214 D.I. Cooper et al. Agricultural and Forest Meteorology 114 2003 213–234
2003; Schuepp et al., 1990; Finn et al., 1996 , and
is a continuing topic of research. A number of dif- ferent approaches have been developed for both
neutral and convective boundary layers using a: La- grangian model, large eddy simulation model, and
analytical solutions to the advection–diffusion equa- tion
Leclerc et al., 1997 . However, verification of
the models with data has previously been limited to point sensors
Finn et al., 1996; Leclerc et al., 2003; Warland and Thurtell, 2000
or airborne line obser- vations
Ogunjemiyo et al., 1997 . Since footprints
and source-area concepts are inherently spatially de- pendent processes, multidimensional remotely sensed
atmospheric data offer a unique opportunity to verify these models. Unlike other remote sensing systems,
the scanning Raman lidar can spatially resolve atmo- spheric phenomena, especially within the boundary
layer, and can thus be used to improve and perhaps verify boundary layer models
Kao et al., 2000 . In
turn, the footprint model results enhance the inter- pretation of lidar data and may help to improve lidar
techniques in the field. The primary objective of this paper is to evaluate the optimum spatial sampling size
for lidar derived variables such as latent energy flux.
In this paper, we first discuss the experimental site, the tower-based instruments used, and the scanning
Raman lidar Section 2
. In next section Section 3
, we describe the Monin–Obukhov method for estimat-
ing latent energy from lidar measured water vapor profiles and independent friction velocity measure-
ments, as well as the dimensionless moisture flux, q
∗
. Section 4
describes three methods for estimating lidar average horizontal sampling size and point-source-
area footprints by A combining lidar-eddy covari- ance, B footprint modeling analysis, C co-spectra
of the vertical wind w and water vapor q time se- ries and D integral length scales. We finally discuss
results and summarize in
Section 5 .
2. Site, instrument, and lidar overview
The study site was in the riparian corridor at the Bosque Del Apache Wildlife Refuge hereafter called
the Bosque in the semi-arid south–central part of New Mexico, 25 km south of Soccoro, NM at ap-
proximately 34
◦
N latitude, 107
◦
W longitude, at an average elevation of 1376 m
Fig. 1 . The Bosque is
adjacent to the Rio Grande river, which was flowing well during the experiment in mid-September 1998,
due to the summer monsoon rains in late July. During the study period, the local climate was typically char-
acterized by clear skies in the morning with stratocu- mulus clouds developing during the afternoon. Winds
were generally mild at 2 ms or less, northerly in the early morning, periodically the wind would reverse
direction by mid-morning to become southerly. Wind directions subsequently oscillate between southeast
and southwest throughout the day and through early evening transition hours. The vegetation at the Bosque
consisted almost entirely of uniformly dense riparian Tamarisk
salt cedar 5 m tall with a height variation of ±1 m. The Tamarisk is a phreatophyte, thus water
is not usually a limiting factor to evapotranspira- tion. The riparian corridor vegetation is adjacent to
the western side of the river, covers a band between 500 and 700 m wide and extends north and south for
several kilometers. During September the salt cedar canopy was closed with a green leaf area index of
approximately two, as measured by a radiative LAI meter Licor 2000.
Two 12 m tall micrometeorological towers were positioned between the western edge of the salt cedars
and the Rio Grande, approximately 300 m from the river. The two towers were placed 685 m apart along
the north–south direction. The following instruments were installed on the towers 2.7 m above the canopy:
three-dimensional sonic anemometers CSI Incorpo- rated CSAT-7,
1
fine-wire thermocouples, Krypton hygrometers KH-20, temperature–humidity probes
Vaisala HMP100a, infrared thermometers Everest, and net radiometers REBS Q-7. Five soil heat flux
plates REBS were buried 0.08 m below the soil sur- face and spatially distributed to represent the range
of shade conditions below the canopy. Soil thermo- couples were located above the heat flux plates at
−
0.02 and −0.06 m. The displacement height d was estimated to be 0.6 the height of the vegetation h. Tur-
bulent fluxes of sensible heat, latent heat, and momen- tum were computed using standard methods with a
rotated re-sampled coordinate system from the above mentioned micrometeorological sensors using eddy
covariance at 20 Hz
CSI Incorporated, 1998; Prueger
1
Indication of manufacturer does not necessarily support an endorsement.
D.I. Cooper et al. Agricultural and Forest Meteorology 114 2003 213–234 215
Fig. 1. Site map showing the location and elevation of the Bosque in relation to New Mexico and the study site in relation to the Bosque. The right panel is a false color image showing the salt cedar, cottonwoods, the drainage levee and the location of the micrometeorological
towers and the lidar. The location of lidar scan azimuths for Fig. 2A–D
as well as the mean wind are shown as black lines on the IR image.
et al., 2000 . In addition, an airborne visiblenear IR
imaging camera operated by the Utah State University collected images of the study site
Neal et al., 2000 .
A sand levee runs roughly north–south and the lidar, sodar, and profiling radar where located in the center
of the levee Fig. 1
. The black strip shown west of the levee is the water in the drainage canal running par-
allel to the Rio Grande. The open channel of the Rio Grande and the sand deposited on its banks are repre-
sented by the high-reflectance patterns at the extreme east edge of the figure. The lidar was situated approx-
imately at the midpoint between the two towers on the western edge of the salt cedar at the top of an adjacent
levee. The levee was high enough that the lidar scan- ning mirror was approximately 3 m above the top of
the canopy. An false-color image shows the relatively uniform, dense salt cedar canopy on the east and the
sparse cottonwoods on the west Fig. 1
. The lidar az- imuthal scan range covered 110
◦
from north to south, and acquired data from the top of the canopy to 40 m
into the atmospheric boundary layer ABL. Several specific lidar scan azimuthal lines have been super-
imposed on the false-color IR image shown in Fig. 1
to illustrate the surface conditions for selected lidar observations. The LANL Raman lidar generates vol-
ume images from multiple azimuth two-dimensional range-height-indicator scans of water vapor, hereafter
called vertical scans. Details on the method and oper- ation of the scanning lidar are found in
Eichinger et al. 1999
. We can operate the lidar to a radial range of about 600 m, a horizontal spatial resolution of 1.5 m,
azimuthal scanning range up to 360
◦
, and a vertical step resolution up to 0.05
◦
. Each vertical scan re- quired 35 s to complete; a set of 22 scans stepped in 5
◦
216 D.I. Cooper et al. Agricultural and Forest Meteorology 114 2003 213–234
increments from +35
◦
in azimuth to −145
◦
in azimuth required about 15 min. The absolute accuracy of the li-
dar was shown to be ±0.34 gkg at the 95 confidence level
Cooper et al., 1996; Eichinger et al., 1994 .
3. Moisture fields and models for lidar
