guide for more on individual commands.

! 7/01 *acm* change uppercase GO LAND to GO land

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CAN REGION
SET MODE VERIFY 
SET DATA coads_climatology
SHOW DATA coads_climatology

! Note that 7 variables are available.  The grid is 2x2 degree, and global.

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SET REGION/Y=60S:80N
SET WIND/SIZE=.6/ASPECT=.86
PPL CROSS 1	! This PPL command causes the equator to be drawn in plots

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! Sea level pressure for the average July

go rgb_rainbow
SHADE/L=7 SLP
GO land		

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! Overlay the July surface wind.

VECTOR/OVER/L=7/LEN=10 UWND,VWND

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! The horizontal equations of motion on the rotating earth show that
! acceleration of a parcel of air is dependent on the pressure gradient,
! the coriolis force, and any friction retarding its motion.  Neglecting 
! friction, a geostrophic wind may be defined, where the pressure gradient
! force is balanced by the coriolis force.  Let's define variables ug and vg
! to be that geostrophic wind, being careful to use common units of measure.

LET RHO = 1.275                    ! Use a constant for air density for now
LET OMEGA = 2*3.14159/86400        ! The angular velocity of the earth
LET DPDX = SLP[X=@DDC]*100         ! Zonal pressure gradient (in SI units)
LET DPDY = SLP[Y=@DDC]*100         ! Meridional pressure gradient
LET C = 3.14159/180                ! Conversion factor -- radians/degree
LET F = 2*OMEGA*SIN(Y[G=SLP]*C)    ! The coriolis parameter

LET/TITLE="VG" VG = IF ABS(Y) GT 5 THEN DPDX/(F*RHO)
LET/TITLE="UG" UG = IF ABS(Y) GT 5 THEN (-1)*DPDY/(F*RHO)

! The geostrophic wind is ill-defined near the equator as f is 0 there.
! This definition specifies ug and vg poleward of latitudes +/- 5 degrees.

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! The geostrophic wind

VECTOR/LEN=10/L=7 UG,VG
GO land

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! A bit noisy so let's smooth it 5 grid points in x

LET UGS = UG[X=@SBX:5]
LET VGS = VG[X=@SBX:5]
VECTOR/LEN=10/L=7 UGS,VGS
GO land

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! Here is the geostrophic wind field superimposed on sea level pressure.

SHADE/L=7 SLP
GO land
VECTOR/LEN=10/OVER/L=7 UGS,VGS

! If we now overlay the observed winds we see that they are often
! subgeostrophic, that is, the coriolis force is not strong enough to 
! balance the pressure gradient force (from high to low pressure),
! possibly indicating presence of friction near the earth's surface.

VECTOR/LEN=10/OVER/L=7 UWND,VWND

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! Here is the same plot for March

SHADE/L=3 SLP
GO land
VECTOR/LEN=10/OVER/L=3 UGS,VGS
VECTOR/LEN=10/OVER/L=3 UWND,VWND

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! And we can look more closely at a hemisphere

SET REGION/Y=5:70
SET WIND/ASP=.5

SHADE/L=7 SLP
VECTOR/LEN=10/OVER/L=7 UGS,VGS
VECTOR/LEN=10/OVER/L=7 UWND,VWND
PPL CROSS 0

! The "land" go tool ("go land") is useful for continental outlines and has
! been used several times in this demo.  There is another tool to fill 
! the land masses with a dark grey color, "go fland".  A land mask is layed
! over the plot, as in:

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GO fland

! The "go land" tool can still be used, too.

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GO land

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! More exploration of global climate is possible using the COADS climatology. 
! This demonstration uses a fixed value for the density of air.  Since 
! pressure, temperature, and specific humidity are available, density could
! be cast in a more complex functional form.  Models of the friction suggested
! above can be tested.  And other explorations that come to mind.

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GO rgb_rainbow
SET WINDOW/ASPECT=.86