C Precipitation and the moisture balance

Longitudinal influences are apparent in the distribution of annual precipitation, although this is in large measure a reflection of the topography. The 600-mm annual isohyet in the United States approximately follows the 100°W meridian (Figure 10.19), and westward to the Rockies is an extensive dry belt in the rain shadow of the western mountain ranges. In the southeast, totals exceed 1250 mm, and 1000 mm or more is received along the Atlantic coast as far north as New Brunswick and Newfoundland.

The major sources of moisture for precipitation over North America are the Pacific Ocean and the Gulf of Mexico. The former need not concern us here, since comparatively little of the precipitation falling over the interior appears to be derived from that source. The Gulf source is extremely important in providing moisture for precipitation over central and eastern North America, but the predominance of southwesterly airflow means that little precipitation falls over the western Great Plains (see Figure 10.19). Over the southern United States, there is considerable evapotranspiration and this helps to maintain moderate annual totals northward and eastward from the Gulf by providing additional water vapour for the atmosphere. Along the east coast, the Atlantic Ocean is an additional significant source of moisture for winter precipitation.

There are at least eight major types of seasonal precipitation regime in North America (Figure 10.20); the winter maximum of the west coast and the transition type of the intermontane region in mid-latitudes have already been mentioned; the subtropical types are discussed in the next section. Four primarily mid-latitude regimes are distinguished east of the Rocky Mountains:

1 A warm season maximum is found over much of the continental interior (e.g. Rapid City). In an extensive belt from New Mexico to the prairie provinces more than 40 per cent of the annual precipitation falls in summer. In New Mexico, the rain occurs mainly with late summer thunderstorms, but May to June is the wettest time over the central and northern Great Plains due to more frequent cyclonic activity. Winters are quite dry over the plains, but the mechanism of the occasional heavy snowfalls is of interest. They occur over the northwestern plains during easterly upslope flow, usually in a ridge of high pressure. Further north in Canada, the maximum is commonly

Figure 10.19 Mean annual precipitation (mm) over North America determined on a 25-km grid as a function of location and elevation. Based on data from 8000 weather stations for 1951 to 1980. Values in the Arctic underestimate the true totals by 30 to 50 per cent due to problems in recording snowfall accurately with precipitation gauges.

Figure 10.19 Mean annual precipitation (mm) over North America determined on a 25-km grid as a function of location and elevation. Based on data from 8000 weather stations for 1951 to 1980. Values in the Arctic underestimate the true totals by 30 to 50 per cent due to problems in recording snowfall accurately with precipitation gauges.

Source: From Thompson et al. (1999). Courtesy of the US Geological Survey.

Figure 10.20 North American rainfall regime regions and histograms showing mean monthly precipitations for each region (January, June and December are indicated). Note that the jet stream is anchored by the Rockies in more or less the same position at all seasons.

Source: Mostly after Trewartha (1981); additions by Henderson-Sellers and Robinson (1986). Copyright © 1961. Reproduced by permission of The Wisconsin Press.

in late summer or autumn, when depression tracks are in higher mid-latitudes. There is a local maximum in autumn on the eastern shores of Hudson Bay (e.g. Inukjuak) due to the effect of open water.

2 Eastward and southward of the first zone there is a double maximum in May and September. In the upper Mississippi region (e.g. Columbia), there is a secondary minimum, paradoxically in July to August when the air is especially warm and moist, and a similar profile occurs in northern Texas (e.g. Abilene). An upper-level ridge of high pressure over the Mississippi valley seems to be responsible for reduced thunderstorm rainfall in midsummer, and a tongue of subsiding dry air extends southward from this ridge towards Texas. However, during the period June to August 1993 massive flooding occurred in the Midwestern parts of the Mississippi and Missouri rivers as the result of up to twice the January to July average precipitation being received, with many point rainfall totals exceeding amounts appropriate for recurrence intervals over 100 years (Figure 10.21). The three summer months saw excesses of 500 mm above the average rainfall with totals of 90 cm or more. Strong, moist southwesterly airflow recurred throughout the summer with a quasi-stationary cold front oriented from southwest to northeast across the region. The flooding resulted in forty-eight deaths, destroyed 50,000 homes and caused damage losses of $10 billion. In September, renewed cyclonic activity associated with the seasonal southward shift of the polar front, at a time when mT air from the Gulf is still warm and moist, typically causes a resumption of rainfall. Later in the year drier westerly airstreams affect the continental interior as the general airflow becomes more zonal.

The diurnal occurrence of precipitation in the central United States is rather unusual for a continental interior. Sixty per cent or more of the summer

Figure 10.21 Distribution of flooding streams and inundation in the US Midwest during the period June to August 1993. Peak discharges for the Mississippi River at Keokuk, Iowa (K) and the Missouri River at Booneville, Missouri (B) are shown, together with the historic annual peak discharge record. The isopleths indicate the multiples of the thirty-year average January to July precipitation that fell in the first seven months of 1993, and the symbols the estimated recurrence intervals (R.I. years) for point rainfall amounts received during June to July 1993.

Figure 10.21 Distribution of flooding streams and inundation in the US Midwest during the period June to August 1993. Peak discharges for the Mississippi River at Keokuk, Iowa (K) and the Missouri River at Booneville, Missouri (B) are shown, together with the historic annual peak discharge record. The isopleths indicate the multiples of the thirty-year average January to July precipitation that fell in the first seven months of 1993, and the symbols the estimated recurrence intervals (R.I. years) for point rainfall amounts received during June to July 1993.

Sources: Parrett et al. (1993) and Lott (1994). Courtesy of the US Geological Survey.

precipitation falls during nocturnal thunderstorms (20:00 to 08:00 True Solar Time) in central Kansas, parts of Nebraska, Oklahoma and Texas. Hypotheses suggest that the nocturnal thunderstorm rainfall that occurs, especially with extensive mesoscale convec-tive systems (see p. 203), may be linked to a tendency for nocturnal convergence and rising air over the plains east of the Rocky Mountains. The terrain profile appears to play a role here, as a large-scale inversion layer forms at night over the mountains, setting up a low-level jet east of the mountains just above the boundary layer. This southerly flow, at 500 to 1000 m above the surface, can supply the necessary low-level moisture influx and convergence for the storms (cf. Figure 9.33). MCSs account for 30

to 70 per cent of the May to September rainfall over much of the area east of the Rocky Mountains to the Missouri River.

3 East of the upper Mississippi, in the Ohio valley and south of the lower Great Lakes, there is a transitional regime between that of the interior and the east coast type. Precipitation is reasonably abundant in all seasons, but the summer maximum is still in evidence (e.g. Dayton).

4 In eastern North America (New England, the Maritimes, Quebec and southeast Ontario), precipitation is distributed fairly evenly throughout the year (e.g. Blue Hill). In Nova Scotia and locally around Georgian Bay there is a winter maximum, due in the latter case to the influence of open water.

In the Maritimes it is related to winter (and also autumn) storm tracks.

It is worth comparing the eastern regime with the summer maximum that is found over East Asia, where the Siberian anticyclone excludes cyclonic precipitation in winter and monsoonal influences are felt in the summer months.

The seasonal distribution of precipitation is of vital interest for agricultural purposes. Rain falling in summer, for instance, when evaporation losses are high, is less effective than an equal amount in the cool season. Figure 10.22 illustrates the effect of different regimes in terms of the moisture balance, calculated according to Thornthwaite's method (see Appendix 1B). At Halifax (Nova Scotia), sufficient moisture is stored in the soil to maintain evaporation at its maximum rate (i.e. actual evaporation = potential evaporation), whereas at Berkeley (California) there is a computed moisture deficit of nearly 50 mm in August. This is a guide to the amount of irrigation water that may be required by crops, although in dry regimes the Thornthwaite method generally underestimates the real moisture deficit.

Figure 10.23 shows the ratio of actual to potential evaporation (AE/PE) for North America calculated by the methods of Thornthwaite and Mather from an equation relating PE to air temperature. It is drawn to highlight varaition in the dry regions of the country. The boundary separating the moist climates of the east, where the ratio AE/PE exceeds about 8 per cent or more, from the dry climates of the west (excluding the west coast), follows the 95th meridian. The major humid areas are along the Appalachians, in the northeast and along the Pacific coast, while the most extensive arid areas are in the intermontane basins, the High Plains, the southwest and parts of northern Mexico. In the west and southwest the ratio is small due to lack of precipitation, whereas in northwest Canada actual evaporation is limited by available energy.

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