Evaluation of a Coupled Land-Surface and Dry Deposition Model through Comparison to Field Measurements of Surface Heat, Moisture, and Ozone Fluxes

EP A/600/A-97/038
20.3 EVALUATION OF A COUPLED LAND-SURFACE AND DRY DEPOSITION MODEL THROUGH COMPARISON TO
FIELD MEASUREMENTS OF SURFACE HEAT, MOISTURE, AND OZONE FLUXES
Jonathan E. Pleim1, Aijun Xiu2, Peter L. Finkelstein1, and John F. Clarke1
'Atmospheric Sciences Modeling Division, NOAA, Research Triangle Park, NC 27711
{on assignment to the National Exposure Research Laboratory, US Environmental Protection Agency)
Environmental Programs, MCNC, Information Technologies Division, Research Triangle Park, NC
1. INTRODUCTION
The Fifth Generation Mesoscale Model (MM5,Grell et
at. 1993) has been modified to include an improved land-
surface scheme with explicit treatment of soil moisture
and evapotranspiration, A key aspect of this work is an
indirect soil moisture nudging scheme which adjusts soil
moisture according to model errors in surface level
temperature and humidity. Since stomatal resistance is
an important component of both evapotranspiration and
dry deposition, a chemical dry deposition model is coupled
to the land surface scheme through the use of several
common elements. Both aerodynamic and bulk stomatal
resistance computed in the evapotranspiration model are
used to compute dry deposition of gaseous species. This
technique has the advantage over many other dry
deposition models of being able to respond to changing
soil moisture conditions. Also, the soil moisture
adjustment scheme should result in more realistic
stomatal conductances and dry deposition velocities.
In a previous evaluation study (Pleim et at., 1996),
model simulations of surface energy fluxes, dry deposition
velocities, and planetary boundary layer (PBL) evolution,
were compared to field measurements made near
Bondville, IL during the summer of 1994. The model
predictions compared quite well to observations for all
parameters, including dry deposition velocity, over a com
field during moderately moist conditions. The current
study focuses on comparison of model simulations to field
measurements made in a soybean field in southern
Kentucky during the summer of 1995.
2. SURFACE/PBL MODEL
Pleim and Xiu (1995) describe the development and
initial testing of a land surface and PBL model for use in
mesoscale models. Since that paper we have continued
development of this model and applied it in full 3-
dimensional form in a modified version of MM5. The key
elements of our modifications are a surface model
including soil moisture and evapotranspiration based on
the work of Noilhan and Planton (1989), and a non-local
closure PBL model developed by Pleim and Chang (1992).
The surface model includes a two layer soil scheme with a
1 cm surface layer and a 1 m root zone layer. Evaporation
has three pathways: direct soil surface evaporation,
vegetative transpiration, and evaporation from wet
canopies. Ground surface (1 cm) temperature is
computed from the surface energy balance using a force-
restore algorithm for heat exchange within the soil.
Stomatal conductance is parameterized according to root
zone soil moisture, air temperature and vapor pressure
deficit, leaf area index (LAI), and photosynthetically
active radiation (PAR). Our modifications to the original
Noilhan and Planton (1989) model include the addition of a
canopy shelter factor to account for shading within denser
canopies and modification of some of the stomatal
functions with respect to environmental parameters.
We have included a soil moisture adjustment scheme
which uses the errors in the predicted values of
temperature and relative humidity for the lowest model
layer as compared with gridded analyses of surface
observations to nudge (correct) root zone and upper layer
soil moisture. The concept is that errors in low level
temperature and humidity may be due to erroneous
partitioning of latent and sensible surface heat fluxes
which in turn may be caused by unrealistic soil moisture
conditions. Nudging coefficients must be carefully
prescribed to act only when and where coupling is strong
between soil moisture and surface heat and moisture
fluxes so that soil moisture nudging is not employed when
model errors have other causes. Therefore, we prescribe
nudging coefficients as functions of model parameters
such as solar insolation, air temperature, leaf area, soil
texture, vegetation coverage, and aerodynamic
resistance.
3. DRY DEPOSITION MODEL
A new dry deposition model has been developed for
use in 3-dimensional chemistry transport models. The
aerodynamic resistance, which is a measure of turbulent
transport within the atmospheric surface layer, and the
stomatal resistance, which is a measure of resistance to
uptake or emission through stomatal pores, produced by
the surface model in MM5 are also used in the dry
deposition model. Since both of these variables depend
only on physical characteristics of the atmosphere and
vegetation, they can be applied to any quantity for which
these processes are important. The only adjustment
needed for gaseous dry deposition is to weight the
stomatal resistance by the ratio of molecular diffusivities
of the chemical species and water vapor. Other dry
deposition resistances, including cuticle resistance,
ground resistance, water resistance, and a pathway
through the vegetative canopy to the ground, are
parameterized according to relative reactivity and
solubility.
4. RESULTS
The modified version of MM5, including the land
surface and dry deposition model (MM5-DD), was run for a
continuous period of August 1 - September 16,1995 with
54x54 km grid cells and 30 vertical layers. In addition to
the soil moisture assimilation scheme described above,
direct data assimilation was used for winds at all levels,
and temperature and humidity above the PBL. Grid cell
values of land use related parameters (LAI, vegetation
coverage, surface roughness length and minimum

-------
H - Model
—— LE - Model
• LE ¦ Observed
X H - Observed

Aug/11
Aug/12
Aug/13
Aug/14
Figure 1. Modeled and measured sensible and latent heat flux at the Keysburg, KY site in 1995.
stomatal resistance) were derived from a detailed
vegetation/land use.
Comparison of modeled and measured sensible and
latent heat fluxes for 4 days in Keysburg, KY are shown in
Figure 1. See Finkelstein et al. (1997) in this volume for a
description of the field experiment. In this early part of the
modeling period the partitioning of available energy is
mostly into latent heat flux due to the relatively moist
conditions. The model clearly did an excellent job of
simulating both latent and sensible heat fluxes. Towards
the end of the period, when the soil was much dryer, the
energy partitioning was reversed with the sensible heat
flux greater than the latent heat flux. The model was also
able to simulate these dry conditions well (not shown)
reflecting the models ability to respond to different
moisture and vegetation regimes.
U
o
CD
>
V)
Q
*
£" n
o °
; -irMM5-DD
-••Obs

I J
t
*
9
4
1
¦ >'r
~ t
* \ *
# # •
/
/ i\
r * V
f * v
*
5 10 15 20
Hour (LT)
Figure 2. Modeled and measured 03 dry deposition.
Figure 2 shows the average diurnal pattern of modeled
and measured ozone dry deposition velocity at Keysburg.
For each hour of the day the modeled and measured
values are averaged over all days. While the modeling
period covered 46 days, the number of days for which data
was available for each hour ranged from 10-26. Figure 2
shows that during daylight hours the MM5-DD, on the
average, compares very well to the observations but often
overpredicts at night. The relatively accurate predictions
during daylight hours, when the plants are transpiring,
demonstrates the advantage of using stomatal
conductance from the new surface model in the MM5-DD.
These results show that a 3-dimensional model with
soil moisture and vegetation simulation can compare well
with field measurements for both moisture and chemical
fluxes.
DISCLAIMER
This paper has been reviewed in accordance with the US
Environmental Protection Agency's peer and
administrative review policies and approved for
presentation and publication. Mention of trade names or
commercial products does not constitute endorsements or
recommendation for use.
5. REFERENCES
Finkelstein, P. F„ J. F. Clarke, T. G. Ellestad, and J. E
Pleim, 1997, Diagnostic evaluation of the multi-layer
deposition velocity model. In this volume, P20.1,
Grell, G.A., Dudhia, J., and Stauffer, D.R., 1993, A
Description of the Fifth-Generation PENN
STATE/NCAR Mesoscale Model (MM5), NCAR
Technical Note, NCAR/TN-398+IA.
Noilhan, J. and Planton, S., 1989, A simple
parameterization of land surface processes for
meteorological models. Mon. Wea. Rev., 117:536-
549.
Pleim, J. E., J. F. Clarke, P. L. Finkelstein, E. J. Cooter, T.
G. Ellestad, A. Xiu, and W. M. Angevine, 1996.
Comparison of measured and modeled surface fluxes
of heat, moisture and chemical dry deposition. In: Air
Pollution Modeling and its Application XI, Ed: Gryning
and Schiermeier, Plenum Press, New York.
Pleim, J. E. and Chang, J. S., 1992, A non-local closure
model for vertical mixing in the convective boundary
layer, Atmos. Environ., 26A:965-981.
Pleim, J. E. and Xiu, A,, 1995, Development and testing of
a surface flux and planetary boundary layer model for
application in mesoscale models, J. Appl. Meteor.,
34:16-32.

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• , • TECHNICAL REPORT DATA
1. REPORT NO.
EPA/600/A-97/038
2.
3 .RE
4. TITLE AND SUBTITLE
Evaluation of a Coupled Land-Surface and Dry-
Deposition Model through Comparison to Field
Measurements of Surface Heat, Moisture, and Ozone
Fluxes
5.REPORT DATE
6.PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Jonathan E. Pleirr.1, Aijun Xiu2, Peter L.
Finkelstein1, and John F. Clarke1
8. PERFORMING ORGANIZATION REPORT NO.
9, PERFORMING ORGANIZATION NAME AND ADDRESS
'Same as Block 12
JMCNC, Information Technologies Division, Research
Triangle Park, NC
10.PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Research and Development
National Exposure Research Laboratory
Research Triangle Park, NC 27711
13.TYPE OF REPORT AND PERIOD COVERED
Preprint FY-97
14. SPONSORING AGENCY CODE
EPA/600/9
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The Fifth Generation Mesoscale Model (MM5,Grell et al. 19935 has been modified to include an improved
land-surface scheme with explicit treatment of soil moisture and evapotranspiration. A key aspect of
this work is an indirect soil moisture nudging scheme which adjusts soil moisture according to model
errors in surface level temperature and humidity. Since stomatal resistance is an important component of
both evapotranspiration and dry deposition, a chemical dry deposition model is coupled to the land
surface scheme through the use of several common elements. Both aerodynamic and bulk stomatal resistance
computed in the evapotranspiration model are used to compute dry deposition of gaseous species. This
technique has the advantage over many other dry deposition models of being able to respond to changing
soil moisture conditions. Also, the soil moisture adjustment scheme should result in more realistic
stomatal conductances and dry deposition velocities.In a previous evaluation study CPleim et al., 1996),
model simulations of surface energy fluxes, dry deposition velocities, and planetary boundary layer (PBL)
evolution, were compared to field measurements made near Bondville, IL during the summer of 1994. The
model predictions compared quite well to observations for all parameters, including dry deposition
velocity, over a corn field during moderately moist conditions. The current study focuses on comparison
of model simulations to field measurements made in a soybean field in southern Kentucky during the summer
of 1995.
17. KEY WORDS AND DOCUMENT ANALYSIS
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b.IDENTIFIERS/ OPEN ENDED
TERMS
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RELEASE TO PUBLIC

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