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ICOADS Web information page (Tuesday, 07-Jan-2014 23:58:38 UTC): ## Detailed Explanation of 4-panel Plots: Near-global (62°N-62°S) Time-series Comparisons between Two Datasets using Colocated 2° boxesThese 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. ---------- * 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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