Page 143 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 4 Atmosphere and Surface Energy Balances 107
NET R
H
High sensible heat
G
LE heat
◀Figure 4.16 Radiation budgets for two stations. H = turbulent sensible heat transfer; LE = latent heat of evaporation; G = ground heating and cooling. [(a) Based on W. D. Sellers, Physical Climatology, © 1965 University of Chicago. (b) Based on T. R. Oke, Boundary Layer Climates, © 1978 Methuen & Co. Photos by Bobbé Christopherson.]
soil to high temperatures. Over a 24-hour period, H is 90% of NET R expenditure; the remain- ing 10% is for ground heating (G). The G component is greatest in the morning, when winds are light and turbulent transfers are lowest. In the afternoon, heated air rises off the hot ground, and convective heat expenditures are accelerated as winds increase.
Compare graphs for El Mirage (Figure 4.16a) and Pitt Meadows (Figure 4.16b). Pitt Meadows is midlatitude (49° N), vegetated, and moist, and its energy expenditures differ greatly from those at El Mirage. The Pitt Meadows landscape is able to retain much more of its energy because of lower albedo
values (less reflection), the presence of more water and plants, and lower surface temperatures than those of El Mirage.
The energy-balance data for Pitt Meadows are from a cloudless summer day. Higher LE values result from the moist environment of rye grass and irrigated mixed orchards, contributing to the more moderate H levels during the day.
The Urban Environment
Urban microclimates generally differ from those of nearby nonurban areas, with urban areas regularly reach- ing temperatures as much as 6 C° hotter than surround- ing suburban and rural areas. In fact, the surface energy characteristics of urban areas are similar to those of des- ert locations, mainly because vegetation is lacking in both environments.
The physical characteristics of urbanized regions produce an urban heat island (UHI) that has, on average, both maximum and minimum temperatures higher than nearby rural settings (Figure 4.17). A UHI has increas- ing temperatures toward the downtown central business district and lower temperatures over areas of trees and parks. Sensible heat is less in urban forests than in other parts of the city because of shading from tree canopies and due to plant processes such as transpiration that
Mojave desert landscape near El Mirage, CA
525 350 175
0 –175
Midnight
(a) Daily net radiation budget at El Mirage, California, near 35°N latitude.
Midnight
4
A.M. A.M.
8
Noon 4
Time P.M. P.M.
8
Irrigated blueberry orchards
near Pitt Meadows, British Columbia
525 350 175
0
–175 Midnight
4 8 Noon 4 8 A.M. A.M. Time P.M. P.M.
NET R
heat
High latent LE
Low sensible heat
H G
Midnight
(b) Daily net radiation budget for Pitt Meadows, British Columbia, near 49°N latitude.
net radiation is able to do the “work” that ultimately produces the global climate system—work such as rais- ing temperatures in the boundary layer, melting ice, or evaporating water from the oceans.
Two Sample Stations Variation in the expenditure of NET R among the processes just described produces the variety of environments we experience in nature. Let us examine the daily radiation budget at two locations, El Mirage in California and Pitt Meadows in British Columbia (Figure 4.16).
El Mirage, at 35° N, is a hot desert location character- ised by bare, dry soil with sparse vegetation. The data in the graph is from a clear summer day, with a light wind in the late afternoon. The NET R value is lower than might be expected, considering the Sun’s position close to zenith (June solstice) and the absence of clouds. But the income of energy at this site is countered by surfaces with relatively high albedo and by hot soil at the ground surface that radiates longwave energy back to the atmo- sphere throughout the afternoon.
El Mirage has little or no energy expenditure through evaporation (LE). With little water and sparse vegetation, most of the available radiant energy dissipates through turbulent transfer of sensible heat (H), warming air and
Low latent
Energy rate (W·m–2) Energy rate (W·m–2)
Sunrise
Sunset
Sunrise
Sunset