IGBP/BAHC field experiment to address scale problems in land-surface parameterization

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Exchange Processes at the Land Surface for a Ranee of Space and lime Scales (Proceedings of the Yokohama Symposium, July 1993). JAHS Publ. no. 212, 1993.

1 Exchange Processes at the Land Surface for a Ranee of Space and lime Scales (Proceedings of the Yokohama Symposium, July 1993). JAHS Publ. no. 212, IGBP/BAHC field experiment to address scale problems in land-surface parameterization M. SUGITA, I. KAYANE & T. HIYAMA Institute of Geoscience, University of Tsukuba, Tsukuba, lbaraki 305, Japan Abstract A three-week intensive field campaign for the IGBP/BAHC program in Japan took place in the summer of 1992 at and around Tsukuba, Japan. The generally flat 16 km by 16 km experimental area can be characterized by the complexity of its land cover that includes various crops, pine tree woods, scattered houses and buildings, paddy fields, and so on. Except for the paddy fields which extended along the rivers, horizontal scale of each surface type was typically of the order of ÎO'-IO 2 m. Five flux stations installed at all major surface types in the area, allowed determination of the local scale surface fluxes, while atmospheric profilings by means of a 200-m meteorological tower, a tethered balloon, and a radiosonde are expected to yield the regional ( m) mean fluxes. In this paper, a summary of the experiment and a brief description of some of the initial findings are presented. INTRODUCTION One of the major concern in current hydrology is the lack of information on how one can aggregate and/or disaggregate various important surface fluxes such as heat, momentum and water vapor from one scale to another. Indeed, such knowledge is essential to make use of existing and accumulated data sets and findings from many previous small-scale hydrologie studies, in a different scientific field such as atmospheric sciences where the scales of problems are somewhat different. To shed some light on this topic, an intensive field campaign was organized in the frame work of International Geosphere-Biosphere Programme/Biospheric Aspects of the Hydrological Cycle (IGBP/BAHC) project in Japan, to acquire a data set which should allow the determination of the surface fluxes at different scales, ranging from 10 1 to 10 4 m, and the investigation on the interrelationship between them. The field phase of the experiment has just been completed and a preliminary data analysis appears to yield some interesting, promising, and yet challenging results. TSUKUBA 92 The field experiment was carried out as part of a larger campaign called Tsukuba 92 (Oki et al., 1993; Sugita et al., 1993). Briefly, it was a joint effort of some ten scientific teams from different scientific projects and groups which include the IGBP/BAHC project and the Subcommittee on Hydrological Processes within the Earth Environment Observation Committee of the Remote Sensing Technology Center of

2 390 M. Sugita et al. Japan, among some others. The three-week field campaign took place for 4-24 August, 1992, in a generally flat 16 km by 16 km experimental area (Fig. 1) at and around Tsukuba city, located some 60 km northeast of Tokyo in the Kanto plain. The experimental area is characterized by the complexity of its surface land cover, which includes patches of woods, open agricultural terrain, scattered houses and small buildings, parking lots, and paddy fields in the lower part of the area. EXPERIMENTAL SETTING Surface Flux Stations In order to determine the local scale surface fluxes, flux stations were installed in five selected sites shown in Fig. 1. In three locations, i.e., one at a pasture field (station A), one in a pine forest (B), and one at an empty parking lot (C), the sensible heat fluxes H were determined directly by means of the eddy correlation method with a sonic anemometer-thermometer; then the latent heat fluxes LE ( = le E where le is the latent Fig. 1 A map showing the 16 by 16 km experimental area. Closed circles represent the surface flux stations, and the square indicates the 200-m tower at the Meteorological Research Institute. Also shown are contour lines of 20 m, rivers, major national roads, and railway.

3 IGBP/BAHC field experiment to address scale problems 391 heat for vaporization and E the evaporation) were evaluated by considering the energy balance with the net radiation Rn and the soil heat flux G. At two other locations at a short grass site (D) and at a paddy field (F), the surface fluxes were determined by the Bowen ratio/energy balance method. Tethered sonde measurements of the lower ABL profiles Measurements of the lower atmospheric boundary layer (ABL) profiles were made at station A by means of a tethered sonde equipped with an anemometer, a barometer, and a set of ventilated wet-and dry-bulb thermometers, from the surface up to some 150 m. Each flight consists of ascending and descending operations of the sonde, during which the sonde was tethered at a fixed height every 5-10 m for one minute. Data acquisition was made every 10 seconds during the flight and it generally took approximately one hour to complete the flight. Observations during the daylight period (i.e., between 9 and 17 Japan Standard Time (JST), i.e., 840 and 1640 LST), when the wind speed was not too strong, produced 64 sets of complete profiles. Radiosonde profiling Some 76 radiosondes were released from station A to obtain profiles of potential temperature 6, specific humidity q, and wind speed V for the entire ABL. The profiles of 6 and q were determined from measurements of pressure and dry-and wet-bulb temperatures, while the V profile was obtained by tracking the movement of a balloon by an optical theodolite. Since they are expected to be used in conjunction with satellite radiance data, the sounding schedule was determined so that release timings corresponded with satellite overpasses over the experimental area. Flux profile determinations by means of a 200-m tower A 200-m meteorological tower of the Meteorological Research Institute (see Fig.l) was used as a platform to determine the profiles of 8 and q as well as those of heat and momentum fluxes by means of an eddy correlation method; necessary data were obtained from sets of a ventilated thermometer, a thin film humidity sensor, and a sonic anemometer-thermometer installed at 7 levels (10, 25, 50, 100, 150, 200, and 213 m) of the tower (Hanafusa et al., 1979). Satellite radiance data In order to investigate a way to utilize satellite data for the determination of regional surface fluxes, some of the available satellite data during the experiment were acquired. The sensors and satellites suitable for this purpose include, AVHRR on board NOAA-11, TM of Landsat-5, the Visible and Thermal Infrared Radiometer (VTIR) of the Marine Observation Satellite-1 (MOS-1), HRV of SPOT, and VISSR of the Geostationary Meteorological Satellite (GMS).

4 392 M. Sugita et al. Table 1 Land use of the experimental area. Whole area Area over the fetch East South (%) (%) (%) Short vegetation Tall vegetation Paddy fields Asphalt and concrete surfaces Water surfaces SOME INITIAL FINDINGS Land cover of the experimental area The latest set of 64 aerial photographs (1:10000), which covers the entire experimental area, and which was taken in November, 1990 by the Geographical Survey Institute, was used to determine the land cover types of the area. These were classified into 5 categories, i.e., (a) short vegetation which includes grass, agricultural crops, and brush, (b) tall vegetation such as pine tree woods, (c) paddy fields, (d) asphalt and concrete surfaces, and (e) water surfaces, by visually examining the photographs and plotting them on a topographic map. Then, the proportion of each land cover type was determined by measuring the area on the map. The land cover in the upwind fetches is probably the most important for the consideration of regional heat fluxes by means of the ABL profile analysis. Since the predominant wind directions during the experiment were south and east, the survey was carried out, in addition to the whole area, to two fan-shaped areas over the fetches extending 8 km in the directions ±15 of east and of south from the station A where the ABL profiles were obtained. The result is listed in Table 1 and it can be seen that the southern part of the area is characterized by the presence of somewhat larger area of short vegetation, while the paddy fields occupy more area in the eastern part. Differences of surface heat balance due to the land cover Examples of the diurnal variation of the energy balance components on August 17 at three selected stations (Station A, B, and C) are shown in Figs 2-4. A comparison indicates that pine tree woods received the largest net radiation, due probably to smaller albedo and to cooler canopy, and thus generated the largest fluxes of heat and water vapor among three. The smallest value of the Bowen ratio (Bo = H/LE) was obtained at station A on the pasture field, followed by station B, D, and C (where LE=0 was assumed).

5 IGBP/BAHC field experiment to address scale problems 393 Fig. 2 An example of the diurnal variation of the energy balance component observed at station A Time (JST) Fig. 3 Same as Fig. 2 at station B. The soil heat flux was estimated as cr Rn where cr is the constant experimentally determined for the site as 0.02 (Sugita, 1984) Time (JST) Fig. 4 Same as Fig. 2 at station C

6 394 M. Sugita et al. Lower boundary layer characteristics Figures 5-7 show examples of the profiles of 6, q, and V, measured between 1237 and 1333 JST on August 17, during which the dominant wind direction was east. Visual inspection of these figures reveals that the surface sublayer of the ABL appears to extend from the surface up to some m, above which 9 begins to increase with height, and q and V stay more or less the same, showing the characteristics of the outer region of the ABL. The extent of this surface layer seems to agree with previous findings. For example, Brutsaert & Sugita (1990) and Sugita & Brutsaert (1992) determined the height range of the layer over a complex hilly grassland as 46 (±32) < (z-d)/z < 101 (±51), where z is the height above the ground, d the displacement height, and z the roughness length. For the experimental area these heights have been determined as d=7.19 m and z o =0.24 m (Nasuno and Nishizawa, 1987), and they would yield the height range of 18 (±7) m < z < 31 (±12) m , (K) Fig. 5 An example of 0 profile. The circles are for an ascending flight, and the triangles for a descending flight. '(D I f q (g/kg) Fig. 6 An example of q profile. The symbols are the same as in Fig. 5.

7 IGBP/BAHC field experiment to address scale problems V (m/s) Fig. 7 An example of V profile. The symbols are the same as in Fig \ S 297 ' JST q (g/kg) 17.8 Fig. 8 An example of plot of 8 versus q. The symbols are the same as in Fig. 5, and the number beside each point denotes the height above the ground. Relationship between regional Bowen ratio and that at local scale The combination of the profiles of 6 and q in the lower ABL allows a determination of the regional Bowen ratio, if the two profiles are similar. The similarity appears to be satisfied for some part of the surface sublayer (Sugita & Brutsaert, 1990), and the idea was tested for the experimental area with one set of the profiles shown in Figs. 5-7, by comparing the profile-derived Bo values with those observed at the surface stations (Fig.8). It can be seen that a couple of points measured in the surface sublayer appear to lay on a straight line and this implies the similarity. On the other hand, there are also one or two points which exhibit an irregular feature, and clearly more studies are needed before this technique can be applied with confidence. Nevertheless, by using some of the points on a straight line (denoted with closed circles and triangles in the figure), tentative values of the regional Bo were determined, such as Bo=0.262 for the ascending flight and Bo=0.207 for the descending flight. These values can be compared with local Bo-values shown graphically in the figure; the slopes of the

8 396 M. Sugita et al. straight lines in the insets represent values of Bo < C p "' (where C p is the specific heat at constant pressure) observed at the five surface stations, indicated by their names, at the start and at the end of the flight. A comparison of the two types of the slope, i.e. one for local scale Bo-value, and another one for regional Bo-value, shows that the regional Bo is in the middle of other Bo values observed at the surface, implying that the fluxes of different magnitude from the various land surfaces are mixed and merged into regional fluxes and thus a profile at higher elevation reflects only average, regional fluxes and does not see the differences of the surfaces. CONCLUDING REMARKS The field phase of the IGBP/BAHC field experiment has just been completed at and around Tsukuba, Japan in the summer of An initial analysis of part of the data sets acquired during the campaign produced some interesting and promising findings to address scale problems in the studies of the land-surface/atmosphere interface, although it is clear that more research is needed to draw more concrete conclusions. Acknowledgements This research has been supported and financed, in part, by the Grant-In-Aid for Scientific Research for IGBP to Univ. of Tsukuba from the Ministry of Education, Science and Culture. Station C and F were operated by teams led by Tian Shao-Fen of Univ. Tsukuba, and by Yoshinobu Harazono of National Inst, of Agro-Environmental Sciences, respectively and we would like to thank them for their providing data sets. REFERENCES Brutsaert, W. & Sugita, M. (1990) The extent of the unstable Monin-Obukhov layer for temperature and humidity above complex hilly grassland. Boundary-Layer Meteorol., 51, Hanafusa.T., Fujitani, T., Banno.N., &Uozu, H. (1979) On the meteorological tower and its observational system at Tsukuba science city. Tech. Rep. Meteorol. Res. Inst., no. 3. Meteorological Research Institute, Ibaraki, Japan. Nasuno, M. & Nishizawa, T. (1987) On the wind profiles and aerodynamic parameters at the experimental field of ERC. Bull. Environ. Res. Center, Univ. Tsukuba, no. 11, 117. Oki, T., UedaS., Ohte, N., Kai, K., Kayane, I., Kawamura, R., Koike, T., Kotoda, K., Kondoh, A., Shimada, J., Sugita, M., Tanaka, T., Tian, S-F., Harazono, Y., Fukami, K., & Yasunari, T. (1993) Tsukuba 92: an experiment to address scale problems in land-atmosphere interface, (1) soil moisture determinations by various methodology, J. Jap. Soc. Hydrol. Water Resour., in preparation. Sugita, M. (1984) Energy and water balance of a pine forest during a Bai-u and a summer season. J. Agricul. Meteorol. 40, Sugita, M. & Brutsaert, W. (1990) How similar are temperature and humidity profiles in the unstable boundary layer?}. Appl. Meteorol., 29, Sugita, M. & Brutsaert, W. (1992) The stability functions in the bulk similarity formulation for the unstable boundary layer. Boundary-Layer Meteorol., in press. Sugita, M., Ueda, S., Ohte, N., Kai, K., Kayane, I., Kawamura, R., Koike, T., Kotoda, K., Kondoh, A., Oki, T., Shimada, J., Tanaka, T., Tian S-F., Harazono, Y., Fukami, K., & Yasunari, T. (1993) Tsukuba 92: an experiment to address scale problems in land-atmosphere interface, (2) fluxes from surface through ABL to the free atmosphere. J Jap. Assoc. Hydrol. Sci., in preparation.

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