Experimental determination of turbulent fluxes over the heterogeneous LITFASS area: Selected results from the LITFASS-98 experiment

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1 Theor. Appl. Climatol. 73, (2002) DOI /s Meteorological Observatory Lindenberg, German Meteorological Service (DWD), Lindenberg, Germany 2 Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands 3 GKSS Research Center, Geesthacht, Germany 4 Meteorology and Air Quality Department, Wageningen University, Wageningen, The Netherlands 5 Dept. of Micrometeorology, University of Bayreuth, Bayreuth, Germany 6 Institute for Hydrology and Meteorology, Technical University Dresden, Tharandt, Germany 7 MCR Lab, University of Basel, Basel, Switzerland 8 National Institute for Meteorology and Hydrology, Sofia, Bulgaria Experimental determination of turbulent fluxes over the heterogeneous LITFASS area: Selected results from the LITFASS-98 experiment F. Beyrich 1, S. H. Richter 1, U. Weisensee 1, W. Kohsiek 2, H. Lohse 3, H. A. R. de Bruin 4, Th. Foken 5,M.G ockede 5, F. Berger 6, R. Vogt 7, and E. Batchvarova 8 With 8 Figures Received June 6, 2001; revised January 15, 2002; accepted April 4, 2002 Published online November 19, 2002 # Springer-Verlag 2002 Summary During the LITFASS-98 experiment, local flux measurements were performed over five different types of underlying surface (grass, barley, triticale, pine forest, water) in a heterogeneous landscape using eddy covariance and profile techniques over a three week time period in June, Estimates of the area-integrated sensible heat flux during daytime were obtained from continuous measurements with a large aperture scintillometer (LAS) along a 4.7 km path. The calculation of a mean diurnal cycle of the fluxes during the experiment revealed significant differences between the main land use classes. A land-use weighted average of the sensible heat flux was found to be in good agreement with the LAS based estimate, which in turn was supported by other regionally integrated flux estimates from budget considerations and aircraft measurements for a few case studies. The profiles of turbulent quantities measured along a 99 m-tower significantly deviate from idealised profiles measured over homogeneous terrain. Peculiarities in the profile structure could be attributed to the heterogeneity of the terrain, namely to the differences in the surface characteristics of the footprint areas for the different tower levels. 1. Introduction A central issue in climate modelling and numerical weather prediction (NWP) is the implementation of adequate parameterisation schemes for the interaction processes between the atmosphere and the underlying surface with special focus on the proper description of the exchange of energy and momentum. Both the development and validation of parameterisation schemes are usually based on measurements. A special problem to be solved when synthesising measurements and grid models is the harmonisation of scales between the two. With respect to the energy and momentum budget this means that the single components (radiative, heat, and momentum fluxes) derived from measurements have to be available as data representative for the scale of a grid cell of a NWP or climate model, which basically is in the meso- scale ( km) for the present generation of numerical models,

2 20 F. Beyrich et al. but increasingly enters the meso- scale (2 20 km). Over land, the surface at this scale is usually characterised by a certain degree of heterogeneity. The task of area-averaging or scale aggregation of surface parameters and fluxes also needs to be solved for using local (in-situ-) measurements at the atmosphere surface interface as ground truth for the validation of satellite data. A number of field experiments was performed over heterogeneous land surfaces in different geographical and climate regions of the earth over the last years (e.g. Tsvang et al., 1991; Andre, 1995; Sellers et al., 1997; Halldin et al., 1998; Harding et al., 2001). Within field programs, the experimental determination of the energy budget over a locally homogeneous surface is usually based on micrometeorological measurements and modelling techniques, like the eddy covariance, profile, gradient, or Bowen-ratio methods (for a summary, see, e.g. Businger, 1986; Kaimal and Finnigan, 1994; Arya, 2001). It should be mentioned that homogeneity of the surface is only apparently in most cases, since the variability of soil and vegetation parameters (which lies in the micro-scale) are usually neglected. Direct measurements of the area- averaged fluxes are possible with aircraft only (e.g. Mahrt and Ek, 1993; Frech and Jochum, 1999; Mahrt et al., 2001). In addition, alternative methods based on optical path measurements of turbulence properties using a scintillometer (e.g. Green et al., 1994; de Bruin et al., 1995) or on the measurements of mean mixed layer variables and budget considerations (e.g. Barr et al., 1997; Gryning and Batchvarova, 1999) have been applied successfully over the past years to determine area-representative flux values. In 1995, the German Meteorological Service (Deutscher Wetterdienst, DWD) started the LITFASS project in order to contribute to the problem of the area-averaging of fluxes over a heterogeneous landscape. LITFASS can be translated as Lindenberg Inhomogeneous Terrain Fluxes between Atmosphere and Surface: a Long-term Study. The overall project strategy is described in Beyrich et al. (2002b, this issue). Experimental investigations of land surface atmosphere interaction processes and atmospheric boundary layer (ABL) structure within a20km 20 km area with heterogeneous land use form a central part of the LITFASS project. The LITFASS study region is situated in the north-eastern part of Germany, about 60 km to the south east of Berlin (at 14 E, 52 N) at the transition between marine and continental climate of the mid-latitudes. The landscape in the area is characterised by a slightly undulating surface, the land use is dominated by forest and agricultural fields (40% to 45% each), lakes cover 6 7%, villages and traffic roads less than 4%. The overall LITFASS project strategy was tested over a three-weeks period during the LITFASS-98 experiment. Different experimental and semi-empirical modelling techniques were used to determine the turbulent fluxes of heat and momentum both over locally homogeneous surfaces and as area-integrated values. A general overview on the experiment is given in Beyrich et al. (2002b, this issue). The present paper gives a synthesis of the flux measurements performed during the experiment. The sites and measurements used for flux determination are characterised in section 2. In section 3, results are presented on the variability of the fluxes measured locally over different types of land use. Flux profiles and area-integrated flux values in relation to the local measurements are discussed in section 4 and 5, respectively. Finally, from a summary of the basic results, conclusions are derived concerning the future realisation of the flux measurement program within the LITFASS project. 2. Methods, measurements and instrumentation According to the overall goals of the LITFASS project, special emphasis in the experiment was put on the determination of turbulent fluxes using different direct and indirect techniques. A summary of the operational characteristics of the different methods employed during the LITFASS-98 experiment is given in Table 1. It should be noted that seven magnitudes of spatial sampling scales and about five magnitudes of footprint area scales are covered by the different measurement systems and methods. The present paper will focus on the discussion of the local flux measurements using eddy

3 Experimental determination of turbulent fluxes over the heterogeneous LITFASS area 21 Table 1. Operational characteristics of the methods used for flux determination (momentum flux M, sensible heat flux H, latent heat flux l v E) in the LITFASS-98 experiment Method (Instruments) Fluxes determined Continuous operation Sampling scale Footprint scale single leaf evaporation (chamber) l v E no 10 2 m eddy covariance (sonic) M, H, l v E yes 10 1 m m profile measurements (mast) M, H, l v E yes 10 1 m m remote sensing systems M, H, l v E (yes) m m (wind profiler, sodar, lidar) laser scintillometer M, H yes 10 2 m m large aperture scintillometer H yes 10 3 m m (bulk) mixed layer model H no 10 3 m m slow aircraft (Helipod, DO128) M, H, l v E no 10 4 m m fast aircraft (Falcon) M, H, l v E no 10 5 m m Table 2. Site characteristics of the micrometeorological sites Site Falkenberg I Lindenberg Herzberg Wulfersdorf Kehrigk Falkenberg II surface type grass barley triticale water pine forest bare soil=maize altitude asl 71 m 89 m 90 m 43 m 49 m 72 m distance from 3km 5km 3km 12km <1km Falkenberg I site undisturbed fetch sector vegetation height leaf area index m, after June 16: 0.10 m 1 3 (estimated) m m no vegetation 13 m 0.10 m no vegetation 5 (estimated) 1.0 covariance and profile techniques and on the area-integrated heat flux estimates based on the operation of a large aperture scintillometer. Airborne measurements will be discussed in more detail in a separate paper (Bange et al., 2002 this issue). An additional paper of the present series is devoted to the analysis of the wind profiler and sodar data including flux estimation (Engelbart and Bange, 2002 this issue). 2.1 Local flux measurements Local micrometeorological and turbulence measurements were carried out at five (temporarily six) sites in order to characterise the interaction between the atmosphere and different types of the underlying surface (grass, agriculture, forest, water). Measurements included radiation, soil and air parameters, and turbulent fluxes using eddy covariance and=or profile techniques. An overview on the site characteristics is presented in Table 2, details on the instrumentation are given in Table 3 for each of the sites. All flux measurement systems had been set up in a way that uniform fetch conditions without internal boundary layers were given for a wind direction sector from SSE (S) to W (NW). The position of the measurement sites in the LITFASS area within the general layout and strategy of the LITFASS-98 experiment is described in Beyrich et al. (2002b, this issue). Near-surface measurements (at measurement heights less than 10 m) of atmospheric parameters at the boundary layer field site near to the village of Falkenberg (a grassland site about 5 km to the south of the Meteorological Observatory Lindenberg) were supplemented by temperature, humidity, wind and flux measurements at a 99 m meteorological tower. Details on the tower instrumentation are given in Table 4. At the forest site, turbulent fluxes were determined from profile measurements of wind,

4 22 F. Beyrich et al. Table 3. Instrumentation of the micrometeorological stations during LITFASS-98 Parameter Site operator Falkenberg I MOL=GKSS Lindenberg GKSS Herzberg IHM TUDD Wulfersdorf GKSS Kehrigk MOL Falkenberg II UBT wind speed sensor manufacturer levels air temperature= humidity shortwave radiation net radiation soil (water) temperature soil moisture soil heat flux sensor manufacturer levels sensor manufacturer levels sensor manufacturer levels sensor manufacturer levels sensor manufacturer levels sensor manufacturer levels F460 Climatronics 0.25, 0.5, 1, 2, 4, 6, 8, 10 m Frankenberger-Psychr. Friedrichs 0.5, 1, 2, 4, 10 m CM 14 Kipp & Zonen Schulze Lange Optik Pt-100=lance TWG=GKSS cm= cm Trime-EZ Imko 15, 30, 45, 60, 90 cm HP3 Rimco=Thies 5, 10 cm Wind Monitor Young 4.7 m HMP35 AC Vaisala 3.0 m SP1110 Skye THRD S5 REBS T-probe 107=lance Campbell=Scanlog 8 80 cm= cm Sens. 253=Trime EZ Campbell=Imko 8, 16, 35, 55, 80 cm HFT1 þ HFT3=SH1 REBS=Hukselflux 8, 10, 16 cm= 6, 9cm USA-1 Metek 3.0 m HMP35 Vaisala 3.0 m CM 21 Kipp & Zonen LXG 500 Lange Optik A100R Vector 4.3 m HMP 35AC Vaisala 3.0 m SP1110 Skye Q6 REBS T-probe 107 Campbell 0cm F460 Climatronics 2, 4, 9.5,13.5, 18, 24, 29.5 m Frankenberger-Psychr. Friedrichs 2.0, 4.0, 9.5, 11.5, 13.5, 18, 20, 24, 29.5 m CM 14 Kipp & Zonen 28 m Schulze Lange Optik 28 m Pt-100 TWG 5 90 cm Trime-EZ Imko 10, 20, 30, 60 cm LWS40 PLE HP3 Rimco=Thies 5cm F460 Climatronics 0.25, 0.5, 1, 2, 4, 6 m Frankenberger-Psychr. Friedrichs 0.25, 0.5, 1.0, CM 14 Kipp & Zonen CNR-1 Kipp & Zonen Pt-100 TWG 2 50 cm gravimetric HP3 Rimco=Thies 5cm turbulence sensor DAT-310 B þ Pt 150= CSAT3 þ KH20= USA-1 þ KH20 CSAT3 þ KH20= CSAT3 þ KH20= manufacturer levels Ly- Kaijo Denki=Air 4.0 m Pt150 Campbell=Air 4.8 m Metek=Campbell 3.0 m Pt150 Campbell=Air 3.5 m Pt150 Campbell=Air 2m

5 Experimental determination of turbulent fluxes over the heterogeneous LITFASS area 23 Table 4. Instrumentation of the 99 m tower at the Falkenberg field site during LITFASS-98 Parameter wind speed Sensor (Manufacturer) Wind Transmitter (Thies) Levels 10, 20, 40, 60, 80, 98 m wind Wind Direction 40, 98 m direction Transmitter (Thies) air temperature air humidity turbulent fluxes Pt-100 (Degussa=Th. Friedrichs) 10, 20, 40, 60, 80, 98 m HMP35D (Vaisala) 10, 20, 40, 60, 80, 98 m DAT-310A (Kaijo Denki) Pt-150 (Air) IR-Hygrometer (KNMI) 10, 30, 70, 90 m 70, 90 m 10, 30 m temperature and humidity using profile fitting techniques based on Monin-Obukhov similarity theory (e.g. Nieuwstadt, 1978; Gerstmann, 1980) and energy budget considerations, procedures used for the analysis were based on Vogt (1995). A special flux instrumentation inter-comparison was performed after the main field phase of the LITFASS-98 experiment. The need for such an exercise comes from the fact that the eddy covariance measurements at the different sites had been performed using different types of sonic anemometers and of fast humidity sensors (see Table 3), using different installations for the set-up of the turbulence measurement systems, employing different algorithms for standard data evaluation=correction. In order to be able to interpret the differences in the values of turbulence parameters derived from the measurements over the different surfaces= at the different tower levels, the relative uncertainty of single parameters must be estimated from an inter-comparison of measurements taken simultaneously under closely the same atmospheric and surface conditions. Selected results of this inter-comparison experiment are summarised in the Appendix. Good agreement was found between turbulence systems of the same type while systematic deviations did occur between different sensor systems. The turbulence measurements performed at the 99 m tower during the main experiment with the same type of sensor should thus be comparable. The same holds for the surface layer measurements over barley, water and bare soil (sparse maize) which were all done with CSAT3 systems. The root mean square differences for the inter-comparison between the different sensor types are of the order of magnitude of Kms 1 and ms 1 for the kinematic sensible heat flux and friction velocity, respectively. 2.2 Scintillometer measurements A large aperture scintillometer (LAS, de Bruin et al., 1995, 1998) was operated during the LITFASS-98 experiment over a distance of 4.7 km between the Falkenberg field site and the observatory site (see also Beyrich et al., 2002a). The mean height of the scintillometer path above ground was 45 m. The system measures light intensity fluctuations at the receiver which are caused by inhomogeneities of the refraction index of the air along the propagation path. Generally, both temperature and humidity fluctuations induced by heat- and moisture eddies along the scintillometer path give rise to the fluctuations of refraction index. However, for the near infrared wavelength at which the LAS operates, the contribution from moisture fluctuations appears to be small, and it can be considered in the data evaluation with the help of a correction term making use of an estimate of the Bowen ratio Bo. This moisture correction is less than 10% if the Bowen ratio is larger than 0.6. Similarity theory is used to derive the sensible heat flux, H, from the temperature structure parameter (CT 2 ) values for daytime unstable atmospheric conditions (based on Obukhov, 1960): pffiffiffiffiffiffi C 2 1 T ðz dþ3u 1 z d 3; H ¼ c p pffiffiffiffi 1 a 2 ð1þ a 1 L in which is the air density, c p the specific heat of air at constant pressure, z is height, d the displacement height, u is friction velocity, L is the Obukhov length, and a 1, a 2 are constants for which different values have been given in the literature (e.g. Wyngaard et al., 1971; Hill et al., 1992; de Bruin et al., 1993). In the present analysis, the values suggested by de Bruin et al. (1993) were used, notably a 1 ¼ 4.9 and a 2 ¼ 9. The friction velocity was estimated from measurements of the mean wind speed using similarity relationships for the vertical wind profile.

6 24 F. Beyrich et al. Under daytime convective conditions, the resulting heat flux is close to that of the free convection case, and the use of friction velocity adds only a minor correction to this. The overall uncertainty in the scintillometer based heat flux estimates originating from both the consideration=neglect of the humidity contribution and from the specific choice of the constants in the similarity relationship was estimated to range between 10% and 20%. 2.3 Budget estimates Mixing-layer height evolution and budget methods have found increasing application over the last decade for an independent estimate of the area-representative heat and moisture flux values from mean ABL data making use of profile data from a sequence of frequent radiosoundings in most cases (e.g. Barr et al., 1997; Gryning and Batchvarova, 1999). The basic idea behind these methods is that the convective ABL grows during the day in response to the regional turbulent fluxes incorporating the aggregation of smallscale processes. In this way the measured growth of the mixed layer could be the input for an inverse solution of a mixed-layer growth model in order to estimate regional values of the sensible heat flux. The method is therefore applicable when the mixed-layer height is well above the layer where the individual surface heterogeneity is felt. For the LITFASS area, this minimum height was estimated using the methods described in Gryning and Batchvarova (1999) as about 200 m. The input parameters necessary for an inverse solution of a mixed-layer growth model (either the height of the ABL and the mean ABL warming rate, or the ABL growth rate and the temperature lapse rate above the convective ABL) were determined from sodar, wind profiler=rass and radiosonde data. Moreover, data on the large-scale vertical velocity (subsidence) are requested, two different estimates of which were obtained for the LITFASS-98 data set. A first value was derived from the mean warming rate of the lower free troposphere, a second value was taken from the NWP model forecast for the Lindenberg grid point. Application of budget methods assumes that the turbulent heat transfer between the surface and the atmosphere is the only source of ABL warming. It will therefore not work for situations where significant advection and=or the formation of ABL clouds occur. 3. Local measurements of the turbulent fluxes In order to illustrate the results of the local flux measurements at the micrometeorological stations, the diurnal cycle of the friction velocity and sensible heat flux over the different surface types for June 18 is shown in Fig. 1. This day represents one of the Intensive Observation Periods of the experiment for which data from flight measurements are available (see section 5 and the paper by Bange et al., this issue). It was characterised by moderate winds from around W WSW. Clear sky conditions were observed early in the morning. However, after 05 UTC medium level cloud fields passed the experimental area Fig. 1. Diurnal cycle of friction velocity (a), and of the sensible (b) and latent heat fluxes (c) over different surface types on June 18, 1998

7 Experimental determination of turbulent fluxes over the heterogeneous LITFASS area 25 and the development of cumulus clouds started between 07 UTC and 08 UTC. Cloud cover was broken over most of the day except for a period between 13 UTC and 14 UTC when a shower line passed the area leading to temporarily overcast conditions. The rain rate during these showers was between 0.0 and 2.4 mm over the LITFASS area. Maximum air temperature was less than 20 C. Stationarity of the turbulence measurements was tested according to Foken and Wichura (1996). Due to the changing cloud cover, considerable non-stationarity was found over large periods of the day, particularly between 05 UTC and 07 UTC (cloud fields) and between 13 UTC and 15 UTC (shower line passage). The time evolution of friction velocity (Fig. 1a) at all sites is characterised by a gradual increase until around noon, a pronounced minimum at around 14 UTC (i.e., after the passage of the shower line), and a sharp decrease after 16 UTC. This is well correlated with the behaviour of the wind speed. As to be expected, the friction velocity is smallest over the lake and largest over the forest. Differences between the various types of low vegetation are not that obvious. For the sensible heat flux (Fig. 1b), all measurements over land show a pronounced diurnal cycle as well, again with a sharp minimum in the early afternoon when the shower passed over the area. At the forest site in the western part of the LITFASS area, this minimum was observed earlier then at the other sites which is in direct response to the radiative forcing reflecting the travel direction of the cloud fields associated with the shower line. The magnitude of the sensible heat flux is highest over the forest exceeding that over low vegetation temporarily by up to 100%. This is attributed to the higher roughness, to the lower albedo (and therefore higher values of net radiation) and is also likely due to a different partitioning of the available energy between the sensible and latent heat fluxes which could not however, be verified since the uncertainty in the estimation of evaporation from the humidity profile measurements at the forest station was too high. The difference in the sensible heat flux between the barley, triticale and grassland sites is comparably small, and no clear systematic deviations could be found. The behaviour of the sensible heat flux over the lake is completely different. It is directed upward during the whole day indicating a positive temperature difference between the water and the air all the time. This is a result of the thermal inertia of the lake and the advection of relatively cold air over the LITFASS area since June 17. Gradual warming of the air during daytime leads to a decrease of the water air temperature difference and hence to a reduction of the heat flux towards the evening. The temporal behaviour of the latent heat flux (Fig. 1c) is similar to that of the sensible heat flux with a pronounced diurnal cycle and a sharp minimum between 13 UTC and 14 UTC. The latent heat flux over the agricultural fields appears to be higher than over grass which is attributed to the higher vegetation density and leaf area index of well developed barley and triticale when compared to short grass. Over the lake, the latent heat flux is directed into the atmosphere the whole day indicating evaporation from a free water surface. Determination of the latent heat flux from the humidity profile measurements at the forest site turned out to be impossible due to the limited accuracy of the profile data. In order to obtain a more general picture, a mean diurnal cycle of the energy and momentum fluxes at the different sites over the whole experimental period (June 01 June 21) was constructed, and a statistical analysis of the time series was performed. Only measurements during conditions with a wind direction between 150 and 300 were considered. This corresponds to the S- and W wind sectors for which the experimental set up had been optimised with respect to the fetch conditions and to the absence of internal boundary layers (see Table 2). The results are shown in Fig. 2, and in Table 5. The error bars in Fig. 2 indicate the variance of the flux measurements during the period of the experiment at a given time of the day, for simplicity, these are shown for the forest, grassland and lake sites only. The statistical comparison (Table 5) is based on half-hourly averaged data, and the measurements over grass at 4 m height were used as a reference. Both Fig. 2 and Table 5 confirm that June 18 was a quite typical case concerning the behaviour of the fluxes (compare with Fig. 1). Friction velocity and sensible heat flux are both highest over the forest. Differences between the different types of low vegetation typically

8 26 F. Beyrich et al. Fig. 2. Mean diurnal cycle of friction velocity (a) and sensible heat flux (b) over different surfaces during the LITFASS-98 experiment (June 01 21, 1998) for the wind direction sector 150 to 300 Table 5. Statistical comparison of flux measurements over different surfaces during the LITFASS-98 experiment (reference: measurements over grass at Falkenberg) Friction velocity (u) inms 1 N r 2 a b Barley field Triticale field Forest (profile) Lake Sensible heat flux (H) in Wm 2 Barley field Triticale field Forest (profile) Lake Latent heat flux (l v E) in Wm 2 Barley field Triticale field Lake N number of data, r 2 square of correlation coefficient, a slope of linear regression line, b offset of linear regression line amount to about 10 25%. Friction velocity is slightly higher over the agricultural crops than over grass which can be attributed to the differences in the vegetation height. In contrast, the sensible heat flux is higher over grass than over the cereals which is compensated by a lower latent heat flux (Table 5). The correlation decreases with increasing distance between the sites. The magnitude of the differences between the different types of agricultural farmland is comparable to earlier results, reported, e.g. in Tsvang et al. (1991) or Kalthoff et al. (1999). Also, the higher sensible heat fluxes over a coniferous forest when compared to grassland or agricultural farmland support the results from previous studies of, e.g. Wicke and Bernhofer (1996) or Gryning and Batchvarova (1999). The closure of the energy balance has increasingly received interest during the last years. Many careful measurements of the components of the surface heat exchange have revealed that the net radiation exceeds the sum of the turbulent fluxes of heat and moisture and the ground heat flux (e.g. Foken and Oncley, 1995; Panin et al., 1998; Foken, 1998). The deficit is typically in the order of about 20 30%. In particular, in long term weather forecasting and in climate research this is a severe problem since relatively nonimpressive errors in the exchange of heat and moisture may add up to an unrealistic drift. In seeking for an explanation for the apparent deficit, researchers have looked at the effects of non homogeneous terrain (Panin and Tetzlaff, 1999), large scale circulation (Mahrt, 1998), convergence and advection (Lee, 1998; Paw U. et al., 2000), heat storage in the upper soil layer (Foken et al., 1999), and instrumental effects (Laubach and Teichmann, 1996). The one-dimensional energy budget closure at the Falkenberg field site in the LITFASS area was investigated in detail during a pre-experiment in 1997 (see Foken et al., 1999), and further investigations were performed for the LITFASS- 98 data set. The turbulent fluxes were taken from the measurements at the 10 m-level of the 99 m tower. These data were collected at a rate of 10 Hz. Ten-minute averages of all relevant quantities were calculated on-line. The turbulence data were corrected afterwards for a number of effects, of which the following ones are of special relevance to the heat and moisture flux data:

9 Experimental determination of turbulent fluxes over the heterogeneous LITFASS area 27 (a) the Webb correction (Webb et al., 1980); (b) correction of the sonic temperature following Schotanus et al. (1983); (c) correction for the transducer shadow effect, varying between 0.95 (underestimate) for flow parallel with a transducer pair to 1.05 for perpendicular flow; (d) low frequency loss on basis of the Kaimal et al. (1972) spectra; (e) axes rotation. The latter two corrections never exceeded a few per cent. The net radiation was measured with a Schulze net radiometer. This instrument was employed for almost two years side-by-side with pyrgeometers (PIR, Eppley) and pyranometers (CM11, Kipp & Zonen) at the KNMI experimental site Cabauw. The deviations were typically in the order of 10 Wm 2 on half hour average basis. Radiation was calculated using the manufacturer-provided sensitivities for the two thermopiles. The manufacturer gives sensitivities for the short-wave and long-wave radiation that differ slightly. We used the short-wave sensitivity only. The error in not doing so is less than 10 Wm 2. The ground heat flux was calculated from the 5 and 10 cm plates following a simple procedure. It was found that the 5 cm flux data could be matched with the 10 cm data by advancing the latter one hour in time and multiplying them with 1.7. Supposing a homogeneous soil (which is a crude first approach only), the phase shift is linear with depth and the ratio of the amplitudes at 0 and 5 cm the same as the ratio of the 5 and 10 cm amplitudes. Thus, the same time shift and multiplication operation was performed on the 5 cm data to infer the ground heat flux at 0 cm. All data presented below are half-hour average values, constructed from the basic data set of 10 minute averages. At least two 10 minute periods were used to calculate the half-hour value, otherwise the half-hour interval was deleted from the data set. In Fig. 3, a scatter plot of all data is presented. Linear regression gives y ¼ 0.81 x 0.71 Wm 2 where y ¼ H þ l v E þ G and x ¼ R n. H is the sensible heat flux, l v E the latent heat flux, G the ground heat flux and R n the net radiation. Here, H, l v E and G are defined positive if the flux is away from the surface (loss), and R n is positive if the flux is towards the surface (gain). Relying on the data for the S W wind sector only, for which the fetch conditions are best defined, the regression equation is y ¼ 0.86 x 3.2 Wm 2. Fig. 3. Scatter plot of H þ l v E þ G versus net radiation for the measurements at Falkenberg field site during LITFASS- 98 (June 01 21, 1998) This indicates a magnitude of the non-closure of the energy budget of about 15 20% for the Falkenberg site which is comparable to previous studies (e.g. Foken, 1998; Foken et al., 1999). A slightly higher value (20 25%) was obtained for the barley site. 4. Turbulence and flux profiles at the 99 m tower The turbulence measurements performed at the 99 m tower during LITFASS-98 are thought to represent the link between the near-surface local flux measurements and the estimates of regionally representative flux values obtained from scintillometer and aircraft measurements, and from budget considerations. As an example of the turbulence measurements at the tower, the diurnal evolution of the momentum and sensible heat fluxes on June 17, 1998, is presented in Fig. 4. It can be clearly seen that both the overall behaviour in time and the absolute magnitude of the flux values is comparable at the four levels. During night-time, negative heat fluxes are generally small. However, during daytime at certain times, significant differences up to between 50% and 100% in magnitude occur. These are definitely higher than one would expect over a height interval of a few tens of meters in a well-developed convective ABL based on general ideas about the vertical ABL structure over basically homogeneous terrain. Thus, for the sensible heat flux a linear decrease with height is often assumed with a change in sign at about 70% of the ABL height (e.g. Garratt, 1992). For an ABL

10 28 F. Beyrich et al. Fig. 5. Mean roughness length in the footprint area of different tower levels and stability parameter z=l (for z ¼ 10 m) on June 17, 1998 Fig. 4. Diurnal cycle of the momentum (a) and heat (b) fluxes at different tower levels on June 17, 1998 height of more than 1 km this would imply variations of the heat flux across the tower range of 10 20% or even less. The momentum flux is usually assumed to decrease with height across the convective ABL as well. In contradiction to this, the one of the most obvious features in Fig. 4 is a significantly smaller momentum flux measured at the 10 m level in comparison with the other heights. Interpretation of the flux measurements at the tower has to include a detailed footprint analysis since one can expect the information on the turbulent structure of the lower ABL measured at different heights to originate from different source areas (surface types) in the inhomogeneous landscape around Falkenberg. For June 17, the source area (where the flux signal comes from) was determined in dependence on surface and stratification using the footprint model of Schmid (1997). Stability was estimated from the local surface layer measurements, and the roughness in the surroundings of the Falkenberg tower was estimated for a grid with 250 m side length according to the European Wind Atlas procedure (Petersen and Troen, 1990). The mean roughness length in the footprint area of the different tower levels on June 17 is shown in Fig. 5. It becomes obvious that the comparably small momentum flux at the 10 m level noted above can be explained by a significantly smaller surface roughness in the corresponding footprint area. Instead of trying to interpret single profiles of turbulence data measured at the tower, it might be equally worth trying to construct averaged profiles for certain weather conditions. As an example, mean profiles of the vertical velocity variance and of the sensible heat flux for different time periods during daytime and for wind directions from the SW sector and also mean profiles of the sensible heat flux around noon for different wind direction sectors are shown in Fig. 6. The vertical velocity variance (Fig. 6a) shows an increase with height for all profiles which is expected in the lower part of a convective ABL (e.g. Garratt, 1992). The increase in time of the values at a given level corresponds to the typical behaviour as well. At around noon, a quasi-stationary stage seems to be reached and the mean profile does not change any more. The sensible heat flux (Fig. 6b) shows a decrease with height in the early morning as it is typical for a shallow convective ABL. Later in the morning, the profiles show some structure and no clear decrease with height. It has to be remarked that the differences between the different height levels are less than the variability of the single

11 Experimental determination of turbulent fluxes over the heterogeneous LITFASS area 29 Fig. 6. Averaged profiles of turbulence parameters during the LITFASS-98 experiment from tower measurements over the lowest 99 m of the atmosphere. (a) Vertical velocity variance for SW flow during different time periods of the day; (b) sensible heat flux for SW flow during different time periods of the day; (c) sensible heat flux between 12 and 13 UTC for different wind direction sectors values at a given height. Around noon, the mean heat flux profile for SW flow is nearly constant with height. The ABL depth around noon in June is usually well above 1 km, one should therefore expect flux variations of not more than 5 10% over the tower range (if the footprint conditions are comparable for the single measurement levels or if flux differences originating from the characteristics of the source area are blended out due to convective mixing) which is close to the uncertainty of the measured data. The sensible heat flux profiles around noon look different for other wind direction sectors (Fig. 6c). A significant decrease of the heat flux with height over the tower range can be noticed for winds from the S, whereas the heat flux increases up to the 70 m level for winds from the W and NW. Again, the differences in the footprint area seem to explain this behaviour. For wind directions from the W- and NW-sector, forested areas can be found between 1.5 and 3 km upstream while a lake is situated about 2.5 to 4 km south of the Falkenberg site. It should be noted however, that the occurrence of different wind directions is (at least) partly associated with different weather situations and a clear separation of the influence of land surface patterns from that of weather and air mass characteristics needs to be based on a larger data set. 5. Regionally integrated fluxes Results from an estimation of the sensible heat flux from both the scintillometer measurements and the budget method for June 18 are presented in Fig. 7. For the scintillometer data, the calculations were performed using different values of the similarity constants in Eq. (1) and neglecting=considering the moisture correction when computing the temperature structure parameter. The grey area in Fig. 7 marks the range where the heat flux was estimated to be within. The global radiation and the heat flux derived from the local measurements over grass are also shown in Fig. 7 for comparison. Finally, the surface flux estimate as derived from the aircraft measurements (see Bange et al., 2002, this issue) performed on that day is also plotted. From Fig. 7, the following conclusions can be drawn: (i) The LAS based heat flux closely follows the global

12 30 F. Beyrich et al. Fig. 7. Diurnal cycle of the sensible heat flux on June 18, 1998 in the LITFASS area: Regionally representative estimates derived from LAS measurements, budget considerations and aircraft data compared to the local measurements over short grass and to the global radiation Fig. 8. Mean diurnal cycle of the sensible heat flux during the LITFASS-98 experiment for wind directions between 150 and 300 over different surface types, as a land use weighted area average and derived from the LAS measurements radiation curve illustrating the reaction of the energy transfer at the surface on the radiative forcing. (ii) The LAS and budget methods provide the same magnitude of heat flux values between around 06 and 12 UTC. During the afternoon, and particularly related to the passage of the shower line at around 13 UTC of that day (see section 3), larger differences occur. These are likely attributed to advection. Both estimates are supported from the aircraft measurements during the time of the flight. (iii) The local fluxes over grass are smaller than the area-averaged estimates. For a more general comparison an averaged diurnal cycle of the sensible heat flux during the period of the experiment was computed as a land-use weighted average of the heat flux measured over the dominant land use classes (forest, agriculture represented by triticale, grassland, water) for wind directions from the sector between 150 to 300 representing undisturbed fetch conditions for the local flux stations. The averaging was performed according to the mean percentages of the different land use classes in the area (see section 1) which is comparable to the partitioning of surface types upwind of the LAS path for the considered wind direction sector. This weighted average then was compared to the LAS heat flux data. The results are shown in Fig. 8. Good agreement between the two estimates can be noticed. This indicates that the mosaic (tile) approach (i.e. the weighted arithmetic averaging of parameters typical for different surface types in an area according to their relative occurrence in the area) appears to be applicable over the LITFASS area for the heat flux during the growing season. 6. Summary and conclusions Both local and regionally integrated flux estimates are an essential part of the measurement program of the LITFASS project. During the LITFASS-98 experiment, local flux measurements over five different types of underlying surface (grass, barley, triticale, forest, water) in a heterogeneous landscape were performed using micrometeorological stations including eddy covariance and profile techniques for the determination of the turbulent fluxes. Estimates of area-integrated fluxes were obtained from LAS measurements along a 4.7 km path, and from budget considerations. For single case studies, airborne measurements are available, the results of which are discussed in detail in Bange et al. (2002, this issue). An inter-comparison of the different eddy-covariance systems was performed after the main field phase of LITFASS-98 in order to determine the uncertainty of the local flux values attributable to the experimental setup. The following conclusions and recommendations can be derived:

13 Experimental determination of turbulent fluxes over the heterogeneous LITFASS area 31 The local fluxes based on eddy covariance measurements are comparable to within 5% if measured with the same type of sensors, this allows inter-comparison of both the near surface and tower based flux measurements. Significant differences of the turbulent fluxes were found between the main land use classes (low vegetation, forest, water), and even differences between different types of low vegetation appear to exceed the measurement uncertainty. This has to be confirmed from long-term measurements over different types of low vegetation taking into account the differences in plant development. Therefore, the operational long-term LITFASS measurement program will include direct (eddy covariance) flux measurements over the main land use types in the area, being supplemented by additional flux measurements over different types of agricultural farmland during the growing season (April July). For the Falkenberg grassland site, a non-closure of the surface energy balance in the order of 15% to 20% was found which is comparable to the results from previous studies over similar terrain. The profiles of turbulent fluxes measured along the 99 m-tower significantly deviate from idealised profiles measured over homogeneous terrain. Peculiarities in the profile structure, like an increase in the momentum flux with height could be attributed to the heterogeneity of the terrain, namely to the differences in roughness in the corresponding footprint areas. Systematic long-time measurements at the 99 m-tower linked to surface measurements in the footprint area of the different tower levels are necessary to establish parameterisations for the flux profiles in the lower ABL over a heterogeneous land surface. Different estimates of area-integrated fluxes based on LAS and aircraft measurements, and on budget considerations support each other in magnitude. Generally, the area averaged sensible heat and momentum fluxes appear to be larger than those found over grass at the Falkenberg field site. A land use weighted average of the sensible heat flux is in good agreement with the LAS based estimate indicating that the mosaic (tile) approach appears to be a feasible concept for the averaging of the sensible heat flux over the LITFASS area during the growing season. Long-term measurements are needed to verify this conclusion for other seasons and for other fluxes. A different behaviour in particular is expected for the averaging of momentum fluxes, e.g., recent studies over the boreal forest in Northern Finland have indicated, that there are basic differences in the formation of area averaged fluxes for heat and momentum (Batchvarova et al., 2001). Acknowledgement The LITFASS-98 experiment has been organised by the Deutscher Wetterdienst (DWD, German Meteorological Service), participation of the different groups in the field experiment was based on own institutional funding which is greatly acknowledged. Critical comments of S.-E. Gryning and A. Raabe on an earlier version of this manuscript were highly appreciated. Special thanks go to Mrs. P. Dereszynski for carefully preparing the most of the figures and the final manuscript. Appendix Selected results from the turbulence inter-comparison experiment The turbulence inter-comparison experiment was performed after the LITFASS-98 main field phase at the boundary layer field site near Falkenberg over short grass. Twelve turbulence measurement systems were set up along a line using basically the same installation and mounting structures as during the main experimental phase, except for those systems which were operated at the 99 m tower before. The line was oriented in a way that flow was normal to it for winds from the WSW (250 ). The distance between the single installations was 3 m, the sensor height was between 4.0 m and 4.3 m. Measurements were performed from June 26 to June 30, but the weather was favourable for the inter-comparison (which means a fair amount of insolation and winds from SW-W)onJune28andJune29,only. For the data analysis of the inter-comparison experiment, a 10 minutes integration time had been defined as a basic measurement interval. Twofold co-ordinate rotation was performed except for the CSAT systems, for which this was not possible since not all of the basic data were available. No further corrections were applied to the data. Results from a statistical analysis of 36 hours of measurements (between June 28, 0730 UTC and June 29, 1930 UTC) for some of the measurements systems and for selected turbulence parameters are summarised in Table A1. All systems were compared against the data from a sonic Kaijo Denki DAT310A operated by DWD. The use of this instrument as a reference system is a partly arbitrary choice and does not indicate a better performance a-priori when compared with

14 32 F. Beyrich et al. Table A1. Selected results from a statistical analysis of the turbulence intercomparison experiment (reference system: Kaijo- Denki 310A, position 2) Operator DWD KNMI GKSS UBT system Kaijo- Denki 310A Kaijo- Denki 310A Kaijo- Denki 310A Kaijo- Denki 310B Kaijo- Denki 310B CSAT3 CSAT3 position in line var (T S ) N r a b rmsd var (w) N r a b rmsd cov (wt S ) N r a b rmsd u N r a b rmsd N number of data, r 2 square of correlation coefficient, a slope of linear regression line, b offset of linear regression line, rmsd root mean square difference the other systems. It was mainly based on the completeness of the data set including availability of non-rotated=rotated data and of temperature fluctuation measurements from a platinum fine-wire sensor. For the CSAT-3 instruments, the comparison was performed using the non-rotated data also for the reference instrument (numbers in Italics in Table A1). Results for a third CSAT-3 system and the USA-1 system which were operated at the lake and triticale sites, respectively, during the main field experiment are not included in Table A1, since measurements from these instruments were available for less than 12 hours during the 36 hours time period. From Table A1, the following conclusions can be drawn: 1. For all parameters, the agreement is best between instruments of the same type, while larger differences were found between different types of sonic anemometers. 2. Best agreement with the reference instrument was usually observed for the Kaijo Denki DAT310A of KNMI which was operated side by side to the reference system during the inter-comparison experiment. This may indicate a certain influence of the experimental set-up on the intercomparison results which is most obvious in the friction velocity data where the highest b-values (indicating a systematic deviation from the reference instrument) were found for those systems which were situated most far away from the reference system during the measurements. 3. No offset (see the small b values) could be noticed for the vertical velocity variance and for the heat flux data. For friction velocity, the offset is smaller than 2 cms 1 for most systems. For the sonic temperature variance, both Kaijo Denki DAT310A instruments of MOL showed an offset when compared to the other systems. This was identified as a result of a not well-adapted resolution during data acquisition. However, the temperature variances from the platinum fine-wire sensor did compare well with the other instruments (not shown here) and were considered for the analysis of the experimental data consequently. 4. The relatively high root mean square differences for the friction velocity of the CSAT systems in comparison to the reference instrument (about 0.06 ms 1 ) may be attributed to the fact that no co-ordinate rotation was applied in this case. 5. The slope of the regression lines is within the interval in most cases for an inter-comparison of systems of the same type. 6. The root mean square differences for the heat flux correspond to an uncertainty of about Wm 2 between systems of the same type. The resulting uncertainty for the friction velocity is less than 0.05 ms 1. For some systems, high resolution temperature measurements had been additionally performed with a fine-wire