ADVECTION AND THE SURFACE ENERGY BALANCE ACROSS AN IRRIGATED URBAN PARK

Transcription

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 20: (2000) ADVECTION AND THE SURFACE ENERGY BALANCE ACROSS AN IRRIGATED URBAN PARK RACHEL A. SPRONKEN-SMITH a, *, TIMOTHY R. OKE b and WILLIAM P. LOWRY c,1 a Department of Geography, Uni ersity of Canterbury, Christchurch, New Zealand b Department of Geography, Uni ersity of British Columbia, Vancou er, BC, Canada c Emeritus, Departments of Ecology and Geography, Uni ersity of Illinois, Urbana, IL, USA Recei ed 2 March 1999 Re ised 21 September 1999 Accepted 21 September 1999 ABSTRACT The surface energy balance in an irrigated urban park in suburban Sacramento, CA is observed. Three sites extend from the edge of the park to its centre, along a transect which is aligned with the prevailing wind. Direct measurements of the fluxes of net radiation, soil heat flux and evaporation are made at each site and the convective sensible heat is found by residual. Strong advective effects on evaporation are observed, especially in the afternoon and evening. The driving forces for this are the differences in surface and air temperature, and humidity, between the cool, wet park and its warmer, drier built-up surroundings. The control of the surroundings on park evaporation is demonstrated by comparing values with those from synchronous observations in the surrounding suburbs and at an irrigated sod farm just outside the city. Greatest evaporative enhancement is observed at the upwind edge. Throughout the afternoon evaporation considerably exceeds the net radiation. This is interpreted to be due to the microscale leading-edge effect which appears to be restricted to a fetch of about 20 m. Further into the park evaporation also exceeds the net radiation in the afternoon due to the oasis effect. At all sites the sensible heat flux density in the afternoon is negative. Daily and daytime total evaporation from the park is more than 300% that from the integrated suburban area, and more than 130% that from the irrigated rural grass site. The unlimited water supply and the high temperatures of the park allow it to behave like a wet leaf in that its surface temperature seems to be thermostatically controlled it never rises more than a few degrees above that of the park air and for much of the day is cooler than the park air. Copyright 2000 Royal Meteorological Society. KEY WORDS: surface energy balance; advection; evaporation; urban parks 1. INTRODUCTION The ameliorating thermal effect of parks can be substantial in hot, dry climates (Spronken-Smith and Oke, 1998) so they can be a powerful tool in climate-sensitive urban design. To intelligently manage the thermal microclimates of urban parks it is first necessary to understand their surface energy balance (SEB). Hence, the primary objective in this paper is to elucidate the surface energetics of an irrigated urban park in a hot dry climate. This is a subject which has received almost no attention in the past. A priori it is to be expected that such a park will experience advective effects, because it is an isolated cool and humid patch in an otherwise extensive warm and dry environment. Thus, we anticipate that the daytime radiative input to the park is supplemented by a downward flux of sensible heat from the warmer air above. This process of heat removal should cool the air traversing the park and hence the urban area lying downstream from it. These expectations underlie the two secondary objectives of the study; firstly, to examine the role of advection in the SEB and its spatial variation, and secondly, to relate the SEB to any thermal differences between the park and its built-up surroundings. * Correspondence to: Department of Geography, University of Canterbury, PO Box 4800, Christchurch, New Zealand; tel.: , ext 7916; fax: ; r.spronken-smith@geog.canterbury.ac.nz 1 Recently deceased. Copyright 2000 Royal Meteorological Society

2 1034 R.A. SPRONKEN-SMITH ET AL. The primary objective is investigated using a transect of stations across a heavily irrigated park in Sacramento, CA in mid-summer. Each site includes observations of SEB fluxes. Together with a conceptual framework these results are used to infer the presence of advection in the park. Ancillary measurements of temperatures in- and outside the park are used to give insight into the processes controlling, and the magnitude of, the thermal behaviour of the park relative to its surroundings. 2. CONCEPTUAL FRAMEWORK 2.1. E aporation and the energy balance o er a wet surface The energy balance for the surface of a wet patch, such as an irrigated park is: Q* Q H Q E Q G Q A =0[Wm 2 ], (1) where the flux densities are: Q*, net all-wave radiation; Q H, turbulent sensible heat; Q E, turbulent latent heat (or evaporation); Q G, conductive sensible heat; and Q A, heat advection (representing the net gain or loss due to transport associated with the spatial heterogeneity of sources and sinks). Equation (1) is conceptual. In practice it is difficult to measure Q A as a separate term. Instead, the first four terms on the left-hand side are measured and advective contributions appear most obviously in the two turbulent terms. The net radiative and soil heat fluxes are likely to be relatively spatially conservative in an open irrigated park, provided there are no trees or other horizon obstructions to interfere with radiative exchange and that the soil is quasi-homogeneous. On the other hand, the local turbulent fluxes are likely to vary considerably across the park, due to advective processes operating on several scales. For example, at the microscale the abrupt change in surface moisture availability at the upstream built-park edge will cause the latent heat flux to vary with fetch distance across the park. Horizontal transport of much drier air from the upstream built-up zone across the wetted park will create a large surface-to-air gradient in humidity. This in turn will force a sudden increase in evaporation at the leading edge and an approximately exponential decline to lower, but still enhanced, rates of Q E further across the park. This is the microscale leading-edge or fetch effect (Oke, 1987). At the local scale a park which is moist and cool compared to its surrounding neighbourhood may also act as an oasis (Tanner, 1957). In this case evaporative enhancement occurs as a result of mechanical subsidence of warmer regional (city) air down over the cooler park due to mass divergence within tens of metres. The extra downward flux of sensible heat supplements the radiative energy supply and permits abnormally high rates of evaporation. Several field studies of evaporation from irrigated fields surrounded by semi-arid areas report leading edge effects (see Rosenberg et al., 1983 for a review). For example, several observational studies at Davis, CA (near the Sacramento site used here) focus on the case of a step change in moisture availability due to irrigation of grass set in the midst of drier fields (e.g. Dyer and Crawford, 1965; Goltz and Pruitt, 1970). In those studies the latent heat flux was estimated from both closure of the energy balance (Q* and Q G were measured and Q H was inferred from changes in a series of air temperature profiles downstream from the leading-edge) and lysimetry. Lang et al. (1974) used lysimetry to study the influence of microscale advection on evaporation from an irrigated field of rice, and Rider et al. (1963) measured evaporation from irrigated grass downwind from a paved airport runway. All these studies show sharp increases in evaporation near the step change, followed by exponential decay with increasing fetch, until the surface layer fully adjusts to the new surface moisture, i.e. Q E becomes approximately constant with distance. Estimates of the horizontal extent of edge effects vary widely; from less than 20 m (Rider et al., 1963) to as great as 200 m (Rijks, 1971). Microscale advection has also been shown to contribute to increased evaporation in urban areas. Oke (1979) found advectively-assisted evaporation over an irrigated suburban lawn in Vancouver, BC. On a daily basis evaporation from a mini-lysimeter exceeded the net radiation. These findings were replicated by Suckling (1980).

3 URBAN PARK ENERGY BALANCE 1035 Oasis effects have been observed over crops at several locations (e.g. Fritschen and van Bavel, 1962; Rosenberg, 1969, 1972; Wright and Jensen, 1972; Blad and Rosenberg, 1974; Rosenberg and Verma, 1978). Even at sufficiently large fetch distances where edge effects can be safely ignored these studies show evaporation is significantly higher than the potential rate of evaporation. For example, in a study of water use by irrigated cotton in the Sudan, Rijks (1971) found evaporation rates 1.8 times greater than the supply of net radiation when both edge and oasis-type advection were important. Even the oasis effect alone elevated Q E /Q* to 1.5. In the present study of an irrigated park in an urban area both leading-edge and oasis effects are expected to be in operation. Further, it is also possible that a third, mesoscale advection of heat and/or moisture, will be involved. This could arise from thermal-driven airflow circulations in the Sacramento region (valley winds, land sea breezes and urban breezes) and large-eddy circulations in the urban boundary layer. In the absence of advection, evaporation from a saturated and extensive surface has a lower limit termed the equilibrium evaporation rate, Q Eq, by Slatyer and McIlroy, (1961): Q Eq = s s+ (Q* Q G), (2) where s is the slope of the saturation vapour pressure versus temperature curve and is the psychrometric constant =c p a /L v where c p is the specific heat of air at constant pressure, L v is the latent heat of vaporization, a is the density of air and is the ratio of the molecular weight of water to that of dry air. For these special conditions the equation gives the fraction of the available energy (Q* Q G ) used in evaporation and since s increases with temperature so does the fraction. In practice evaporation over large regions where surface water is freely available exceeds the equilibrium rate. Priestley and Taylor (1972) referred to this as potential evaporation (Q Ep ): Q Ep = Q Eq, (3) where is an empirical coefficient, which for oceans and other extensive wet surfaces, is about The increase over the equilibrium rate has been attributed to the fact that the atmospheric boundary layer is hardly ever in equilibrium with the saturated surface it is continually responding to changing weather, and is in receipt of drier air by entrainment from the free atmosphere above (McNaughton and Spriggs, 1989). In fact no unique value of is to be expected because it is a function of absolute temperature (effect on s), the aerodynamic conductance of the local surface (effects of wind speed and surface roughness) and the areal surface conductance of the surrounding region (Garratt, 1992; Lhomme, 1997). For the case of oasis-type evaporation from a saturated patch surrounded by a non-saturated region Lhomme (1997) suggests the local potential evaporation Q E = Q Eq, with: = (Q* Q G) Q* Q G + 1 (s/ )+1 r c (r a ), (4) where r c and r a are the surface and aerodynamic resistances, respectively and indicates an areal average for the surrounding region and indicates the local surface value. For an extensive grass site (i.e. where the available energy for both the patch and its surroundings are the same, and therefore their ratio is unity) and where both r c and r a are about 50 s m 1, at 25 C (s/ =2.82) Equation (4) collapses to the observed Q Ep value of Equation (4) also shows that the drier the surroundings, and the rougher the local surface, the greater the local potential evaporation, Q E. values 1.26 are expected for an irrigated urban park because the first term on the right-hand side is greater than unity (greater heat storage in the built fabric is likely to outweigh any differences in net radiation between the suburb and the park), and so is the ratio r c /r a. Therefore, only when air temperature is well above 25 C is the counter influence of rapidly increasing s able to reduce. Few researchers distinguish between edge and oasis effects, i.e. microscale and local scale advection. Often the confounding effects of proximity to a leading edge are strategically avoided by siting instrument arrays away from borders in an extensive homogeneous area. Here, for an urban park we make the

4 1036 R.A. SPRONKEN-SMITH ET AL. Figure 1. Hypothesized variation of the latent heat flux with distance of fetch as air traverses from a residential suburb across an irrigated urban park distinction between edge and other advective effects using a slight modification of the conceptual framework suggested by Brakke et al. (1978). At sufficient distances to be downwind of any edge effects they infer that advective effects exist if Q E exceeds the available energy (Q* Q G ). Here we simply replace (Q* Q G ) by the equilibrium evaporation (Q Eq ) this has the merit of accounting for temperature dependence, and separates the energy from the advective terms in the Penman evaporation formula for a saturated surface (e.g. Oke, 1987, p. 385). Hence (Figure 1), we assume that evaporation from an irrigated urban park builds from the base rate for a saturated surface with no advection (Q Eq ), plus one or more of the three advective contributions: (i) mesoscale and deep boundary layer convection, (ii) local scale oasis effects, and (iii) microscale edge effects. The contribution from any edge effect is likely to be shown by an approximately exponential decay in evaporation from the border into the park. Any evaporation between Q Eq and the asymptotic tail of the decay curve attributable to edge effects can be assumed to arise from oasis or mesoscale effects. It is not possible to separate these effects. Nor can we be certain that the park corresponds to a truly saturated surface because radiative heating caused grass blades to lose most or all their external water droplets (from irrigation) so some physiological control on water exchange must be involved Temperature of a wet surface in the presence of hot, dry air In order to see the dependence of the surface temperature (T 0 ) of a wet park upon the SEB terms we can re-write the observed version of Equation (1), i.e. without Q A as: (Q* Q G ) C a(t 0 T a ) C as r H (T 0 T d ) =0, (5a) r E which, after re-arrangement gives: T 0 = (Q* Q G )+ C at a + C ast d n r H r E, r H r E C a r E +C a sr H and after making the assumption r=r H =r E to aid simplification gives: T 0 = s+ Ta + r (Q* Q C G )+ s a dn T, (5b) where, C is the heat capacity, the subscripts a, 0 and d refer to the air, surface and dew-point, respectively, and r is the resistance between the surface and the airstream. The assumption of equality of resistances ignores physiologic control by the grass, but it may be reasonable for a wet park. Equation (5b)

5 URBAN PARK ENERGY BALANCE 1037 emphasizes the fact that T 0 depends upon the energy availability, the sign and magnitude of the driving gradients of temperature and humidity between the surface, and the aerodynamic resistance, which in turn is controlled by wind speed, roughness and stability over the park. In the case of a wet park in warmer, drier environs, advection (the hidden Q A term in (5a)) causes T a T 0 and T d T 0 (i.e. e a e 0 )sothe sensible heat term would become a source of heat and the latent heat an even larger sink. The SEB of a single leaf, can be written in a manner analagous to Equation (5b), except that the resistances are those of the laminar boundary layer of the leaf and of its stoma rather than the surface boundary layer and the plant canopy. Solving the equation for the leaf air temperature difference gives insight into the roles played by the net radiation, the resistances and the atmospheric vapour deficit and temperature (see Jones, 1983, pp ): T 0 =T a + r H Q*r E sr H+ r (e E a L s(t a ) e a ), (6) v where the surface-air vapour gradient has been replaced by the vapour pressure deficit of the air (vpd a ) using Penman s linearization of the saturation vapour pressure versus temperature curve, and the terms superscripted by a dot indicate leaf rather than canopy resistances. Of special interest to the advective case is the response of T 0 to increases in T a and vpd a. Firstly, Equation (6) shows that T a is the reference temperature for the surface if other forcing is small. Secondly, since s increases with temperature there is a tendency for any excess of T 0 over T a to decrease at higher temperatures. Thirdly, since vpd a increases with temperature, if water is available this will increase Q E, and thereby lower T 0. Fourthly, although not evident in Equation (6), it is a fact that for each increment in temperature the difference of temperature that drives Q H increases linearly, but because of the relation between e and T, the difference of vapour pressure driving Q E increases exponentially. These relations underlie the thermostat-effect the observed tendency of a wet leaf to maintain almost constant T 0 in an increasingly hot environment (Gates et al., 1964). This occurs at air temperatures above C (Jones, 1983, p. 205). Here we speculate that an irrigated park, in a hot, dry city may show thermostatic behaviour as if it were acting as a very large leaf. In addition to the leaf analogy, Equations (2) and (3) show that, even in the absence of local advection, at the park scale a wet surface channels an increasingly large fraction (s/(s+ )) of the available energy (Q* Q G ) into Q E. So at temperatures beyond 30 C, where more than 77% of energy goes to Q E, and especially where the heat supply is boosted by advection, conditions may support thermostatting by a park. 3. EXPERIMENTAL Measurements of the SEB were conducted in Orville Wright Park (Figure 2) in the suburb of Mission Oaks, Sacramento, CA (38 39 N, W). The park is relatively small (3.6 ha), covered with short grass, and surrounded by a border of trees and hedges (5 10 m high). The park is irrigated at night from sprinkler heads installed in the ground. It is located in a residential area of predominantly single-family dwellings with well-irrigated (mesiscape) vegetation. Simultaneous measurements of the SEB were also made in the nearby suburb of Carmichael, and at two rural sites; one an irrigated sod farm the other a dry grassland ranch (for details see Grimmond et al., 1993). The suburban land cover around the park is similar to that at Carmichael, hence the fractional land cover of the three-dimensional surface is approximately: 42% vegetated, 33% roofs, 15% walls, and 10% impervious ground such as roads, sidewalks and parking lots. The mean height of the buildings is about 4.8 m, and of the trees about 6.7 m. Based on morphometric analysis the site has a mean roughness length in the range m, and a neutral aerodynamic conductance in the range mm s 1 (Grimmond and Oke, 1999). The advective processes identified earlier dictate that field observations be conducted at several positions across the park, not just at its centre. Therefore, the SEB was sampled at three sites, spaced approximately exponentially along a transect aligned with the prevailing wind (Figure 2). Site 1 is at the edge of the park 4minfrom the windward boundary; Site 2 is 27 m from the edge and Site 3 is near the centre of the park, 96 m from the boundary.

6 1038 R.A. SPRONKEN-SMITH ET AL. Figure 2. Map of Orville Wright Park including location of SEB observation sites during August 1991, as well as the microclimate observation sites (air and surface temperature, and relative humidity) during August Note that in August 1993 some flux measurements were also made at Site 3 (see text for details) Net all-wave radiation was measured at each site with a net pyrradiometer (Swissteco, model S1) mounted 0.8 m above the ground. Soil heat flux was monitored at each site with two soil heat flux plates buried at 0.05 m depth (Middleton and Pty. Ltd.). Heat flux divergence between the surface and plates was accounted for (Leuning et al., 1982). Latent heat flux was measured at the three sites by mini-lysimeters. Each lysimeter is a soil monolith in a container, and its weight is continuously measured by a load cell (Grimmond et al., 1992). Specifications of the mini-lysimeter are given in Table I. The hourly evaporation rate, E (kg m 2 h 1 ), is calculated: E= w P W/A, (7) where P is precipitation (m h 1 ), W is the change in weight (kg h 1 ), A is the surface area of the lysimeter (m 2 ) and w is the density of water (kg m 3 ) and the flux of water mass (E) is converted to energy equivalent (Q E ): Q E =L v E. (8) The turbulent sensible heat flux (Q H ) at each site is estimated by residual in Equation (1) and therefore includes any measurement errors in the other fluxes. This approach to solving the SEB has the merit that three terms are measured directly at the surface (Q*, Q G and Q E ) and the turbulent components are found without resort to assumptions regarding the existence of spatial equilibrium. Fluxes determined in this way contain all advective contributions and hence there is no need to include a separate Q A term when writing the measured balance since it is subsumed in the others. The approach, however, does not allow the role of advection to be singled out. Table I. Specifications of the mini-lysimeters Feature Specifications Weighing mechanism Single point I load cell (Interface Inc., model SP-I 50) Internal diameter (m) 0.27 Surface area (m 2 ) Depth (m) 0.25 E resolution (mm h 1 ) Q E 20 C (W m 2 ) 11.6

7 URBAN PARK ENERGY BALANCE 1039 Soil moisture was sampled along the transect every 3 days. Moisture content is not uniform across the park, partly due to the existence of hard pan at different depths, and proximity to irrigation sprinkler heads. This must contribute to spatial variability of the SEB but it is reassuring that lysimeter monoliths were found to possess similar wetness to that of their surrounding soil. (At the end of the observation period the lysimeter at Site 1 had a soil moisture content of 19% by weight, compared with 16% nearby; at Site 2 it was 17% both in and around the lysimeter; and at Site 3 it was 24% compared with 20% nearby.) We take this to indicate that the monoliths were representative of the park state. Air temperatures were measured at a height of 1.3 m using shielded thermistors at all sites, and relative humidity was monitored at Site 3 using a capacitance sensor (Vaisala, model HMP 35C). Wind direction at 1 m above ground was measured at Site 2 during the latter half of the research period. The fluxes and climatic variables were sampled every 20 s, averaged every 15 min and later composited to give hourly values. The full field programme was conducted in Sites were installed in Orville Wright Park on August 22 (Year Day (YD) 91/234) and operated for the following 7 days (YDs 91/234 91/240). Continuous observations were obtained at Site 3, but Site 2 started later due to the need to relocate the lysimeter (a broken irrigation pipe saturated the first lysimeter pit), and data at Site 1 were lost on YD 91/238. The average air temperature for August was 22.9 C (compared with the normal 23.7 C), and it was drier than normal with only 0.25 mm rain recorded in the 6 weeks prior to the measurement period (compared with an average August precipitation of 1.8 mm). Synoptic conditions were otherwise typical for the region; cloudless skies by day with occasional night cloud. A cold front extended into the Sacramento region on YD 91/237 giving some high cloud, and another front passed through on YD 91/240. In August 1993 a further 2 days of measurements (YDs 93/225 and 93/226) were conducted under conditions very similar to those encountered in There were two objectives: firstly, to corroborate that 1991 estimates, which estimated Q H by residual, were correct. In the 1993 project Q H was measured directly by eddy correlation using a one-dimensional sonic-anemometer thermometer (SAT) system (Campbell Scientific Inc., model CA27) mounted 0.8 m above the surface at the centre of the park. Fluctuations of vertical wind velocity and air temperature were sampled at 10 Hz, with covariances determined over 15 min. Flux corrections were made for density effects (Webb et al., 1980). Kristensen and Fitzjarrald (1984) suggest that in unstable conditions it is not necessary to correct for line-averaging effects down to heights four to five times the length of the sonic path. The SAT has a path length of 0.1 m therefore allowing measurement down to 0.4 m. However, in stable conditions Q H may be underestimated. The second objective of the 1993 observations was to assess the strength of the driving forces for park evaporation the differences of surface temperature and vapour pressure between the park and its surroundings. Two sites, one near the centre of the park, the other over the concrete driveway of a residence located west of the park, were used to characterize suburban-park differences. Air temperature and relative humidity were measured with combined temperature and capacitance probes (Vaisala, model HMP35C) mounted in radiation shields. Surface radiant temperature was measured with an infra-red thermometer (Everest, model 4000A) and surface emissivity was taken from tables (Wittich, 1997) which could lead to errors of 1 2 C (Oke, 1987). Other measurements at the park site in 1993 included Q* with a pyrradiometer, and wind speed and direction at the height of 1.5 m (R.M. Young Wind Sentry). The net radiation and standard climatic variables were sampled at 0.1 Hz and the average was recorded every 15 min. 4. RESULTS AND DISCUSSION As already noted, the results relate to two different observation periods. Data from the August 1993 observation period are reported in the sections dealing with the advective driving force and the thermostat effect, while the SEB data is from August Given the similarity of meteorological conditions and fluxes during the two observation periods, it was deemed acceptable to consider the driving forces in 1993

8 1040 R.A. SPRONKEN-SMITH ET AL. to be a reasonable surrogate for those in operation during the SEB observations in It is important to bear this in mind in the following discussion. SEB partitioning showed a daily cycle at each park site that was consistent from day-to-day throughout the period in Given this, the discussion of energy balance partitioning and of the influence of fetch on Q E uses 3-day averages of the SEB at each site. Continuous data records were obtained for YDs 91/237, 91/239 and 91/240 so they were chosen to form the ensemble average Ad ecti e dri ing force The observed surface and air temperature and humidity both in the park, and over a driveway adjacent to the park, illustrate the surface climate differences driving advective interactions between the two environments (Figure 3). At the surface the wet park is cooler than the nearby dry paved area throughout the day and night. The contrast is strongest in the late afternoon ( T 0 16 C) and provides a potentially significant source of warm air advection to the park. The air temperature difference between the two ( T a ) is much smaller (only 1 2 C) but, as noted by Spronken-Smith and Oke (1998), the park is consistently cooler at all times of day. The reason for the relatively constant difference in the air, in the face of the strongly varying surface difference, is explored in Section 4.4. The strength of the humidity driving force ( e a ) is almost zero near sunrise because dew is present on both surfaces, but it grows rapidly in the morning to a peak near 15:00 Local Apparent Time (LAT) Energy balance partitioning The ensemble 3-day average SEB components at each park site are given in Figure 4, and a statistical summary of the daily (24 h) and daytime (Q* 0) fluxes and several non-dimensional ratios, is given in Table II. The SEB at Site 1 at the park border clearly shows strong advective effects. After mid-morning Q E exceeds Q* for the rest of the daytime and on an absolute scale Q E is very large up to 600 W m 2. Q H is only positive during the morning, in the afternoon it becomes a significant (c. 200 W m 2 ) source of heat which supplements Q*, making such high evaporation rates possible. Evaporation continues throughout the night at about W m 2. Heat storage change in the substrate is negligible by comparison with the other SEB terms. The daily course of the SEB at Site 1 is temporarily complicated Figure 3. Hourly differences in diurnal air and surface temperature, T a and T 0 ( C), and vapour pressure, e a (Pa), between suburban paved surfaces and adjacent Orville Wright Park. Data averaged over 2 days (YD 93/225 and 93/226)

9 URBAN PARK ENERGY BALANCE 1041 by shadows from the park border (Figure 2). The shade only affected the radiometer not the lysimeter, so there is a marked dip in Q* between 10:00 and 13:00 LAT, but not in Q E (Figure 4(a)). This temporary discrepancy makes calculation of the residual (Q H ) invalid, so it is not shown for this period. At Sites 2 and 3, edge influences are less and the SEBs are broadly similar to each other in terms of heat partitioning and the dominance of Q E. At these sites the ensemble mean Q* peaks at about 530 W m 2 and Q E reaches a maximum of W m 2 in the early afternoon. The sensible heat flux peaks at about 100 W m 2 just before midday, and in the afternoon becomes negative (i.e. the flux is directed towards the surface). This pattern is a common feature of advective environments. In order to be certain that this feature is not an artefact of errors in the residual approach, we repeated observations at Site 3 in 1993 under broadly similar conditions, but this time taking direct measurements of Q H by eddy correlation. The course of Q H in the two periods, is very similar (Figure 4(d)), as were the average daytime ratio Q H /Q* (0.05 in 1991; 0.09 in 1993). We conclude the 1991 results were real and therefore that even near the centre of the park, Q H is into the air in the morning, but later as T 0 between the cool park and the warmer neighbourhood increases (Figure 3), warmer air advects over the park and Q H is directed down the inversion towards the surface. This inversion develops in the late afternoon and the surface may be up to 6 C cooler than the air (see the temperature difference between the curves for the park surface and 1 m above the park surface in Figure 8). The nocturnal evaporation seen at Site 1 is evident for a few hours at Site 2, but not at all at Site 3. The spatial variation of net radiation across the park is relatively conservative; each site has a daytime average receipt of approximately 13 MJ m 2. The latent heat flux, on the other hand, shows both spatial Figure 4. (a c) Three-day average of hourly SEBs at three sites in Orville Wright Park, Sacramento in August The sharp dip in Q* in the middle of the day at Site 1 is due to local shading of the net radiometer by a tree. The Q H curve is omitted for this period (see text). The standard error of the measurements is indicated by the vertical bars. (d) Net radiation and sensible heat fluxes measured at Site 3 in August 1991 and 1993

10 1042 R.A. SPRONKEN-SMITH ET AL. Table II. Daily and daytime (Q* 0) averages and ratios for energy balance fluxes at sites in Orville Wright Park, Sacramento a Site Q* Q E Q H Q G =Q H /Q E Q E /Q* Q Eq Q Ep =Q E /Q Eq Daily Park/Suburban Park/Wet rural Daytime Park/Suburban Park/Wet rural a All fluxes in MJ m 2 day 1. Ratios of park fluxes to those measured at the suburban and wet rural sites are shown for comparison. and temporal differences with increasing fetch across the park (Figure 4). Q E is greatest at Site 1 due to microscale advection of sensible heat due to the edge effect at the built-park boundary. This drives anomalously high rates of evaporation which are sustainable because of regular and copious irrigation. The peak at Site 1 occurs at 14:00 LAT which corresponds approximately to the time of maximum forcing by T 0 and e 0 between the park and the adjacent built-up surfaces (Figure 3). Evaporation at Sites 2 and 3 is less than at Site 1 by about 20% (Table II). Daytime and daily evaporation in the centre of the park is more than three times that measured at a suburban tower site in the nearby Carmichael suburb (Table II). The tower observations at 29 m are above the roughness sub-layer or blending height (Grimmond and Oke, 1999) and therefore give the spatially-integrated evaporation of the park s surroundings. Perhaps even more notable, park evaporation is 40% greater than from the rural irrigated site during daytime (Table II). Since both are heavily irrigated short grass surfaces the difference is attributable to differences in their surroundings. We calculate for the irrigated park and rural grass sites using Equation (4) and the same r a and (Q* Q G ) values for grass. Further, we assume the rural area is surrounded by like irrigated surfaces, but the park is surrounded by drier built-up surfaces with daytime values Q* Q G as measured at Carmichael (Table II), and r c =350sm 1. At 25 C, the approximate daytime air temperature during the observations (see Grimmond and Oke, 1995), Equation (4) predicts park should be about 2.2, and rural about 1.3, i.e. the urban surroundings of the park should boost values by about 69%. The observed values of are about 1.3 for the centre of the park and 0.92 at the farm, i.e. the park is boosted by about 41%. The lower than predicted absolute values may be due to daytime drying, so that neither grass surface conforms to the saturated condition assumed in the derivation of Equation (4). The fluxes normalized by Q* show distinct trends in energy partitioning by the park in the daytime (Figure 5). During the morning Q E remains a fairly constant proportion of Q*. But in the afternoon Q E continues to increase for a few hours beyond the midday peak in Q*. Thereafter Q E /Q* exceeds unity and steadily increases throughout the afternoon. This suggests that evaporation is more restricted by the availability of heat than of water. As already outlined, the advective supply of sensible heat from drier built surfaces upstream boosts the evaporative forcing. Since Q G is relatively insignificant the trend of Q H /Q* is essentially the mirror image of Q E /Q* and becomes increasingly negative later. The Bowen ratio ( ) generally decreases through the day at all sites (Figure 5(c)). Small positive values in the morning reflect the dominance of the latent heat flux, and in the afternoon turns negative at all sites, due to the change in sign of Q H. The time of sign change becomes later the further the site is into the park (at Site 2 by 13:00 LAT and at Site 3 by 15:00 LAT). At Site 1 the daytime average is 0.18 due to the strong edge effect and at Sites 2 and 3 values are as small as for open water ( 0.10) (Table II).

11 URBAN PARK ENERGY BALANCE The influence of fetch on Q E An appreciation of the advective influence is gained by comparing the measured latent heat flux Q E, with the equilibrium value Q Eq, which should occur from an extensive moist surface without advection, and Priestley and Taylor s (1972) potential evaporation Q Ep from a saturated surface (i.e. r c 0 and typically 1.26) (Table II). Except for a few hours in the morning, evaporation at all sites easily exceeds the equilibrium rate, indicating the presence of some form of advection (Figure 6). Since Q E Q Ep at all sites in the afternoon, there is evidence of enhanced local scale advective effects. Since evaporation continues through the night daily is even higher than the daytime value. On a daily basis evaporation at the edge of the park is two times Q Eq and 1.6 times Q Ep. At the two sites inside the park the daily rate is about 1.6Q Eq and 1.3Q Ep. The similarity of Sites 2 and 3 suggests the main influence of edge effects is confined to a narrow strip near the park edge. These results, however, do not incorporate the true fetch because fixed sites aligned along a transect are not necessarily parallel to the wind direction. The true effect of fetch on Q E throughout the daytime is shown in Figure 7. This shows that at 09:00 LAT Q E is fairly constant across the park with no evidence of microscale edge effects. At 11:00 LAT Q E is still similar at Sites 1 and 2 but higher in the centre of the park. This may reflect shading effects near the park edge that reduce the available energy for evaporation. By 13:00 LAT some microscale edge effects are apparent with increased Q E at Sites 1 and 2. This edge effect strengthens in the afternoon peaking at 15:00 LAT corresponding to the time when the built-park driving force is at its greatest (Figure 3). From 16:00 until at least 22:00 LAT, Q E exceeds Q Ep by up to 240 W m 2 at Site 1, 120 Wm 2 at Site 2, and 150 W m 2 at Site 3 (Figure 6). During these hours a strong edge-effect (estimated to be about 105 W m 2 at Site 1 based on Figure 5. Three-day average of the hourly daytime SEB flux ratios at three park sites in August To facilitate interpretation, the data for Site 1 near midday are omitted because of local shading (see Figure 4(a))

12 1044 R.A. SPRONKEN-SMITH ET AL. Figure 6. Comparison of the measured hourly Q E, Q Eq and Q Ep at three sites in Orville Wright Park, Sacramento in August 1991 the shape of the decay curve) is superimposed upon a considerable oasis effect (about 135 W m 2, based on Q E Q Eq at Site 3). During the period of advective forcing in the afternoon the rapid decrease in Q E from the edge of the park towards the centre occurs within a distance of about 20 m. Therefore, we conclude that evaporation trends across Orville Wright Park are consistent with the conceptual model of an irrigated urban park illustrated in Figure 1: there is an exponential decay of Q E with distance from a dry wet leading edge into the park, and the centre of the park experiences local-scale oasis effects, at least during the afternoon and evening period. There is also some evidence that the presence of a vegetation and houses at the border alters the turbulence regime, producing a quiet zone (Raine and Stevenson, 1977) which affects the decay of Q E.. Evaporation is slightly reduced at Site 2 (compared to Site 3), perhaps without the shelter the eddy diffusivity near the park edge would be greater and Q E considerably enhanced at both Sites 1 and Thermostat effect of an irrigated park The thermostat effect outlined in Section 2.2 requires that the surface be wet and the air temperature high. Such prevailed at Orville Wright Park during the measurement period. The standard of comparison here are conditions on the surface and 1 m above a paved driveway adjacent to the park. From 09:00 to 18:00 LAT T a increased from 16 to about 31 C. During the same period T 0 became increasingly hotter than the air, and by late afternoon there was as much as a 16 C difference over the lowest metre before the difference started to decline (Figure 8). In fact the surface of the driveway always appeared to be warmer than the air above. This is consistent with the behaviour of a dry system able to store sufficient

13 URBAN PARK ENERGY BALANCE 1045 Figure 7. The variation of Q E with distance of fetch from the park edge, at several times of day. Data from three sites are averaged over 3 days in August 1991 heat by day to maintain a positive heat flux at night and thereby to offset the development of a surface inversion. However, the uncertainty of surface emissivities, especially for the concrete driveway, do not allow us to be certain of this interpretation. In contrast T 0 of the wet park never became more than a few degrees warmer than the reference T a and later in the day it even became cooler than the reference. This behaviour is clearly thermostatting. It therefore acted like a very large wet leaf. The cooler surface of the park imparted a slight coolness ( 2 C) to the air 1 m above, relative to the reference. Figure 8. Hourly course of temperatures observed on a paved surface, a wet park surface and in the air at 1 m above the park, relative to the air temperature 1 m above the paved surface. Data are averaged over 2 days (YD 93/225 and 93/226). The early peak in the surface temperature of the park, and its later dip below the reference temperature, is interpreted to be due to thermostatting (see text)

14 1046 R.A. SPRONKEN-SMITH ET AL. 5. CONCLUSIONS The SEB of Orville Wright Park, a well-irrigated grassed area set in a residential suburb of a city in a hot climate, is dominated by evaporation. Comparison of synchronous observations at the park and two other sites show the wet park evaporates three times more water than from the surrounding residential neighbourhood, and 1.3 to 1.4 times more than an irrigated sod farm in the rural environs. Very high Q E occurs at the upwind edge of the park. The probable shape of the decay with increasing fetch across the park suggests this is a microscale edge-effect. In this park the pattern is slightly disrupted by trees, bushes and houses at the periphery which disturbs the flow in their lee. Sensible heat is a moderate heat sink in the park in the morning, however, by early afternoon it becomes a significant heat source. This advective heat supply allows the park to act as an oasis thereby boosting Q E even well away from the edge. These results are consistent with the theoretical framework of Lhomme (1997) which predicts that the local evaporation from a wet surface depends not only on the available energy and resistances of the local surface but also those of its surrounding area, and it is also significantly related to the absolute temperature. A logical extension of these ideas shows that the park acts like a large leaf and exhibits thermostatting. At increasingly high temperatures the thermal difference between the surface of the park and the air above become smaller and may even reverse, so that the park surface is cooler than the air, near the time of peak heat input (Q*+Q H ). ACKNOWLEDGEMENTS We are grateful to Dr T.A. Black for discussions, Mark Belding, Dr Sue Grimmond and Harold Spronken for assistance with field work, and Michelle Rogan and Tim Nolan for help in preparation of diagrams. The study was supported by a University Graduate Fellowship from the University of British Columbia and funding from the New Zealand Meteorological Service to R.A. Spronken-Smith. T.R. Oke was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. REFERENCES Blad BL, Rosenberg NJ Evapotranspiration by subirrigated alfalfa and pasture in the east central Great Plains. Agronomy Journal 66: Brakke TW, Verma SB, Rosenberg NJ Local and regional components of sensible heat advection. Journal of Applied Meteorology 17: Dyer AJ, Crawford TV Observations of the microclimate at a leading edge. Quarterly Journal of the Royal Meteorological Society 91: Fritschen LJ, van Bavel CHM Energy balance components of evaporating surfaces in arid lands. Journal of Geophysical Research 67: Garratt JR The Atmospheric Boundary Layer. Cambridge University Press: Cambridge. Gates DM, Heisey WM, Milner HW, Nobs MA Temperatures of Mimulus leaves in natural environments and in a controlled growth chamber. Carnegie Institute Washington Yearbook 63: Goltz SM, Pruitt WO Spatial and Temporal Variations of E apotranspiration Downwind from the Leading Edge of a Dry Fallow Field, Div. Technical Report ECOM68-G10-1, Department of Water Science and Engineering, University of California at Davis. Grimmond CSB, Isard SA, Belding MJ Development and evaluation of continuously weighing mini-lysimeters. Agricultural Forest Meteorology 62: Grimmond CSB, Oke TR Comparison of heat fluxes from summertime observations in the suburbs of four North American cities. Journal of Applied Meteorology 34: Grimmond CSB, Oke TR Aerodynamic properties of urban areas derived from analysis of urban form. Journal of Applied Meteorology 38: Grimmond CSB, Oke TR, Cleugh HA The role of rural in comparisons of observed suburban-rural flux differences, in Exchange Processes at the Land Surface for a Range of Space and Time Scales. Proceedings Yokohama Symposium, July, 1993, IAHS Publication No. 212, Jones, H.G Plants and Microclimate, Cambridge University Press, Cambridge, p Kristensen L, Fitzjarrald D The effect of line averaging on scalar flux measurements with a sonic anemometer near the surface. Journal of Atmospheric Oceanic Technology 1: Lang ARG, Evans GN, Ho PY The influence of local advection on evapotranspiration from irrigated rice in a semi-arid region. Agricultural Meteorology 13: Leuning R, Denmead OT, Lang AR, Ohtaki E Effects of heat and water vapour transport on eddy covariance measurement of CO 2 fluxes. Boundary-Layer Meteorology 23:

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