Detailed Explanation of 4-panel Plots: Near-global (62°N-62°S) Time-series Comparisons between Two Datasets using Colocated 2° boxes

These plots are used to compare 2° summaries from two datasets (dataset1 and
dataset2).  Each 4-panel plot provides comparison information for a single
variable (e.g., sea surface and air temperature, scalar wind, pressure, and
total cloudiness).  A description of each of the four panels follows:

a) Departures
For each variable, 2° latitude/longitude boxes in the region 62°N-62°S were
included in the comparison only if they possessed data "concurrently" (i.e.,
for a given year-month) from both dataset1 (left) and dataset2 (right).  This
ensured a comparable grid within each monthly time step, but not a frozen grid
through time.  Departures were calculated for each year-month-2° box, and each
dataset, of monthly means with respect to a basic 1950-79 COADS Release 1
long-term monthly mean (LTM).  (Note that the data used to construct the LTM
were also means, not medians.)  Each colocated 2° box departure value was
cosine-weighted,* from which the area weighted average was computed.  The top
panel contains two separate curves of the area-weighted average departures
(black = dataset1; green = dataset2).  It should be emphasized that this type
of comparison does not reveal anything about data patterns in either dataset
outside of the region defined by the set of colocated boxes for each
year-month.

b) Differences
The green curve is the area-weighted average, as calculated for plot a),
of the difference between 2° monthly means for dataset1 minus dataset2 (i.e.,
both datasets must possess a monthly mean for a given 2° box to be included in
the difference).  This set of boxes may be larger than the set of colocated
boxes used for the departures.** The black curve, which corresponds almost
exactly (and is thus invisible on many of the plots), is the non-area-weighted
average of the differences.

c) 2° boxes
The black curve shows the number of colocated*** 2° boxes; the green curve
shows that number plus the number of non-colocated 2° boxes containing only
data from dataset2 (the number of non-colocated 2° boxes in dataset1, if any,
is not shown).  The green curve also includes any 2° boxes containing data in
dataset2 that were not available in the 1950-79 LTM; such boxes were also not
included in plot a).

c) Numbers of observations
Using only the set of colocated**** boxes for each year-month, the green curve
shows the number of observations for dataset2, and the black curve shows the
corresponding number of observations for dataset1.  Curves are not shown for
any additional observations falling in 2° boxes represented in either dataset
outside of the set of colocated boxes.

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* The method used by GrADS employs the "delta of the sin of the latitudes at
  the edges of the grid box," rather than the central latitude of the box.
** The set of boxes used for the differences (plot b) may be a superset of
   the set of colocated boxes used for the departures (plot a), because we
   did not require that the LTM exist for a given box-month for it to be
   included in the differences (coverage differences may arise, for example,
   if new or untrimmed data fall outside the original Release 1 LTM coverage).
   The set of colocated boxes used for the counts of boxes and observations
   (plots c and d) is also that defined by the differences, not departures.
*** The set of colocated boxes is that defined by the differences, which may
    be a superset of that defined by the departures (see ** footnote).
**** The set of colocated boxes is that defined by the differences, which may
     be a superset of that defined by the departures (see ** footnote).

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