The role of evaporation in the energy balance of an open-air scaled urban surface

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 29: (2009) Published online 26 August 2008 in Wiley InterScience ( The role of evaporation in the energy balance of an open-air scaled urban surface D. Pearlmutter, a *E.L.Krüger b and P. Berliner a a Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Israel b Departamento de Construção Civil, Universidade Tecnológica Federal do Paraná, Brazil ABSTRACT: Surface evaporation is an important component of the urban energy balance, and its role in arid-zone cities may significantly differ from that observed in more temperate regions. However, the quantification of evapotranspiration is difficult in a complex urban setting, given the heterogeneity of the terrain and its various dry and wet elements. Here an open-air scaled urban surface (OASUS) is employed to quantify latent heat flux as a function of the surface area available for evaporation and the three-dimensional (3-D) urban geometry. The OASUS model consists of an extensive urban-like building/street array located in an arid Negev region of southern Israel (30.8 N, 480 m above sea level). Measurements were carried out during the summer month of August, and flux partitioning was analysed using evaporation pans of varying surface areas embedded in arrays of varying height. Results indicated that the increase in latent heat removal with respect to equivalent vegetative surface area was nearly linear. Increasing latent heat flux with vegetated fraction was offset in approximately equal measure by decreases in storage and turbulent sensible heat flux. The proportion of the radiant energy budget represented by evaporative heat loss was also linked to the 3-D geometry of the array, increasing linearly with the ratio between vegetative cover and the complete urban surface area. Copyright 2008 Royal Meteorological Society KEY WORDS scale-modelling; urban climate; surface energy balance; evaporation Received 04 June 2007; Revised 25 March 2008; Accepted 06 July Introduction Cities are often viewed as dry terrestrial surfaces, dominated by building and paving materials. Indeed, the relative lack of surface moisture and transpiring vegetation in many urban areas has been cited as one of the factors contributing to the urban heat island, as evaporative heat removal in such cases is limited. Observations have shown, however, that the actual magnitude of evapotranspiration varies widely both between and within cities, and that the turbulent flux of latent heat is often a significant component of the urban surface energy balance (Grimmond and Oke, 1991, 1999a; Oke et al., 1998; Arnfield, 2003). The familiar roles played by surface evaporation in temperate-zone cities may be significantly altered in arid regions, where the rate of potential evapotranspiration relative to precipitation is especially high (Bruins and Berliner, 1998). Owing to the sparsity of natural vegetation, the proportion of evaporative heat loss may be considerably smaller than in more humid regions. In the arid city of Mexicali (in northern Mexico, 32 N), the daytime latent heat flux (Q E ) was found by Garcia-Cueto et al. (2003) to account for just over 10% of the net radiation * Correspondence to: D. Pearlmutter, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Israel. davidp@bgu.ac.il (Q ), and even lower values have been recorded during the marked dry season of densely built core areas of Mexico City (Oke et al., 1999) and Marseilles (Lemonsu et al., 2004). These values may be compared with a daytime Q E /Q range of 25 35% typically recorded for North American suburban areas (Grimmond and Oke, 1999b). On the other hand, this higher range includes the desert city of Tucson, Arizona, where local landscape irrigation (and to some extent the use of evaporative coolers on buildings) leads to a substantial daytime latent heat contribution. In fact, cities in arid regions may often be more highly vegetated than the dry desert terrain surrounding them reversing the typical urban scenario in which a relative lack of transpiring vegetation and available moisture, as well as soil waterproofing, are counted among the causes of the urban heat island. Evidence for the type of cool island effect which this might cause has been found in Phoenix, Arizona, but predominantly as a daytime phenomenon rather than as a nocturnal one (Brazel et al., 2000). The relative contribution of latent heat to the surface energy balance is in fact dictated by particular characteristics of the urban landscape at the local (land-use) scale, and understanding the influence of these features is consequential not only for climatological description but also for urban design. The most obvious factor of influence is the relative spatial extent of vegetative cover Copyright 2008 Royal Meteorological Society

2 912 D. PEARLMUTTER ET AL. within the urban area, which Grimmond and Oke (1999a) have sought to correlate with the proportional latent heat flux (Q E /Q ). Data from about a dozen North American cities (Grimmond and Oke, 2002) have suggested that Q E /Q tends to rise sharply with the introduction of green space to a city, and then levels off beyond a vegetated fraction of about (i.e. when 20 30% of the horizontal area is covered by vegetation). To date, however, this relationship has been described on the basis of limited data especially within the lower end of the vegetated fraction range, which is especially applicable to arid cities lacking in natural vegetation. In such cases, irrigated vegetation may have an especially potent impact in terms of evaporative cooling because of the high potential evapotranspiration, and at the same time carries a significant price in terms of water requirements relative to the available precipitation. Recent energy balance measurements in non-arid locations such as Basel, Switzerland (Christen and Vogt, 2004) and Lodz, Poland (Offerle et al., 2006) have shown that latent heat flux totals may also vary considerably within a particular city, reaching the lowest levels in the most densely built industrial and commercial areas and increasing as impervious cover is replaced by vegetated area. These studies have quantified an inverse relationship between the vegetated fraction and the Bowen ratio (β = Q H /Q E ), finding that the reduced Q E in city centres is to a large extent counterbalanced by a higher sensible heat removal. In terms of the heat ultimately stored in the urban fabric, the influence of green space is less clear-cut: while daytime storage heat flux ( Q S ) does tend to be higher at dry urban sites than at highly vegetated rural ones, the proportional values ( Q S /Q ) recorded in these dry sites are similar to those found in relatively green suburban residential areas. In fact, the actual partitioning of the radiant energy input between latent heat and other fluxes is dependent on a host of features, and influenced not only by the thermal and optical properties of the various building, paving and planting materials but also by the three-dimensional (3- D) geometry of the rough urban terrain. The rugosity of the city atmosphere interface influences its radiation balance by producing internal shading of both dry and wet surfaces, and by lowering the effective urban albedo, and it also imparts a large active surface area for the exchange of sensible heat (Voogt and Oke, 1997; Oke et al., 1999; Voogt and Grimmond, 2000). In actual urban settings, a wide variety of these surface characteristics are liable to vary simultaneously from site to site making it difficult to isolate the effects of individual features (such as vegetation) from on-site observations alone. The identification of particular urban effects may be complicated by the idiosyncracies of both urban and rural reference sites (Oke et al., 1998), and by uncertainties surrounding the homogeneity of the source area influencing flux measurements (Schmid et al., 1991). A number of meso-scale modelling schemes have been developed to parametrize these effects (Masson, 2000; Kusaka et al., 2001; Martilli et al., 2002), though challenges remain in describing the complex exchanges between the atmosphere and the modelled urban surface (Masson et al., 2002). Because the geometrical gap between mathematically modelled urban canopies and real cities is still wide, the use of physical hardware models provides a powerful alternative for studying urban climate and validating numerical models, provided that similarity requirements are met in terms of radiation, air flow and thermal inertia (Richards, 2002; Kanda, 2006). Such an alternative, complementary approach to modelling was developed by Pearlmutter et al. (2005), making use of an outdoor scaled urban array. Although the scale model was shown to be aerodynamically and thermally similar to an actual urban setting and was able to reproduce realistic flux patterns, previous experiments were limited to the analysis of energy exchanges in and above the modelled urban canopy in the absence of any significant surface moisture. Considering that one of the main justifications of using such a model is the potential to systematically alter physical features affecting the urban climate, the present research aims to evaluate the role of evaporation from controlled moisture sources within the scaled urban canopy on the overall surface energy balance. 2. Methods 2.1. Description of the hardware model The open-air scaled urban surface (OASUS) has been described previously by Pearlmutter et al. (2005) as a novel approach for physically modelling the influence of urban structure on surface energy balance. It was emphasized that the use of this type of hardware model to compare simplified urban-like configurations offers more flexibility than on-site urban observations, where the ability to draw generalized conclusions is limited by the complexity of actual cities, and can yield more realistic results than either laboratory or mathematical modelling that tend to isolate or oversimplify complex and interrelated climatic processes. The OASUS model is located at the Sede-Boqer Campus of Ben-Gurion University, in the arid Negev region of southern Israel (30 50 N, E; 475 m elevation). Experiments were performed in the summer season, which is characterized by strong diurnal temperature fluctuations (average daily range of C in July), dry air and virtually no cloud cover (global radiation averages 7.7 kwh m 2 day 1 during June and July). Prevailing winds in summer are consistently from the north-west (NW) and strongest in the late afternoon and evening (Bitan and Rubin, 1991). The basic model arrangement consists of an extensive small-scale urban building/street array, composed of 0.2 m 0.2 m 0.4 m hollow concrete masonry blocks, set in parallel rows on a level surface of compacted local loess soil. A homogeneous fetch of about 17 m is created by building rows upwind of the main measurement location, which are uniform in height and spacing and

3 ROLE OF EVAPORATION IN THE URBAN ENERGY BALANCE 913 aligned perpendicular to the prevailing north-westerly summer wind (Figure 1). The entire array is constructed at a flat open site, unobstructed for at least 250 m upwind. The experiments were made with two different geometrical configurations, one with a single-height layer of blocks (H = 0.2 m) and the second with a doubled height (H = 0.4 m). This represents a height-to-fetch ratio of about 0.01 for the shallower array and double that for the deeper array. These two arrangements correspond to values of λ = 0.21 and 0.42 for the frontal area density, which is defined as the ratio of the total windward area of buildings to the total plan area, and has been found to correlate with surface roughness (Grimmond and Oke, 1999c). The materials used in the hardware model were found to have thermal and optical properties analogous to those of common local construction materials in the Negev Figure 1. Layout of regular array (a) showing location of evaporation pans and instrument mast, and (b) view of double-height block rows with evaporation pans. region. An in-depth analysis of the model s thermal inertia showed that the overall heat capacity of the active surface, as well as the distribution of mass between built and subsurface elements, is representative of typical lowrise medium-density residential construction (Pearlmutter et al., 2005). This preliminary analysis showed a semilogarithmic vertical wind speed profile to develop under neutral stratification between 2 and 5 times the canopy height, indicating the location of an inertial sub-layer (ISL) in which spatially averaged surface energy fluxes could be measured with negligible net heat advection. The results of these previous experiments also showed that the diurnal partitioning and temporal development of surface energy fluxes followed a distinctly urban hysteresis pattern, with flux totals and ratios falling within the ranges observed in actual cities [based on the multicity hydrometeorological database of Grimmond and Oke (1999b)] The addition of moisture to the scaled array As mentioned, previous experiments with the hardware model were limited to the analysis of energy exchanges in the absence of significant surface moisture. Measurements showed the latent heat flux density above the scaled urban surface during summer to be under 10 W m 2 at all hours, reflecting the dryness typical of the Negev landscape; field experiments in the region have shown that the dry loess soil has an average volumetric water content of only 2% during the summer months (Ninari and Berliner, 2002). In an actual urban scenario, more substantial latent heat fluxes may be generated by sources such as irrigated trees, shrubs, grass or other vegetative ground cover (as well as open pools and evaporative coolers). The simulation of these elements at small scale, however, is problematic due to the complex physical structure of the plants and the dependence of transpiration on actual leaf area. Therefore, a proxy method was sought which could represent with sufficient similarity the spatially averaged evapotranspiration from a given area of irrigated landscaping. One known approach to this problem is to measure the rate of water loss directly from evaporation pans, which can be correlated with evapotranspiration from well-watered vegetation over a sufficient time period (Brutsaert, 1982). While micro-scale measurements using pans or lysimeters are usually precluded in urban areas due to the extreme heterogeneity of the landscape, the regular structure of the OASUS model made the employment of such pans a more viable option. (It may be noted that for small-scale wind tunnel models, methods have recently been developed to measure turbulent flux coefficients in an urban array using naphthalene sublimation (Barlow and Belcher, 2002) and water evaporation (Narita, 2007), though the effects of radiative loading and heat storage are obviously absent from these studies.) In the present study, evaporation pans of defined area (0.1 m wide and 2.0 m long, with a depth of 0.03 m)

4 914 D. PEARLMUTTER ET AL. were fabricated from galvanized sheet metal and installed in the ground surface of the model array adjacent to the concrete block rows. The pans were sunk in the soil and refilled with water on a daily basis in order to guarantee a constant water surface exposure, and the extent of evaporating area was varied by adding one or two pans to the repeating array module for each geometric configuration (Figures 1 and 2). The evaporation rate was monitored by weighing small control pans (0.24 m 0.10 m 0.03 m) located at a series of points (3- and 6-row intervals, respectively, for single- and double-pan configurations) along the central axis within the array and at a reference point upwind. Hourly rates of evaporation from the upwind reference pan and from a standard Class A evaporation pan at an adjacent meteorological station were measured simultaneously over the course of a typical summer day, and found to be similar in terms of both the daily total and the hourly variations during the daytime (Figure 3) Surface energy flux measurements In the absence of significant anthropogenic heat flux (Q F ) and assuming that net heat advection ( Q A ) is negligible in the ISL, the urban surface energy balance may be expressed as: Q = Q H + Q E + Q S (1) where Q is net all-wave radiation, Q H and Q E are the respective turbulent fluxes of sensible and latent heat, and Q S is the storage heat flux (net gain or loss of energy in urban canopy air layer, buildings, vegetation and ground). The latent heat flux component of the energy balance (Q E, expressed in W m 2 ) is (with appropriate corrections) directly equivalent to its hydrological counterpart E (Arnfield, 2003), which is the hourly evaporation rate measured in mm h 1 (or kg m 2 h 1 ). The spatially averaged value of Q E is computed as: Q E = L V E(A W /A H ) (2) Figure 2. Schematic layout of the scaled array looking from the north, with the repeating module detailed in three dimenssions. The surface area of evaporation pans is represented by A W1 for the single-pan configurations and by A W1 + A W2 for the double-pan configurations. Similarly the windward façade area is represented by A F1 for single-height arrays and by A F1 + A F2 for double-height arrays, and A H is the horizontal area. Figure 3. Correlation of cumulative evaporation (E) in small upwind reference test pan with that measured simultaneously in standard Class A evaporation pan. where L V is the latent heat of vapourization (which varies with temperature and pressure, and at 30 C and 100 kpa equals 2.43 MJ kg 1 with a water density of kg m 3 )anda W /A H is the proportion of water surface area to total horizontal area. The value of E was measured hourly during the daytime (and as a sum total for the night hours) by weighing the control pans and calculating the volumetric water loss from the change in mass over the period, and taking the average value for all the control pans measured simultaneously (i.e. within approximately 5 min of the given hour, as required to manually remove, weigh and return them without disturbing other instruments). As a secondary measure, surface-atmosphere fluxes of latent heat were measured by eddy covariance with an open-path gas analyser (Licor LI-7500) and sonic anemometer (Campbell CA27) placed at a height of 0.9 m (in the ISL, between 2 and 5 times the canopy height), with a sampling frequency of 10 Hz and averaging period of 5 min. However, these Q E values proved less reliable than those computed from the actual pan evaporation and were thus used exclusively to estimate hourly variations of Q E at night (when both absolute values, and differences between the totals yielded by the two approaches, were minimal) and in dry mode (prior to installation of evaporation pans). The relatively low Q E values produced by eddy covariance may reflect asynchrony between the high-frequency signals from the two instruments (Wolf et al., 2008), as well as loss of low-frequency turbulence from larger eddies due to the averaging period (Kaimal and Finnigan, 1994; Finnigan et al., 2003) which is shorter than that recommended over full-scale rough urban surfaces (Roth, 2000).

5 ROLE OF EVAPORATION IN THE URBAN ENERGY BALANCE 915 The sonic anemometer with its integrated fine-wire thermocouple was used as the primary means for measuring the turbulent sensible heat flux Q H. Net all-wave radiation was measured with an REBS (model Q7-1) net radiometer, placed together with upward- and downwardfacing Kipp and Zonnen pyranometers at a height of 1.5 m (where surface albedo was found to be spatially homogeneous). All instruments were located approximately 17 m downwind of the array s leading edge (Figure 1(a)). The storage heat flux Q S was calculated primarily as the residual in the energy balance (Equation (1)), though supplemental measurements were made in dry mode (before installation of evaporations pans) using heat flux plates embedded in the soil surface and in thin cement plaster covering the blocks vertical and horizontal facets. 3. Results and discussion 3.1. Energy balance closure Measurements of energy uptake and release from the ground and scaled building facets made with embedded heat flux plates provided a measure of the energy balance closure, or the extent to which all flux components were accounted for prior to introduction of the evaporation Figure 4. Daily surface energy fluxes for four array configurations in dry mode (prior to installation of evaporation pans), showing energy balance closure as represented by correlation between storage heat flux as measured and as calculated as a residual. Fluxes shown for (a) single-height and (b) double-height arrays (canyon diagrams show frontal area density and direction of prevailing wind). pans. As seen in Figure 4, these measured storage heat flux values are closely matched with residual Q S values for most hours in both single- and double-height arrays (canyon diagrams in Figures 4 7 show frontal area density and direction of prevailing wind). While storage heat flux as a residual is noticeably overestimated during early afternoon, the mean hourly deviation from measured Q S over the daily cycle is below 10 W m 2 in both array configurations Daytime evaporation rates Figure 5 shows cumulative evaporation from test pans in four different array configurations, over the course of the summer daytime hours. Simultaneous evaporation from the reference pan upwind of each array is also shown, reflecting ambient conditions that were similar for the first two configurations (single-height arrays), and for the second two configurations (double-height arrays), but somewhat different between these two periods. The observed differences of 1 2 mm in daytime evaporation may be attributed to prevailing conditions that were warmer and drier during the one-story measurement period, with daytime peak temperatures of approximately 35 C and relative humidity as low as 20 25% while in the second period temperatures reached only C and about 35% humidity (as measured at screen height at the adjacent meteorological station). It may be seen that when evaporation pans were placed only on the downwind side of the street (adjacent to the NW face of the blocks, Figure 5(a) and (c)), hourly evaporation rates were virtually identical for all six evenly-spaced points of measurement along the central axis of the array. With pans on both sides of the repeating module (Figure 5(b) and (d)), hourly rates diverged considerably depending on the exposure of the water surface: pans with a south-east (SE) exposure had much higher rates in the morning, such that their cumulative total at noon was as much as double that of the NW-facing pans. In the afternoon hours the relationship reversed, such that similar daytime totals were reached for all points in the array. The asymmetry of these temporal patterns clearly indicates the effect of direct radiation, with hourly evaporation closely correlated to direct solar incidence. A secondary influence of wind exposure may be detected by comparing single- and double-height arrays. In the former case, evaporation rates are especially high for NW-facing points during the afternoon hours, when wind speeds from that direction are substantial such that the daytime total for points of this orientation slightly surpasses those facing SE. This crossover does not occur in the case of a double-height array, where an increase in canyon aspect ratio from 0.33 to 0.66 suggests that air flow would follow a wake interference rather than an isolated roughness pattern (Oke, 1987), and the influence of directional exposure to wind would presumably be less pronounced owing to shelter effects. When taken as a per-pan average, the rate of evaporation is not appreciably changed for an array of a given

6 916 D. PEARLMUTTER ET AL. Figure 5. Cumulative evaporation during summer daytime hours, for four different array configurations: single-height (a and b) and double-height (c and d) arrays, with single (a and c) and double (b and d) evaporation pans. For each configuration simultaneous evaporation is shown for a series of points within the array and for the upwind reference pan. height when the water area is doubled (though the total surface evaporation is of course greater owing to the larger evaporating area). However, a significant quantitative difference between array configurations observed in Figure 5 is the lower daytime evaporation in doubleheight arrays, which totals only 7 8 mm, or 70 80% of the upwind reference. Evaporation in the shallower array, where water surfaces are more directly exposed to both radiant and turbulent effects, reaches a daytime total of about 10 mm, or about 85% of the simultaneously measured reference Surface energy balance Figure 6 shows the diurnal progression of surface energy flux components for the four array configurations with evaporation pans. Storage heat flux values are derived as the residual in the energy balance, and all other fluxes are measured (as described in Section 2). Given the relative consistency of meteorological conditions during the summer season, incoming short-wave radiation K varied by under 10% on a daily basis during the overall measurement period (August 2006). All hourly fluxes were normalized to account for these small deviations, by dividing the given value by the ratio between the simultaneously measured K and the mean K value for the same hour on all measurement days. The daytime (Q > 0) and daily (24 h) flux totals and flux ratios are summarized in Table I for arrays in both dry and wet modes (prior to and following the installation of evaporation pans). As observed in previous experiments (Pearlmutter et al., 2005) and in dry mode (Figure 4), both daytime and daily total net radiation are consistently higher in the higher-density (double-height) array than in the shallower arrangement. This is due primarily to the lower surface albedo of the deeper array and consequent trapping of short-wave radiation in the daytime, and to some extent by diminished long-wave radiant loss due to the relatively constricted sky view factor of its scaled street canyons. In all four array configurations with evaporation pans, a typical hysteresis pattern is observed in the temporal progression of the storage and turbulent sensible heat flux components, with Q S reaching a maximum in midmorning and Q H peaking in the early afternoon. The peak value of Q E is also reached in the afternoon, lagging that of Q H by up to 1 h. As may be expected, latent heat flux reached increasingly higher daytime peaks with the addition of freewater surface area. For single-height configurations (Figure 6(a) and (b)), a single evaporation pan per module (A W /A H = 0.1) produced a maximum of nearly 100 W m 2 and a doubling of the available water (A W /A H = 0.2) yielded a peak value that was two-thirds higher. In the double-height configuration, the larger

7 ROLE OF EVAPORATION IN THE URBAN ENERGY BALANCE 917 Figure 6. Time series of daily surface energy fluxes for four array configurations with varying block height and evaporation area. TableI. Daytime(Q > 0) and daily (24 h) surface energy flux totals and flux ratios for both dry and wet array configurations (the latter with varying proportional area of evaporation pans). Energy fluxes (MJ m 2 day 1 ) Flux ratios Q Q H Q E Q S Q H /Q Q E /Q Q S /Q β = Q H /Q E Daytime (Q > 0) 1-storey (λ F = 0.21) A W /A H = A W /A H = A W /A H = storey (λ F = 0.42) A W /A H = A W /A H = A W /A H = Daily (24 h) 1-storey (λ F = 0.21) A W /A H = A W /A H = A W /A H = storey (λ F = 0.42) A W /A H = A W /A H = A W /A H = water surface produced a similar peak value (approximately 166 W m 2 ), which was a full 100% increase over the peak value of about 80 W m 2 for this denser array with only half the water area (Figure 6(c) and (d)) Latent heat flux versus vegetated fraction The relationship between latent heat flux and evaporating area may be quantified by the correlation between total

8 918 D. PEARLMUTTER ET AL. Q E (over an appropriate time period) and the vegetated fraction, or the relative horizontal area responsible for evapotranspiration within the urban fabric. Evapotranspiration from vegetation has been correlated with water loss from Class A evaporation pans in a large number of observational studies, and as summarized by Brutsaert (1982), these empirical findings suggest a pan coefficient of 0.8 as a usefully representative value (representing the ratio of evapotranspiration from well-watered grass to pan evaporation). Owing to the temporal variability in transpiration from the stomata of leaves, however, this correlation is only applicable at time periods of at least a full diurnal cycle. This pan coefficient was applied to the scaled array by calculating the vegetated fraction (A V /A H ) as the actual proportion of horizontal area covered by evaporating water (A W /A H ), divided by 0.8 (i.e. increased by 25%). In Figure 7, the different vegetated fractions measured are used as a basis for comparing variations in the total daily latent heat flux component Q E /Q, along with simultaneous variations in the sensible and storage heat flux components. It can be seen that for both single- and double-height arrays, Q E /Q increases in an approximately linear fashion with increasing A V /A H from a near-zero value in the absence of pans to values of for the single-pan configuration (A V /A H = 0.13), and to for the double-pan array (A V /A H = 0.26). These values may be compared with observed correlations in actual urban areas, such as those reported by Grimmond and Oke (2002) for North American cities with a wide range of vegetated fraction values (but with a conspicuous lack of data points in the range of ). The trend line passing through this range would suggest a Q E /Q value of about 0.20 for a vegetated fraction of 0.13, increasing to a value of about 0.30 (with points scattered up to 0.50) for higher fractions. Thus the Q E /Q values in Figure 7 are roughly in line with these urban field studies, though the values yielded by the double-pan array fall at the high end of the expected range possibly reflecting the especially high potential for evapotranspiration in the arid conditions of the Negev region. The systematic increase of latent heat flux with vegetated fraction in the surface energy partitioning means that an increasing portion of energy is being removed from the system, which would otherwise either be dissipated as sensible heat or stored within the urban fabric. Figure 7 shows that both these flux components (Q H /Q and Q S /Q ) are decreasing in a near-linear fashion as evaporation increases, though the decline in net storage heat flux follows a slightly steeper slope in both singleand double-height arrays, reaching minimum Q S /Q values of The implication is that additional surface evaporation does enhance the rate of heat removal from the system (i.e. diminished heat storage), but that a large portion of the energy would in any case be dissipated by turbulence (as sensible, rather than latent, heat). The latter point is illustrated by the substantial decreases (a) (b) Figure 7. Correlation between 24-h flux ratios and the vegetated fraction of single-height (a) and double-height (b) arrays. Flux ratios shown as individual data points and corresponding linear trend lines. in the Bowen ratio (β = Q H /Q E ) seen in Table I, with 24-h values of β decreasing by about 60% when available surface moisture is doubled, and in the case of maximal moisture reaching values close to or less than 1.0. It may also be observed that the effect of surface moisture on increasing Q E /Q, and decreasing Q H /Q and Q S /Q, is considerably more pronounced when the array height is doubled. This could be taken as an indication that a larger 3-D surface, particularly one dominated by vertical, non-vegetated wall surfaces, would preferentially increase sensible heat exchange and diminish the role of latent heat within the energy balance (owing to greater interception of solar radiation, and the combined removal of heat by wind turbulence and conduction of heat into the material). To quantify this relationship, the vegetated fraction may be described as the ratio of A V to the 3-D, or complete urban surface area A C (Christen and Vogt, 2004), which is the total exposed area of horizontal and vertical array facets in the repeating array module. Thus in Figure 8(a) the variation in daily Q E /Q is shown for increasing values of the complete vegetated fraction A V /A C, with a near-linear correlation emerging from the measured values. This correlation, however, is based on latent heat totals from different measurement periods, during which meteorological conditions were not identical (see Section 3.2).

9 ROLE OF EVAPORATION IN THE URBAN ENERGY BALANCE 919 geometric dependency of Q. As described previously, net radiation has been found to increase systematically with array density, primarily on account of the lowered albedo and radiation trapping in deeper canyons. To compensate for this, Figure 8(b) shows a comparison similar to that in Figure 8(a) but with Q E as a flux density rather than as a proportion of Q (though all values are normalized, as mentioned previously, for incoming short-wave radiation). Here the correlation between latent heat and complete vegetated fraction is somewhat less pronounced, especially after normalization for differences in meteorological conditions between measurement periods. 4. Conclusions The above-mentioned experimental findings may be summarized in a number of operative conclusions, as follows: Figure 8. Correlation between daily latent heat flux and the complete (three-dimensional) vegetated fraction, as a proportion of net radiation (a) and as an absolute value (b). Values are shown both with and without normalization for varying meteorological conditions. Therefore, the Q E data for each array configuration were normalized with respect to the daily evaporation rates measured simultaneously in the upwind reference pans, which reflect the observed variations in meteorological conditions. By dividing the daily (24 h) total Q E by the ratio between the simultaneous reference value (Q E ref ) and the mean Q E ref for all of the daily measurement periods, normalized daily Q E totals were derived for each configuration that varied by up to 10% from the actual measured values. As seen in Figure 8(a), this normalization does not substantially alter the linear correlation between Q E /Q and A V /A C, and thus the availability of water with respect to the complete 3-D urban surface area does indeed correlate closely with the dissipation of radiant energy as latent heat. An additional factor influencing the comparison of Q E /Q between arrays of varying geometry is the 1. The OASUS model provides a practical means for quantitatively assessing not only dry surfaceatmosphere energy fluxes but also latent heat flux from defined moisture sources. The estimation of storage heat flux as a residual was validated using heat flux plates to measure the energy balance closure prior to the addition of evaporation pans, and evaporation from small test pans was found to be highly similar in its daily progression and total to that measured simultaneously from a standard Class A pan. 2. The latent heat flux reached peak mid-day values of about 160 W m 2 with the maximum supply of surface water (20% coverage of horizontal area), and with 10% water coverage, peak Q E values varied between 80 and 100 W m 2 depending on array geometry. 3. As expected, the daily total latent heat flux increased consistently with consecutive additions of surface water, which were expressed as increasing vegetated fractions using a pan coefficient to relate pan evaporation to evapotranspiration from vegetative ground cover. When taken as a fraction of the net radiation budget (Q E /Q ), this increase in latent heat removal with respect to vegetative surface area was nearly linear. 4. Increasing latent heat flux with vegetated fraction was offset in approximately equal measure by decreases in storage ( Q S ) and turbulent sensible heat flux (Q H ). The latter is also represented by a declining Bowen ratio, which, in the single-height array with the maximum evaporating area, dropped below unity. This reflects the fact that for these wetter conditions, more incoming radiation is required by the process of evaporation than is required by its conversion to turbulent sensible heat. In this same configuration, however, the daily total storage flux was a mere 7% of net radiation. 5. The proportion of the radiant energy budget represented by evaporative heat loss was also linked to the 3-D geometry of the array, as the value of Q E /Q

10 920 D. PEARLMUTTER ET AL. increased linearly with the proportion of vegetative cover to the complete urban surface area. This correlation was only marginally affected when normalizing for differences in meteorological conditions between measurement periods. It may be largely explained, however, by the effects of array geometry on the radiant income itself: as the deeper of the two arrays has a higher net radiation balance, its absolute latent heat flux total is not substantially higher than that measured in the shallower array. Acknowledgements The authors wish to acknowledge the valuable contribution made to the research by Mr Wolfgang Motzafi- Haller. References Arnfield JA Two decades of urban climate research: a review of turbulence, exchanges of energy and water, and the urban heat island. International Journal of Climatology 23: Barlow JF, Belcher SE A wind tunnel model for quantifying fluxes in the urban boundary layer. 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