Turbulence structure of a tropical forest

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  • TURBULENCE STRUCTURE OF A TROPICAL FOREST

    R. T. PINKER and J. Z. HOLLAND

    Department of Meteorology, University of Maryland. College Park, MD 20742, U.S.A.

    (Received in final form 10 August, 1987)

    Abstract. A detailed analysis is presented of the horizontal wind fluctuations with periods 20 s to 1 hr, and their vertical structure as measured with light three-cup anemometers in a tropical forest environment. Information collected during the TREND (Tropical Environmental Data) experiment in a monsoon dominated region, was utilized. A special attempt was made to extract information relevant for dispersion modeling. Variability parameters within and above the forest canopy under different stability conditions were derived. A similar analysis was performed for a nearby clearing, to facilitate comparison between relatively smooth and rough surfaces, under identical ambient conditions. A limited sample of data (7 days) was utilized, initially, to develop a methodology to be later applied on a comprehensive data base, spanning the whole monsoon cycle.

    cd

    5

    CT d l-T g 94 H k L

    L Ri T T 4 0, w

    Nomenclature

    drag coefficient, specific heat at constant pressure, clearing tower, displacement height (m), forest tower, acceleration due to gravity (m s - 2), universal functions of z/L, sensible heat flux (m s - K), von K&m&r constant, eddy viscosity (m2 s - ), eddy length scale, Monin-Obukhov length (m), Richardson number, temperature, average layer temperature in K, wind velocity components in the alongwind (except eastward in Section 4.4.1X crosswind to the left (except northward in Section 4.4. l), and vertically upward directions, time-mean values of U, v, deviations of instantaneous values of T, u, v, w from mean values, friction velocity (m s - I), wind speed, mean wind speed, and deviation of instantanteous wind speed from mean, ith measurement height, roughness height (m), height above ground (m), error of fitted wind speed profile at ith level, time-mean potential temperature, mean wind direction, density of air (kg m - ), standard deviations of eastward (or alongwind), northward (or crosswind to left), and upwards components of wind velocity, wind speed, alongwind and crosswind components (m s ), and wind direction (deg), shearing stress, usually assumed equal to surface value.

    Boundary-Layer Meteorology 43 (1988) 43-63. 0 1988 by Kluwer Academic Publishers.

  • R. T. PINKER AND J. 2. HOLLAND

    1. Introduction

    The unique micrometeorological data collected during the Tropical Environmental Data (TREND) experiment, conducted in a tropical forest environment in Thailand in 1970, provide a basis for obtaining information on parameters that control turbulent transports in forests. A comparison between a forested and relatively smooth terrain will also be possible by utilizing measurements taken along a tower in an adjacent clearing. While these data have the advantage of covering long observational periods, the instrumentation used is not of high time response as could be provided, for instance, by sonic anemometers. Therefore, the data do not allow us to resolve all the scales of turbulence.

    Quantities of interest are: the velocity variances, represented by the standard devia- tions by, a,, cr,, , and a,,,, ; the turbulence intensities represented by or,/v and the related coefficients of linear regression of a, vs 7; the stress at the canopy surface represented by U, , and the related forest parameters z, and d; the scaled alongwind and crosswind turbulence components o,,/u * and a,,/~ * . Of special interest is the variation of the turbulence with height and its dependence on stability, within the forest and in the clearing. Information required to compute or estimate all the commonly used stability parameters (i.e., the Monin-Obukhov length scale; the Richardson number; the Pasquill-Turner categories) is also available.

    The anemometer response time needed to measure turbulent velocity variances in a forest is not well established. Both Allen (1968) and McBean (1968) used instruments with response cutoffs at about 1 Hz. They found a suppression of the high frequency portions (N 0.04 to 1 Hz) of the spectra of w and V, respectively, at low levels in a dense forests, compared to spectra taken over open ground or above the forest. McBean suggested that at least 10 Hz response would be necessary to verify the reality of this. Allen suggests that at the floor of the forest most of the variation in horizontal air flow is due to pressure waves associated with large-scale eddies. At frequencies greater than 0.1 Hz, the relative contribution to the total variance of wind speed appeared to increase upward to the canopy top. In the present study, we examine explicitly only the lower- frequency portion of the horizontal velocity variance, the Nyquist frequency of the TREND anemometer data being 0.05 s - .

    The advantages of our data set are the number of observation levels, the long periods of complete high quality data providing representative sampling of a range of wind speed and stability conditions, and the availability of supporting data on vertical temperature gradients and solar radiation.

    The profiles of mean wind speed and temperature and their diurnal and seasonal variations for the TREND forest and clearing tower sites have been presented by Thompson and Pinker (1975) and Pinker (1980). The effect of the two-story structure of the forest on the mean wind speed profile was examined by Pinker and Moses (1982) in the framework of the exponential extinction model of Cionco (1965). The present report covers a study of a 7-day June 1970 data sample preparatory to the statistical analysis of the data from all four seasons.

  • TURBULENCE STRUCTURE OF A TROPICAL FOREST 45

    2. Site and Data

    The site of the TREND experiment was a forest area of about 80 km2 in Thailand, at approximately 1431 N, lOl55 E, about 190 km northeast of Bangkok. The region is under a monsoonal regime with two distinct seasonal circulations: a dry winter outflow from a cold continental anticyclone, and a moist summer inflow into a continental heat low (Pinker, 1980). The two-story tropical evergreen forest had a dense upper crown canopy at 20-35 m and a lower canopy with tops at 5-17 m.

    Fig. I. Measurement levels along the forest and clearing towers. Temperature: all levels. Wind speed, direction: levels with solid lines. Radiation levels 1, 46 m in the forest and level 1 m in the clearing.

    Two towers (Figure l), - 50 m high and 500 m apart, one (CT) in a clearing of diameter 500 m, and the other (FT) in the forest, were instrumented to collect the following data: temperature, dew point, wind speed and direction, rainfall and radiation. The elevation at the site was 535 m. A more detailed description of the site, instrumen- tation and climate elements can be found in Pinker et al. (1980). In the present study, only data on wind velocity, temperature, and global solar radiation will be utilized.

    The wind speed sensors were Climet three-cup anemometers having a threshold velocity of 0.3 m s- . Temperature measurements were taken with Hewlett-Packard quartz thermometers placed in shielded motored aspirators. The accuracy of the sensors was up to 0.01 C. Measurements were sampled as follows: temperature, every 10 s; wind speed, totalizing signal read every 10 s; wind direction, every 20 to 30 s. Wind direction and wind speed data were written on two separate tape systems with no correspondence between start and stop times for the tape files. We developed a procedure to merge the two data sets. Vector components were computed at the wind speed times, in order to take advantage of the greater frequency of wind speed data. A linear interpolation was performed between successive directions to derive a direction at the time of each wind speed measurement. Directions were digitized to approximately

  • 46 R. T. PINKER AND J. Z. HOLLAND

    0.01 radian increments and a table look-up method was used for the sine and cosine components. More details can be found in Pinker and Kaylor (1982). The radiation data used in the present study were acquired at the top of the forest tower (N 15 m above the canopy). The global shortwave radiation (0.3-3 urn) was measured by an Eppley precision pyranometer (2-junction model). An automatic data acquisition system (HP Model 7259) was used in an analog and digital mode. The radiation signals were sent to the digital voltmeter which integrated the voltage every 20 s and obtained an average value that was digitized. From this information, half-hourly averages were obtained.

    To enable us to classify the data according to several of the commonly used stability criteria, we selected days for which concurrent information on wind velocity, tempera- ture, and solar radiation was available. The selected days and relevant information on the global solar radiation are presented in Table I.

    TABLE I

    The hourly average values of global solar radiation in (mly min- ) for each of the seven June days, and the daily totals in (ly day- I). The duration of sunshine for each day is indicated.

    Day 20 23 24 25 26 2-l 28

    0600 214 0630 566 0700 770 0730 817 0800 1014 0830 686 0900 1095 0930 146 1000 1018 1030 781 1100 691 1130 855 1200 1194 1230 1160 1300 1237 1330 856 1400 442 1430 184 1500 112 1530 112 1600 86 1630 63 1700 44 1730 140

    Total 448 UY day - ) Sunshine 3-8 (hr)

    39 85 135 240 289 443 300 543 399 685 505 641 023 1048 868 924

    1198 1203 1290 1076 1283 1049 1154 1318 1238 1244 192 1090 883 1050 667 805 520 1236 359 756 384 694 372 601 281 603 122 431 41 249 20 102

    422 546

    3-8 9-2

    a Precipitation of between O-10 mm.

    91 158 209 341 591 582 352 538 419 515 846

    1051 921 909

    1527 941

    1005 823 640 788 648 517 343 162

    450

    3-8 3-8 2.5 1.3

    142 60 253 102 386 152 602 231 741 333 924 490

    1102 414 524 424 549 627

    1162 719 591 931 52 692 68 996

    126 1274 159 1417 285 1012 489 558 868 416 512 453 215 393 129 332 103 318 102 126 49 45

    304 319

    95 134 153 286 352 433 351 465 566 562 613 632 128 676 929 192 200 196 418 194 179 150 19 81

    278

  • TURBULENCE STRUCNRE OF A TROPICAL FOREST 41

    3. Procedures and Results

    3.1. PRELIMINARIES

    In order to characterize the stability conditions over the forest and the clearing in a coherent manner, appropriate layers for conducting meaningful comparisons had to be selected. Time series of vertical temperature gradient (y) in the 32-46 m layer above the forest and the 1-16 m layer above the clearing were found to exhibit similar responses to the external heating (Figures 2a-c). The time series of the temperature (T) at the 1 m level in the clearing and the 30 m level in the forest and the wind speed (WS) at the 46 m level in the clearing are also illustrated.

    The close agreement between the 1 m CT and 30 m FT temperature curves confirms that the air near the effective radiative surface level responds in a qualitatively similar manner to the diurnal radiative cycle and to most of the superimposed perturbations (presumably associated with clouds and convective disturbances). The lapse rates (y as defined here is positive when the temperature increases with height) show a general pattern as follows. Absolute values are very small in all four layers during the night hours. On the days shown, the strongest winds at the top level of the clearing tower (WS) tended to occur during the night hours. This may be due to reinforcement of the SW monsoon by a nocturnal drainage flow down the gentle slope towards the northeast, and opposing upslope flows in the daytime. Thus nighttime, during this period, was

    CT - FT -m-

    1%

    .05.

    2 3

    o-.. .-y __. I

    e. r -.05-

    F -1.1 -

    2 .os- - -./ - 2

    3 0. 3 .-y.Y-/. TM

    0 I 2 3 4 5 6 7 6 6 a II 12 I3 I4 I5 16 .R m I3 20 2l 22 23

    TIM-

    Fig. 2a.

    Fig. 2. The diurnal variation of vertical temperature gradient (Lapse rate y = + aT/%Z), in the 1-16 m layer and the 32-46 m layer at both the clearing tower (CT) and the forest tower (FT); the diurnal variation of temperature, (T) at 1 m in the clearing and 30 m in the forest and wind speed (WS) at the 46 m level

    in the clearing for June 23, 1970 (a); June 26, 1970 (b); and June 28, 1970 (c).

  • R. T. PINKER AND .I. Z. HOLLAND

    CT - FT -.-

    0 I 2 3 4 5 6 7 6 9 D II 12 I3 14 I5 I6 I7 16 I9 20 21 22 23

    TIME -

    Fig. 2b.

    CT - FT -.-

    -2 0 3 t-

    Y-.---- ,-e __- w .-._.___

    k

    - -05 -

    w -1.1 L t Ll6m. ./Y (L .os .A % 0 I- -v .-,

    Y-- .-.- .--I./ \,.-.-- .-.-.-.

    % -I To5 -.lI* * * * * *. . . . . . . . . . . . . .

    0 I 23466769Dll~l3l4~l6R~B202l2223

    TIME-

    Fig. 2c.

    usually characterized by near-neutral conditions rather than by strong inversions over both the clearing and the forest canopy. During midday the lapse rate becomes very unstable (negative) in the layer 1-16 m over the clearing, less so in the layer 32-46 m over the forest, still less so in the layer 32-46 m over the clearing, and markedly stable (positive) in the layer 1-16 m in the forest.

  • TURBULENCE STRUCTURE OF A TROPICAL FOREST 49

    Disturbances indicated by sharp increases in wind speed with sharp decreases in solar radiation (Table I) and temperature occurred at about 1200 on June 26 (Figure 2b) and 1430 on June 28 (Figure 2~). On both occasions the lapse rates (y) at 1-16 m at CT and FT converged and, in the tirst case, crossed the layer within the forest becoming unstable (negative lapse rate) while at the same heights over the clearing the air became stable, as did that above the forest canopy on both occasions.

    The lapse rate affects the supply of energy to turbulence. Therefore, an enhancement of turbulence by buoyancy can be expected during sunny days near the surface in the clearing and above the effective radiative surface of the forest. At the same time, the stability within the forest might tend to suppress turbulence. On the basis of these findings, the 1-16 m level above the clearing and the 32-46 m level above the forest were selected for characterizing the free air surface-layer parameters.

    3.2. STABILITY CLASSIFICATION

    The gradient Richardson Number (Pi) defined as:

    ti = 5 am T (a2.4/az)2 (1)

    was used for stability classification. The layer 1-16 m was used for the clearing tower and 32-46 m for the forest tower. Half-hourly averaged data were used to compute Pi. The range of the hourly values of Ri was divided into three intervals characterized as:

    Ri < - 0.03 unstable, - 0.03 I Ri < 0.03 neutral,

    Ri 2 0.03 stable.

    All hourly observations were sorted into these categories. According to the Monin-Obukhov (M-O) similarity theory, assuming horizontal

    homogeneity and steady state, the standard deviations of vertical and horizontal wind direction fluctuations are functions of z/z0 and z/L, where

    L= - u, c*p

    kgH . (2)

    Since both z/L and Ri measure the relative importance of buoyant suppression (or production) and mechanical (shear) production of turbulence, we expect a functional relationship. Several relationships between L and Ri have been sugge...