Ferret Demonstration

This tour introduces and briefly explores the COADS climatological data set using FERRET.

COADS is the comprehensive ocean-atmosphere data set compiled 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. The recently updated COADS is a joint effort between NOAA's Climate Diagnostics Center (CDC) and the Cooperative Institute for Research in Environmental Sciences (CIRES), the National Center for Atmospheric Research (NCAR), and NOAA's National Climatic Data Center (NCDC)

Advanced use of FERRET commands in the tour will give you a good idea how useful the program can be exploring data sets. Commands to FERRET below will be in capital letters. FERRET prompts for input with the word

"yes?".

    yes? SET DATA coads-climatology
    yes? SHOW DATA coads-climatology

         currently SET data sets:
        1> ./coads-climatology.des  (default)
     name     title                             I         J         K         L
     SST      SEA SURFACE TEMPERATURE          1:180     1:90      1:1       1:12
     AIRT     AIR TEMPERATURE                  1:180     1:90      1:1       1:12
     SPEH     SPECIFIC HUMIDITY                1:180     1:90      1:1       1:12
     WSPD     WIND SPEED                       1:180     1:90      1:1       1:12
     UWND     ZONAL WIND                       1:180     1:90      1:1       1:12
     VWND     MERIDIONAL WIND                  1:180     1:90      1:1       1:12
     SLP      SEA LEVEL PRESSURE               1:180     1:90      1:1       1:12

Note that 7 variables are available. The grid is 2x2 degree, and global. For information on FERRET's grid indices click here.

The following commands will generate a plot of sea level pressure and winds for the average July.

    yes? SET REGION/Y=60S:80N
    yes? SHADE/L=7 SLP
    yes? GO land
    yes? VECTOR/OVER/L=7/LEN=10 UWND,VWND

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.

The commands below produce a plot of the geostrophic wind. 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.

    yes? LET RHO = 1.275                     Use a constant for air density for now
    yes? LET OMEGA = 2*3.14159/86400         The angular velocity of the earth
    yes? LET DPDX = SLP[X=@DDC]*100          Zonal pressure gradient (in SI units)
    yes? LET DPDY = SLP[Y=@DDC]*100          Meridional pressure gradient
    yes? LET C = 3.14159/180                 Conversion factor -- radians/degree
    yes? LET F = 2*OMEGA*SIN(Y[G=COADS1]*C)  The coriolis parameter

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

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

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

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

Here is the geostrophic wind field superimposed on sea level pressure:

    yes? SHADE/L=7 SLP
    yes? GO land
    yes? 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:

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

Here's the same plot for March:

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

And we can look more closely at a hemisphere:

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

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

Much more exploration of the global climate is possible using the COADS climatology. This tour 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.

oar.pmel.contact_ferret@noaa.gov