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Effects of “latent waste heat” on the atmospheric moisture field
Energy and Buildings, 4 (1982) 129 - 133
129
Effects of "Latent Waste Heat" on the Atmospheric Moisture Field K. HOSCHELE
Meteorologisches Institut Universitlft Karlsruhe (F.R.G.)
Our knowledge o f water vapor or latent heat fluxes and the distribution o f humidity in the urban atmosphere is much less complete than that o f (sensible) heat transport and the characteristics o f the urban temperature field, popularly known as a "heat island". The reasons for this unequal treatment o f interdependent processes may be the differ. ent means o f measurement for the water vapor content o f the air, the influence o f temperature on saturation values, and instrumental problems. The mass transport o f water vapor is accompanicd by a transfer o f latent heat, a considerable part o f the energy budget at the surface o f the earth. Modifications are possible by choosing one o f several categories o f land Use;
-- by changing the energy source, the radiation balance, via reflectivity ; -- by changing the conditions for heat transfer; -- by changing the availability o f water for vaporization; by artificial (anthropogenic) heat sources. O f these procedures, all o f similar relevance to the urban atmosphere, the last is discussed more thoroughly, with examples o f typical investigations and an estimation o f water vapor fluxes over urban settlements. As a result the reduced surface evaporation from built up areas is compensated in many cases by artificial sources o f water vapor, in combination with waste heat release. -
about 1 km and more in the horizontal-, and some 100 m to about 3 km in the vertical direction. This scale is adequate for problems in urban and regional planning. A still higher resolution, reaching into the microscale, is needed for individual building problems. Until the early seventies "the effects of the city on atmospheric moisture was somewhat confused and lacking in research" [2]. Despite new studies with more consistent results there are still many problems in teaching and extending this additional knowledge to the field of application. The same is true for the role of atmospheric moisture in the thermal comfort of humanity under neutral conditions or in situations of heat stress and sultriness. In numerous more or less complex measures for the thermal environment, besides ambient temperature, and in some cases wind and solar radiation, the weight of humidity varies over a very wide range [ 3 ].
-
INTRODUCTION Our interest in inadvertent artifical modifications of moisture content in the atmosphere has its origin in urban climatological studies and in model calculations of human thermal comfort [1]. This study mainly deals with influences in the mesoscale, extending to 0378-7788)8210000-0000/$02.75
MEANS OF MEASUREMENT OF ATMOSPHERIC HUMIDITY The amount of water vapor in the atmosphere is governed by the temperature dependent upper (saturation) limit, E = f(T), of the vapor pressure, e (in mbar), on a global as well as on a mierometeorologlcal scale. Vapor pressure alone is used as a possible measure for sultriness (e = 19 mbar). The actual vapor pressure, e, is normally calculated from dry and wet bulb temperatures. Another, more direct, method is the measurement of dew point temperature (Td), where e = E (Td). Dew point temperatures are an illustrative measure for estimating the onset of condensation and for predicting minimum temperatures near the ground. Presentations of vertical profiles from radiosonde data normally use T - Td, in addition to T, as a measure of © Elsevier Sequoia/Printed in The Netherlands
130
humidity above the ground. A serious problem in building climatology is dew point (or frost point) temperatures reached in walls, with damaging effects from condensing liquid water or ice. Vapor pressure and dew point temperatures are constant properties of a quantity of air only in isobaric processes. This is often neglected in studies under conditions near the ground. For certain engineering purposes, for example, in air conditioning, or in problems dealing with attenuation of electromagnetic radiation, the water vapor density Pw or absolute humidity a (in g/m s) is preferred. For calculation, the vapor pressure e has to be divided by the gas constant of water vapor Rw and the temperature: Pw = a = e/RwT. A constant property of moist air, f o r anisobaric processes too, is the dimensionless mass mixing ratio m = Pw/Pa and the specific humidity S = Pw/(Pa +
Pw),
often simplified to m ~ s ~ 0.622
e/p
where p is the atmospheric pressure. This measure provides an adequate treatment of problems such as cooling tower plumes, w i t h a vertical extension corresponding to 10 - 100 mbar and more deviations from normal pressure. When comparing humidities in settlements at different altitudes, this influence of air pressure on some measurements of moisture must also be borne in mind. With E instead of e the saturation value S o f specific h u m i d i t y can be calculated. The saturation deficit S -- s (better than E -e) controls the velocity of vaporization and condensation. The popular relative humidity U = 100 e/E or 100 s/S, a direct output of simple hair hygrometers, is important as a limit for physical or biological processes proceeding at air temperatures, as in drying or swelling (of wooden material, for instance) or the growth of mould. For homotherm systems this measurement must be combined with the corresponding air temperatures, since it is necessary when assessing the conditions for human thermal comfort.
CHARACTERISTICS OF MOISTURE FIELDS IN THE ATMOSPHERE OVER URBAN AND RURAL AREAS
The so-called "dry urban atmosphere" is largely a consequence of presenting horizontal moisture fields in cities in terms of relative humidity. The extreme differences between U < 60% in the city centre and U > 90% in the surrounding agricultural or forest area, as evening values in mid-latitude summer, are mainly due to heat island effects. With vapor pressure, absolute or specific humidity, night moisture values in the city are even higher than over rural areas. For understanding horizontal humidity patterns or diurnal and annual variations in the atmosphere near the ground some knowledge of typical vertical distributions of water vapor are necessary. As a consequence of limitations by temperature, decreasing with height, specific humidity is reduced to about 50% of the ground values under standard conditions. As specific humidity is a constant property of quantities of air, strong, vertical turbulent mixing will create a layer with a nearly equal distribution of specific humidity with height; possibly influenced by an onset of condensation in the upper part in consequence of the adiabatic temperature lapse rate developing simultaneously. In the lower part near the ground moisture is reduced by this typical daytime process. Under night-time conditions in rural areas a strong temperature inversion may reduce specific humidities near the ground by the condensation of water vapor on cold surfaces. The humidity and temperature profiles in Fig. 1 demonstrate this effect. A strong temperature inversion of about 5 K in the lowest 100 m layer is paralleled by a reduction in humidity when the saturation deficit near the ground approaches zero at 23 CET. We have made these measurements over farmland in the centre of the upper Rhine valley during the MESOKLIP experiment. Simultaneous measurements at some 10 km distance gave similar results. Over urban areas, with higher night temperatures near the ground, the humidity profile is not influenced by this effect. Advection of rural air will, as a consequence of vertical mixing above the city area, result in a more equal vertical humidity distribution:
131 Z IN M.L
. . . . . . . . . .
SrtQX
Z
200
20(
15C
IS(
10(3
tO©
5~
SO
200
284
268 T IN K PHL 1"7.09.1979 18:00
292 MEZ
12 T IN K PHL 1 " / . 0 9 . 1 9 7 9 2 1 ; 0 0
I'IEZ
T IN K PHL 1 " / . 0 9 . 1 9 7 9 2 3 : 0 ]
rtEZ
Fig. 1. Typical vertical prof'fles of temperature T, specific humidity s, and saturation deficit SMAX -- s (hatched field), with reduced humidity near the ground at night. (z, height above ground.)
moisture values near the ground will be increased as compared with rural conditions. INFLUENCE
OF
SURFACE
PROPERTIES
ON
HUMIDITY PATTERNS A well proved approach to atmospheric problems near the ground, under the influence of large energy transformations at the earth's surface, is to make a budget o f incoming (+) and outgoing (--) energy fluxes. For an element of surface area without storage the budget equation is
Q+B+H+E=O where Q is the radiation balance, B the heat flux from/to the ground, and H the sensible and E the latent heat fluxes from/to the atmosphere. Here latent heat flux E, connected with evaporation or condensation o f water, is of primary interest. A change in any term of this budget equation will modify other terms. Mean values of B for time intervals of one day and more become very small and can be neglected in a simplified consideration. In diurnal situations however, different extreme values of B play an important role. Higher values o f B due to underground thermal properties
reduce amplitudes and displace phases of diurnal surface temperature courses, accompanied by similar changes in H and E. With B near zero, sensible and latent heat, H and E (both negative in the daytime), a r e a direct outflow of radiation balance Q. So it is possible to calculate a mean latent heat flux E (or water vapor mass flux)~ assuming a relation with sensible heat flux H, the Bowen ratio b ffi / t / ~ or the modified Bowen ratio [4]
b* =E/(H + E ) = E / Q = i/(1 + b). Different values of the Bowen ratio are typical for climatic regions, on a global or local scale, and possibly for seasons. The annual mean for locations in central Europe, b = 0.5 or b* = 0.67, indicates the dominant role of latent heat release (2/3 of net radiation income). A still more marked predominance in rainy tropics, with b* > 0.8 is contrasted by b* < 0.1 in desert areas. Obviously, the local change of surface properties in urban settlements, compared with a rural environment, will have an influence not only on net radiation b u t also on the Bowen ratio. The common designation of city areas as "rocky deserts" is probably too extreme in this respect.
132
A reduced availability of water, by better drainage, for instance, will depress As, the difference between the specific humidities at the surface and in the ambient air. In a very simplified form, latent heat release is
E=c~XcXAs corresponding with H=~XAT with a constant c and the heat transfer index ~, depending on wind velocity and surface properties such as roughness, characterized by the roughness length Zo. For a certain wind velocity at a height z above ground, an approximate assumption for the increase in heat transfer with roughness is ~ ~ (ln z / z o ) - 2 under neutral stability conditions. As a change of ~ will change both E and H, higher values for ~ will reduce B in the diurnal course. Table 1 lists typical values for some of the properties having an influence on the heat budget for various surface categories. TABLE 1 Surface category
Albedo (%)
Roughness parameter, z o (era)
Availability of water (qualitative 0 - I )
Urban Farmland Forest Sandy desert Water
25 20 15
300 5 150
0.5 • 1 ~ 1
35 10
0.1 0.01
~, 0 1
All these figures, especially for Zo, vary over a wide range in the same category. Availability may be defined as representative of the evaporating part of the area as a whole.
ROLE OF SENSIBLE AND LATENT WASTE HEAT IN THE ENERGY AND WATER BUDGET OF URBAN AREAS
The "natural" energy budget of an urban area, as described above, will be modified to a large extent by so,ailed artificial, anthropogenic or waste heat release. In a broad sense this term comprises heat emissions from all kinds of human activity: industrial, traffic, heating, metabolism. The energy balance
equation must be completed by a further term, B', acting as a kind of additional heat flux from the ground with an annual and diurnal course in the positive range. In consequence, in long term mean values, the positive net radiation Q will suffer a small reduction by enhanced long wave emission, and the amount of (negative) sensible and latent heat flux will grow. The weight of the latent heat component will depend on the role of water vapor emissions connected with these artificial heat releases. An example with an extremely high latent component, from about 60% to more than 90% under hot environmental conditions, is the wet cooling tower process. For large industrialized areas the latent part of the waste heat is near 20 - 25%; in city centres, releasing total waste heat of an order of magnitude comparable with that of net radiation, it is near 10% [5]. An analysis of the water budget of cities in addition to the energy budget will help to reduce uncertainties in estimating latent waste heat flux. In the budget equation, consisting of precipitation P, balanced by evaporation E and net run off R (without a storage term in long time considerations), for areas with an artificial water supply an input term P' will be balanced by enhanced E or R. Part of this "consumption" of supplied water will be contained in the budget of latent waste heat release (wet cooling for instance). Another portion of the water consumption, without any connection with "energy consumption" in an economic sense, will be neglected in a waste heat cadastre, for example, irrigation of park or garden areas in cities. Nevertheless, this additional water flux in a city system will play a powerful role. We could take into account a higher availability of water for evaporation, hence, a reduced Bowen's ratio, and enhanced quasi natural latent heat release. Another reason for the comparatively large amounts of evaporated water, with regard to areas of vegetation in cities, is the "oasis effect", caused by higher transfer indices for small areas. In an area mean, this effect has the same result as a further increase in avail-
ability. For a mid-latitude city such as Karlsn~e, 260000 inhabitants, with industrial parks (oil refineries, power station) Table 2 presents some data of relevance to latent heat release [6,7].
133 TABLE 2
Net radiation (year) (winter/summer) Waste heat (year) (winter/summer) Precipitation Evaporation of environment area Water consumption Public supply Private (refineries)
/
55 5/110
W/m 2 W/m 2
30 40/20
W/m 2 W/m 2
750
mm/a
460 1350 330 1020
A m o u n t of open areas in total contiguous settlement area
mm/a mm/a~ mm/a~ | ram/a)
vatervapar prsmure i/i I/I
/
q l I t l l VII ] I i ,LI/1 i i
referred to settlement area
/I I I,t' /I I IZI /111XI I /Ill/fill, /H J,/i.I II/ /{ I "I-XI '1"11,4 I 111~11tXIII ,,&l ~1 IJ,/l II I I. /¢1LM I IA41 II I~/P 1¢1L,lll I L,,!d I I I L.,?'F I I IA,,'i'G L,V'PI I I I J.,,M I I I 1 T IA-'M I I IA-4"T I I I I I I I. "PI I1~!t II I ~ -I-"M-I I I I ~ IIII .I-.I.--H"M-T IIII Ilrlllllllltllllll
IIII
0
2O
S
t0
I1
,14"1 il li
ii li il li
t l 1 I l
t2
8 i
I
4
II
I[ II il 25
20
[I f
IIII I] !.4 III t1~/111 .IXII I1
IIII
II
30
o 35
/*0
tern#erahire
*C
All 0 AO 6 14 22 CET
Fig. 2. Comparison of atmospheric humidities and temperatures between city centre and rural environment at Karisruhe during clear days in August/ September.
which reduce evaporation in cities are compensated to a large extent.
ACKNOWLEDGEMENT
The MESOKLIP experiment (data contained in Fig. 1) was supported by "Deutsche Forschungsgemeinschaft".
REFERENCES 1 K. H6schele, Die Ermittlung optimaler thermiseher
2 CONCLUSION 3
The direct release o f latent waste heat in cities has some importance in winter. Water supplied from the environment to the city system plays a decisive role as a source for additional latent heat or water vapor release over the year and especially in s u m m e r using part o f the total waste heat emission as an energy source for vaporization, in addition to net radiation. So the well known effects
I i I I i
City centre rural
20%
Based on net radiation + waste heat and a modified Bowen's ratio o f b* = 0.5 (reduced from 0.67 for environment) latent heat release is 42.5 W/m I for 530 mm/a, with 2 0 0 250 mm/a from total water consumption and about 300 mm/a from precipitation. These raw figures contain 3 W / m 2 o r 37 mm/a genuine latent waste heat with 18 mm/a o f water from burning fossil fuels. This estimated latent heat or water vapor release, similar to rural values, fits the experimental results o f humidity measurements in cities. Figure 2 illustrates vapor pressure and temperature data from measurements in Karlsruhe during clear days in August/September. Early afternoon and evening values of vapor pressure are somewhat lower in the city. In the early morning the temperature drop in the rural environment is accompanied by a further reduced vapor pressure.
15
,fJ Ill
tuber
4
5
6 7
Bedingungen in Geb~uden aus einem Model1 des menschlichen W~rmehaushaltes, Teaching the Teachers on Building Climatology, Preprints Tirol. 2, No. 38, T h e National Swedish Institute for Buiding Research, Stockholm 1972. T. R. Oke, Review of Urban Climatology 1973 1976, WMO Tech. Note No. 169, Geneva, 1979. H. E. Landsberg, The Assessment of Human Bioclimate, WMO Tech. Note No. 123, Geneva, 1972. R. Keller, Hydrologischer Atlas der Bundarepublih Deutschland, Boppard, 1979 (Deutsche Forsehungsgemein~haft). G. Bartholom~i and W. Kinzelbach, Das Abw~frmehatoster Oberrheingebiet, KfK 2869, Kernforsehungszentrum, Karlsruhe, 1979. Stadt Karlsruhe, Landsehaftsplan, Mitteilungen des Baudezernats, Kartsruhe, 1975. F. Fiedler, Modifikation der Luftfeuchte in einem Stadtgebiet, Pro Met, 9 (4) (1979) 12 - 16.
129
Effects of "Latent Waste Heat" on the Atmospheric Moisture Field K. HOSCHELE
Meteorologisches Institut Universitlft Karlsruhe (F.R.G.)
Our knowledge o f water vapor or latent heat fluxes and the distribution o f humidity in the urban atmosphere is much less complete than that o f (sensible) heat transport and the characteristics o f the urban temperature field, popularly known as a "heat island". The reasons for this unequal treatment o f interdependent processes may be the differ. ent means o f measurement for the water vapor content o f the air, the influence o f temperature on saturation values, and instrumental problems. The mass transport o f water vapor is accompanicd by a transfer o f latent heat, a considerable part o f the energy budget at the surface o f the earth. Modifications are possible by choosing one o f several categories o f land Use;
-- by changing the energy source, the radiation balance, via reflectivity ; -- by changing the conditions for heat transfer; -- by changing the availability o f water for vaporization; by artificial (anthropogenic) heat sources. O f these procedures, all o f similar relevance to the urban atmosphere, the last is discussed more thoroughly, with examples o f typical investigations and an estimation o f water vapor fluxes over urban settlements. As a result the reduced surface evaporation from built up areas is compensated in many cases by artificial sources o f water vapor, in combination with waste heat release. -
about 1 km and more in the horizontal-, and some 100 m to about 3 km in the vertical direction. This scale is adequate for problems in urban and regional planning. A still higher resolution, reaching into the microscale, is needed for individual building problems. Until the early seventies "the effects of the city on atmospheric moisture was somewhat confused and lacking in research" [2]. Despite new studies with more consistent results there are still many problems in teaching and extending this additional knowledge to the field of application. The same is true for the role of atmospheric moisture in the thermal comfort of humanity under neutral conditions or in situations of heat stress and sultriness. In numerous more or less complex measures for the thermal environment, besides ambient temperature, and in some cases wind and solar radiation, the weight of humidity varies over a very wide range [ 3 ].
-
INTRODUCTION Our interest in inadvertent artifical modifications of moisture content in the atmosphere has its origin in urban climatological studies and in model calculations of human thermal comfort [1]. This study mainly deals with influences in the mesoscale, extending to 0378-7788)8210000-0000/$02.75
MEANS OF MEASUREMENT OF ATMOSPHERIC HUMIDITY The amount of water vapor in the atmosphere is governed by the temperature dependent upper (saturation) limit, E = f(T), of the vapor pressure, e (in mbar), on a global as well as on a mierometeorologlcal scale. Vapor pressure alone is used as a possible measure for sultriness (e = 19 mbar). The actual vapor pressure, e, is normally calculated from dry and wet bulb temperatures. Another, more direct, method is the measurement of dew point temperature (Td), where e = E (Td). Dew point temperatures are an illustrative measure for estimating the onset of condensation and for predicting minimum temperatures near the ground. Presentations of vertical profiles from radiosonde data normally use T - Td, in addition to T, as a measure of © Elsevier Sequoia/Printed in The Netherlands
130
humidity above the ground. A serious problem in building climatology is dew point (or frost point) temperatures reached in walls, with damaging effects from condensing liquid water or ice. Vapor pressure and dew point temperatures are constant properties of a quantity of air only in isobaric processes. This is often neglected in studies under conditions near the ground. For certain engineering purposes, for example, in air conditioning, or in problems dealing with attenuation of electromagnetic radiation, the water vapor density Pw or absolute humidity a (in g/m s) is preferred. For calculation, the vapor pressure e has to be divided by the gas constant of water vapor Rw and the temperature: Pw = a = e/RwT. A constant property of moist air, f o r anisobaric processes too, is the dimensionless mass mixing ratio m = Pw/Pa and the specific humidity S = Pw/(Pa +
Pw),
often simplified to m ~ s ~ 0.622
e/p
where p is the atmospheric pressure. This measure provides an adequate treatment of problems such as cooling tower plumes, w i t h a vertical extension corresponding to 10 - 100 mbar and more deviations from normal pressure. When comparing humidities in settlements at different altitudes, this influence of air pressure on some measurements of moisture must also be borne in mind. With E instead of e the saturation value S o f specific h u m i d i t y can be calculated. The saturation deficit S -- s (better than E -e) controls the velocity of vaporization and condensation. The popular relative humidity U = 100 e/E or 100 s/S, a direct output of simple hair hygrometers, is important as a limit for physical or biological processes proceeding at air temperatures, as in drying or swelling (of wooden material, for instance) or the growth of mould. For homotherm systems this measurement must be combined with the corresponding air temperatures, since it is necessary when assessing the conditions for human thermal comfort.
CHARACTERISTICS OF MOISTURE FIELDS IN THE ATMOSPHERE OVER URBAN AND RURAL AREAS
The so-called "dry urban atmosphere" is largely a consequence of presenting horizontal moisture fields in cities in terms of relative humidity. The extreme differences between U < 60% in the city centre and U > 90% in the surrounding agricultural or forest area, as evening values in mid-latitude summer, are mainly due to heat island effects. With vapor pressure, absolute or specific humidity, night moisture values in the city are even higher than over rural areas. For understanding horizontal humidity patterns or diurnal and annual variations in the atmosphere near the ground some knowledge of typical vertical distributions of water vapor are necessary. As a consequence of limitations by temperature, decreasing with height, specific humidity is reduced to about 50% of the ground values under standard conditions. As specific humidity is a constant property of quantities of air, strong, vertical turbulent mixing will create a layer with a nearly equal distribution of specific humidity with height; possibly influenced by an onset of condensation in the upper part in consequence of the adiabatic temperature lapse rate developing simultaneously. In the lower part near the ground moisture is reduced by this typical daytime process. Under night-time conditions in rural areas a strong temperature inversion may reduce specific humidities near the ground by the condensation of water vapor on cold surfaces. The humidity and temperature profiles in Fig. 1 demonstrate this effect. A strong temperature inversion of about 5 K in the lowest 100 m layer is paralleled by a reduction in humidity when the saturation deficit near the ground approaches zero at 23 CET. We have made these measurements over farmland in the centre of the upper Rhine valley during the MESOKLIP experiment. Simultaneous measurements at some 10 km distance gave similar results. Over urban areas, with higher night temperatures near the ground, the humidity profile is not influenced by this effect. Advection of rural air will, as a consequence of vertical mixing above the city area, result in a more equal vertical humidity distribution:
131 Z IN M.L
. . . . . . . . . .
SrtQX
Z
200
20(
15C
IS(
10(3
tO©
5~
SO
200
284
268 T IN K PHL 1"7.09.1979 18:00
292 MEZ
12 T IN K PHL 1 " / . 0 9 . 1 9 7 9 2 1 ; 0 0
I'IEZ
T IN K PHL 1 " / . 0 9 . 1 9 7 9 2 3 : 0 ]
rtEZ
Fig. 1. Typical vertical prof'fles of temperature T, specific humidity s, and saturation deficit SMAX -- s (hatched field), with reduced humidity near the ground at night. (z, height above ground.)
moisture values near the ground will be increased as compared with rural conditions. INFLUENCE
OF
SURFACE
PROPERTIES
ON
HUMIDITY PATTERNS A well proved approach to atmospheric problems near the ground, under the influence of large energy transformations at the earth's surface, is to make a budget o f incoming (+) and outgoing (--) energy fluxes. For an element of surface area without storage the budget equation is
Q+B+H+E=O where Q is the radiation balance, B the heat flux from/to the ground, and H the sensible and E the latent heat fluxes from/to the atmosphere. Here latent heat flux E, connected with evaporation or condensation o f water, is of primary interest. A change in any term of this budget equation will modify other terms. Mean values of B for time intervals of one day and more become very small and can be neglected in a simplified consideration. In diurnal situations however, different extreme values of B play an important role. Higher values o f B due to underground thermal properties
reduce amplitudes and displace phases of diurnal surface temperature courses, accompanied by similar changes in H and E. With B near zero, sensible and latent heat, H and E (both negative in the daytime), a r e a direct outflow of radiation balance Q. So it is possible to calculate a mean latent heat flux E (or water vapor mass flux)~ assuming a relation with sensible heat flux H, the Bowen ratio b ffi / t / ~ or the modified Bowen ratio [4]
b* =E/(H + E ) = E / Q = i/(1 + b). Different values of the Bowen ratio are typical for climatic regions, on a global or local scale, and possibly for seasons. The annual mean for locations in central Europe, b = 0.5 or b* = 0.67, indicates the dominant role of latent heat release (2/3 of net radiation income). A still more marked predominance in rainy tropics, with b* > 0.8 is contrasted by b* < 0.1 in desert areas. Obviously, the local change of surface properties in urban settlements, compared with a rural environment, will have an influence not only on net radiation b u t also on the Bowen ratio. The common designation of city areas as "rocky deserts" is probably too extreme in this respect.
132
A reduced availability of water, by better drainage, for instance, will depress As, the difference between the specific humidities at the surface and in the ambient air. In a very simplified form, latent heat release is
E=c~XcXAs corresponding with H=~XAT with a constant c and the heat transfer index ~, depending on wind velocity and surface properties such as roughness, characterized by the roughness length Zo. For a certain wind velocity at a height z above ground, an approximate assumption for the increase in heat transfer with roughness is ~ ~ (ln z / z o ) - 2 under neutral stability conditions. As a change of ~ will change both E and H, higher values for ~ will reduce B in the diurnal course. Table 1 lists typical values for some of the properties having an influence on the heat budget for various surface categories. TABLE 1 Surface category
Albedo (%)
Roughness parameter, z o (era)
Availability of water (qualitative 0 - I )
Urban Farmland Forest Sandy desert Water
25 20 15
300 5 150
0.5 • 1 ~ 1
35 10
0.1 0.01
~, 0 1
All these figures, especially for Zo, vary over a wide range in the same category. Availability may be defined as representative of the evaporating part of the area as a whole.
ROLE OF SENSIBLE AND LATENT WASTE HEAT IN THE ENERGY AND WATER BUDGET OF URBAN AREAS
The "natural" energy budget of an urban area, as described above, will be modified to a large extent by so,ailed artificial, anthropogenic or waste heat release. In a broad sense this term comprises heat emissions from all kinds of human activity: industrial, traffic, heating, metabolism. The energy balance
equation must be completed by a further term, B', acting as a kind of additional heat flux from the ground with an annual and diurnal course in the positive range. In consequence, in long term mean values, the positive net radiation Q will suffer a small reduction by enhanced long wave emission, and the amount of (negative) sensible and latent heat flux will grow. The weight of the latent heat component will depend on the role of water vapor emissions connected with these artificial heat releases. An example with an extremely high latent component, from about 60% to more than 90% under hot environmental conditions, is the wet cooling tower process. For large industrialized areas the latent part of the waste heat is near 20 - 25%; in city centres, releasing total waste heat of an order of magnitude comparable with that of net radiation, it is near 10% [5]. An analysis of the water budget of cities in addition to the energy budget will help to reduce uncertainties in estimating latent waste heat flux. In the budget equation, consisting of precipitation P, balanced by evaporation E and net run off R (without a storage term in long time considerations), for areas with an artificial water supply an input term P' will be balanced by enhanced E or R. Part of this "consumption" of supplied water will be contained in the budget of latent waste heat release (wet cooling for instance). Another portion of the water consumption, without any connection with "energy consumption" in an economic sense, will be neglected in a waste heat cadastre, for example, irrigation of park or garden areas in cities. Nevertheless, this additional water flux in a city system will play a powerful role. We could take into account a higher availability of water for evaporation, hence, a reduced Bowen's ratio, and enhanced quasi natural latent heat release. Another reason for the comparatively large amounts of evaporated water, with regard to areas of vegetation in cities, is the "oasis effect", caused by higher transfer indices for small areas. In an area mean, this effect has the same result as a further increase in avail-
ability. For a mid-latitude city such as Karlsn~e, 260000 inhabitants, with industrial parks (oil refineries, power station) Table 2 presents some data of relevance to latent heat release [6,7].
133 TABLE 2
Net radiation (year) (winter/summer) Waste heat (year) (winter/summer) Precipitation Evaporation of environment area Water consumption Public supply Private (refineries)
/
55 5/110
W/m 2 W/m 2
30 40/20
W/m 2 W/m 2
750
mm/a
460 1350 330 1020
A m o u n t of open areas in total contiguous settlement area
mm/a mm/a~ mm/a~ | ram/a)
vatervapar prsmure i/i I/I
/
q l I t l l VII ] I i ,LI/1 i i
referred to settlement area
/I I I,t' /I I IZI /111XI I /Ill/fill, /H J,/i.I II/ /{ I "I-XI '1"11,4 I 111~11tXIII ,,&l ~1 IJ,/l II I I. /¢1LM I IA41 II I~/P 1¢1L,lll I L,,!d I I I L.,?'F I I IA,,'i'G L,V'PI I I I J.,,M I I I 1 T IA-'M I I IA-4"T I I I I I I I. "PI I1~!t II I ~ -I-"M-I I I I ~ IIII .I-.I.--H"M-T IIII Ilrlllllllltllllll
IIII
0
2O
S
t0
I1
,14"1 il li
ii li il li
t l 1 I l
t2
8 i
I
4
II
I[ II il 25
20
[I f
IIII I] !.4 III t1~/111 .IXII I1
IIII
II
30
o 35
/*0
tern#erahire
*C
All 0 AO 6 14 22 CET
Fig. 2. Comparison of atmospheric humidities and temperatures between city centre and rural environment at Karisruhe during clear days in August/ September.
which reduce evaporation in cities are compensated to a large extent.
ACKNOWLEDGEMENT
The MESOKLIP experiment (data contained in Fig. 1) was supported by "Deutsche Forschungsgemeinschaft".
REFERENCES 1 K. H6schele, Die Ermittlung optimaler thermiseher
2 CONCLUSION 3
The direct release o f latent waste heat in cities has some importance in winter. Water supplied from the environment to the city system plays a decisive role as a source for additional latent heat or water vapor release over the year and especially in s u m m e r using part o f the total waste heat emission as an energy source for vaporization, in addition to net radiation. So the well known effects
I i I I i
City centre rural
20%
Based on net radiation + waste heat and a modified Bowen's ratio o f b* = 0.5 (reduced from 0.67 for environment) latent heat release is 42.5 W/m I for 530 mm/a, with 2 0 0 250 mm/a from total water consumption and about 300 mm/a from precipitation. These raw figures contain 3 W / m 2 o r 37 mm/a genuine latent waste heat with 18 mm/a o f water from burning fossil fuels. This estimated latent heat or water vapor release, similar to rural values, fits the experimental results o f humidity measurements in cities. Figure 2 illustrates vapor pressure and temperature data from measurements in Karlsruhe during clear days in August/September. Early afternoon and evening values of vapor pressure are somewhat lower in the city. In the early morning the temperature drop in the rural environment is accompanied by a further reduced vapor pressure.
15
,fJ Ill
tuber
4
5
6 7
Bedingungen in Geb~uden aus einem Model1 des menschlichen W~rmehaushaltes, Teaching the Teachers on Building Climatology, Preprints Tirol. 2, No. 38, T h e National Swedish Institute for Buiding Research, Stockholm 1972. T. R. Oke, Review of Urban Climatology 1973 1976, WMO Tech. Note No. 169, Geneva, 1979. H. E. Landsberg, The Assessment of Human Bioclimate, WMO Tech. Note No. 123, Geneva, 1972. R. Keller, Hydrologischer Atlas der Bundarepublih Deutschland, Boppard, 1979 (Deutsche Forsehungsgemein~haft). G. Bartholom~i and W. Kinzelbach, Das Abw~frmehatoster Oberrheingebiet, KfK 2869, Kernforsehungszentrum, Karlsruhe, 1979. Stadt Karlsruhe, Landsehaftsplan, Mitteilungen des Baudezernats, Kartsruhe, 1975. F. Fiedler, Modifikation der Luftfeuchte in einem Stadtgebiet, Pro Met, 9 (4) (1979) 12 - 16.