Precipitation frequency and phase

"Present weather" codes that represent part of the synoptic reports in COADS were used by Serreze et al. (1997b) to examine the characteristics of precipitation frequency and phase (solid, liquid, mixed) over the Arctic Ocean. The techniques follow those

Figure 6.6 Modeled annual precipitation averaged for 1985 though 1999 (mm). The contour interval is 200 mm, but 100 mm if smaller than 400 mm, and 300 mm if larger than 1000 mm (from Bromwich et al., 2001b, by permission of AGU).

described by Petty (1995). An application of the present weather codes and other COADS information to define regional Arctic Ocean climates objectively is described by Clark et al. (1996). Figure 6.7 gives a feel for the contrasts between January and July precipitation frequency and phase. These maps bin COADS reports from 1950 to 1999 into a very coarse grid cell array. Over the central Arctic Ocean, the COADS records are primarily from the NP stations. Precipitation frequency represents the percentage of all COADS observations for which any precipitation was observed. Similarly, the frequency of moderate to heavy precipitation is the percentage of all COADS observations when moderate to heavy precipitation was observed. The maps for phase are based on the percentage of all COADS observations of precipitation for which the precipitation was in solid or liquid form (because of mixed precipitation, the two may not sum to 100%). The COADS records do not provide precipitation amounts -only occurrence, relative intensity and phase.

As expected from precipitation totals (Figure 6.3) January precipitation frequency is higher over the Atlantic sector as compared to the central Arctic, i.e., there are more events. There is an opposing pattern in July. Nevertheless, it shows that while total winter precipitation in the central Arctic is quite light, the actual occurrence of precipitation is still common. For January, the contrast between the Atlantic sector and central Arctic Ocean also holds with respect to the frequencies of moderate to heavy precipitation. In July, moderate to heavy precipitation is more evenly distributed across the Arctic Ocean. The annual cycle in precipitation frequency (not shown) only roughly follows that of total precipitation over the central Arctic Ocean.

Over the central Arctic Ocean, nearly all January precipitation is in solid form. On rare occasions, the incursion of a very warm airmass can foster liquid precipitation in parts of this region. The January frequency of solid (liquid) precipitation is sharply lower (higher) over the northern North Atlantic where temperatures are highest. Even in July, about half of the precipitation near the Pole falls in solid form. These snow

Rain Frequency

Figure 6.7 Precipitation frequency, the frequency of moderate to heavy precipitation, solid precipitation frequency and liquid precipitation frequency, based on present weather reports in COADS records over the period 1950-95. Maps are provided for January and July. See text for details (from Serreze et al., 1997b, by permission of NSIDC, Boulder, CO).

Figure 6.7 Precipitation frequency, the frequency of moderate to heavy precipitation, solid precipitation frequency and liquid precipitation frequency, based on present weather reports in COADS records over the period 1950-95. Maps are provided for January and July. See text for details (from Serreze et al., 1997b, by permission of NSIDC, Boulder, CO).

July: precipitation freq. July: freq. moderate to heavy precipitation

July: precipitation freq. July: freq. moderate to heavy precipitation

July: freq. solid precipitation July: freq. liquid precipitation
Figure 6.7 (Cont.)

events help to maintain a high surface albedo in this area. Solid precipitation over the Atlantic sector in July is very rare.

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