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Figure 4.11 World record rainfalls (mm) with an envelope line prior to 1967. The equation of the line is given and the state or country where important records were established.

Source: Modified and updated after Rodda (1970).

Figure 4.12 Maximum expected precipitation (mm) for storms of one-hour and twenty-four-hour duration occurring once in ten years and once in 100 years over the continental United States, calculated from records prior to 1961.

Source: US National Weather Service, courtesy NOAA.

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200 500

Area (km2)

1000 2000

5000 10,000

Figure 4.13 The relationship between area (km2) and frequency of occurrence, during a five-year period, of rainstorms that produce (A) 25-year and (B) 100-year or heavier rain amounts for six- to twelve-hour periods over 50 per cent or more of each area in Illinois.

Source: Chagnon (2002), by permission of the American Meteorological Society.

200 500

Area (km2)

1000 2000

5000 10,000

, Baguio City, Philippines

Lagos, Nigeria

Blaenau Ffestiniog, U.K.

Cleveland, Ohio Shoeburyness, U.K. H5 Jordan

1.001 1.1 1.5 2 5 10 20 50 100 200 300 1,000 Return Period (Years)

Figure 4.14 Rainfall/duration/ frequency plots for daily maximum rainfalls in respect of a range of stations from the Jordan desert to an elevation of 1482 m in the monsoonal Philippines.

Source: After Rodda (1970); Linsley et al. (1992); Ayoade (1976).

southwest England is shown in Figure 4.15. The twenty-four-hour storm had an estimated 150 to 200-year return period. By comparison, tropical rainstorms have much higher intensities and shorter recurrence intervals for comparable totals.

3 The world pattern of precipitation

Globally, 79 per cent of total precipitation falls on the oceans and 21 per cent on land (Figure 4.1). A glance at the maps of precipitation amount for December to February and June to August (Figure 4.16) indicates that the distributions are considerably more complex than those, for example, of mean temperature (see Figure 3.11). Comparison of Figure 4.17 with the meridional profile of average precipitation for each latitude (Figure 4.18) brings out the marked longitudinal variations that are superimposed on the zonal pattern. The zonal pattern has several significant features:

1 The 'equatorial' maximum, which is displaced into the northern hemisphere. This is related primarily to the converging trade wind systems and monsoon regimes of the summer hemisphere, particularly in South Asia and West Africa. Annual totals over large areas are of the order of 2000 to 2500 mm or more.

2 The west coast maxima of mid-latitudes associated with the storm tracks in the westerlies. The precipitation in these areas has a high degree of reliability.

3 The dry areas of the subtropical high-pressure cells, which include not only many of the world's major deserts but also vast oceanic expanses. In the northern hemisphere, the remoteness of the continental interiors extends these dry conditions into mid-latitudes. In addition to very low average annual totals (less than 150 mm), these regions have considerable year-to-year variability.

4 Low precipitation in high latitudes and in winter over the continental interiors of the northern hemisphere. This reflects the low vapour content of the extremely cold air. Most of this precipitation occurs in solid form.

Figure 4.17 demonstrates why the subtropics do not appear as particularly dry on the meridional profile in spite of the known aridity of the subtropical high-pressure areas (see Chapter 10). In these latitudes, the eastern sides of the continents receive considerable rainfall in summer.

Figure 4.15 Distribution of rainfall (mm) over Exmoor, southwest England, during a twenty-four-hour period on 15 August 1952 which produced catastrophic local flooding at Lynmouth. The catchment is marked (by dashes). Seventy-five per cent of the rain fell in just seven hours.

Source: Dobbie and Wolf (1953).

In view of the complex controls involved, no brief explanation of these precipitation distributions can be very satisfactory. Various aspects of selected precipitation regimes are examined in Chapters 10 and 11, after consideration of the fundamental ideas about atmospheric motion and weather disturbances. Here we simply point out four factors that have to be taken into account in studying Figures 4.16 and 4.17:

1 The limit imposed on the maximum moisture content of the atmosphere by air temperature. This is important in high latitudes and in winter in continental interiors.

2 The major latitudinal zones of moisture influx due to atmospheric advection. This in itself is a reflection of the global wind systems and their disturbances (i.e. the converging trade wind systems and the cyclonic westerlies, in particular).

3 The distribution of the landmasses. The southern hemisphere lacks the vast, arid, mid-latitude continental interiors of the northern hemisphere. The oceanic expanses of the southern hemisphere allow the mid-latitude storms to increase the zonal precipitation average for 45°S by about one-third compared with that of the northern hemisphere for 50°N. Longitudinal irregularities are also created by the monsoon regimes, especially in Asia. 4 The orientation of mountain ranges with respect to the prevailing winds.

4 Regional variations in the altitudinal maximum of precipitation

The increase of mean precipitation with height on mountain slopes is a widespread characteristic in mid-latitudes, where the vertical increase in wind speed augments the moisture flux. An increase may be observed up to at least 3000 to 4000 m in the Rocky Mountains in Colorado and in the Alps. In western Britain, with mountains of about 1000 m, the maximum falls are recorded to leeward of the summits. This probably reflects the general tendency of air to continue rising for a while after it has crossed the crestline and the time lag involved in the precipitation process after condensation. Over narrow uplands, the horizontal distance may allow insufficient time for maximum cloud buildup

Figure 4.16 Mean global precipitation (mm per day) for the periods December to February and June to August. Source: From Legates (1995), copyright © John Wiley & Sons Ltd. Reproduced with permission.

Figure 4.16 Mean global precipitation (mm per day) for the periods December to February and June to August. Source: From Legates (1995), copyright © John Wiley & Sons Ltd. Reproduced with permission.

Figure 4.17 Mean precipitation (mm/yr) over (A) the oceans, (B) the land and (C) globally for December to February, June to August and annually.

Source: Peixoto and Oort (1983). From Variations in the Global Water Budget, ed. A. Street-Perrott, M. Beran and R. Ratciffe (1983), Fig. 23. Copyright © D. Reidel, Dordrecht, by kind permission of Kluwer Academic Publishers.

and the occurrence of precipitation. However, a further factor may be the effect of eddies, set up in the airflow by the mountains, on the catch of rain gauges. Studies in Bavaria at the Hohenpeissenberg Observatory show that standard rain gauges may overestimate amounts by about 10 per cent on the lee slopes and underestimate them by 14 per cent on the windward slopes.

In the tropics and subtropics, maximum precipitation occurs below the higher mountain summits, from which level it decreases upward towards the crest.

Observations are generally sparse in the tropics, but numerous records from Java show that the average elevation of greatest precipitation is approximately 1200 m. Above about 2000 m, the decrease in amounts becomes quite marked. Similar features are reported from Hawaii and, at a rather higher elevation, on mountains in East Africa (see Chapter 11H.2). Figure 4.18A shows that, despite the wide range of records for individual stations, this effect is clearly apparent along the Pacific flank of the Guatemalan highlands. Further north along the coast, the occurrence of a precipitation maximum below the mountain crest is observed in the Sierra Nevada, despite some complication introduced by the shielding effect of the Coast Ranges (Figure 4.18B), but in the Olympic Mountains of Washington precipitation increases right up to the summits. Precipitation gauges on mountain crests may underestimate the actual precipitation due to the effect of eddies, and this is particularly true where much of the precipitation falls in the form of snow, which is very susceptible to blowing by the wind.

One explanation of the orographic difference between tropical and temperate rainfall is based on the concentration of moisture in a fairly shallow layer of air near the surface in the tropics (see Chapter 11). Much of the orographic precipitation seems to derive from warm clouds (particularly cumulus congestus), composed of water droplets, which commonly have an upper limit at about 3000 m. It is probable that the height of the maximum precipitation zone is close to the mean cloud base, since the maximum size and number of falling drops will occur at that level. Thus, stations located above the level of mean cloud base will receive only a proportion of the orographic increment. In temperate latitudes, much of the precipitation, especially in winter, falls from stratiform cloud, which commonly extends through a considerable depth of the troposphere. In this case, there tends to be a smaller fraction of the total cloud depth below the station level. These differences according to cloud type and depth are apparent even on a day-to-day basis in mid-latitudes. Seasonal variations in the altitude of the mean condensation level and zone of maximum precipitation are similarly observed. In the Pamir and Tien Shan of Central Asia. for instance, the maximum is reported to occur at about 1500 m in winter and at 3000 m or more in summer. A further difference between orographic effects on precipitation in the tropics and the mid-latitudes relates to the high instability of many tropical airmasses. Where mountains obstruct the flow of moist tropical airmasses, the upwind turbulence may be sufficient to trigger convection, producing a rainfall maximum at low elevations. This is illustrated in Figure 4.19A for Papua New Guinea, where there is a seasonally alternating

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Renewable Energy 101

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