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PC-03 Abstract

Influence of Land Surface Processes/Land Cover Changes in Amazon on Regional Hydrometeorology

Sin Chan Chou — CPTEC - Centro de Previsão do Tempo e Estudos Climáticos (INPE) (SA-PI)
Yongkang Xue — University of California, Los Angeles (US-PI)

1) The general goals

In this project, we propose to use the regional Eta model coupled with

a biosphere model, SSiB, to investigate hydrometeorological prediction

in the Amazon Basin at seasonal to interannual time scales and the effects

of deforestation on circulation and rainfall, in particular, the annual

cycle of convective activity. The land cover and the soil properties will

be specified using remotely sensed data and derived data based upon observations,

respectively. Several important parameterizations, including soil hydrology,

stomatal resistance, and convective processes, will be investigated to

elucidate the mechanisms of land-atmosphere interactions in South America

and evaluate the sensitivity and uncertainty of these interactions in model

simulations. The observational data from LBA will be applied to these studies.

The ultimate goal is to improve the hydrometeorological prediction in the

LBA region by improving the land surface components in a coupled regional


In our research efforts, the prediction of rainfall in the Amazon Basin

will be a major focus. The spatial and temporal distribution of precipitation

in the Amazon Basin largely depends on convective activity. In a recent

study, Zhou and Lau (1998) infer that the austral spring, summer, and fall

rainfall activity in South America has characteristic features of a monsoon

climate system. Our main focus in this proposal is on seasonal-to-interannual

prediction, especially prediction of this annual convective activity, and

its relation to land surface processes.


2). Models and predictions for this study

The NCEP Eta regional model will be used for this study. In a recent

study, conducted with a later version of the Eta/bucket model by Dr. Chou(Co-I)

and her colleagues at the Center for Weather Prediction and Climate studies (CPTEC), a one-month forecast was performed for November 1997. The general

pattern of the forecast precipitation is comparable with the observations,

however a dry bias can be identified over Northeast Brazil and the Northern

coast (Figure 1). A coupled Eta/SSiB model (Xue et

al., 1996) will be used for LBA study. Figure 2 is

the South American vegetation map for Eta/SSiB.

In this proposal, we will use Eta/SSIB to conduct 12 month integrations

with climatological sea surface temperature (SST) data as surface boundary

conditions over the ocean. The role of land surface processes in simulating

the annual hydrologic cycle will be investigated. The El Nino and La Nina

have substantial impact on South American precipitation. After the completion

of the above- mentioned integrations, SSTs from a typical El Nino year

and a typical La Nina year will be used as the boundary conditions for

additional integrations. During the LBA, measurements will be taken at

different scales. These data will allow the establishment of a profile

of the vertical structure of the troposphere inclusive at canopy levels.

The model will be validated against the observations and analyses.


3). Impact of land cover/land cover change on the South American


Amazon is the largest continuous region of tropical rainforest in the

world, and a large amount of the original tropical forest has been lost

to deforestation. Sensitivity study has shown it may have substantial impact

on regional climate (e,g,, Xue et al., 1996a). With satellite remote sensing

data, the scales and locations of Amazonian deforestation have been quantified

and assessed with reasonable accuracy. Using these remote sensing assessments

instead of the hypothetical estimates of the deforestation, the GCM studies

on the hydrometeorological impacts of deforestation will be more realistic.

In this study, satellite data will be used for experimental designs

in deforestation scenarios. Data from Thematic Mapper (TM) and Multispectral

Scanner System (MSS) of Landsat are being used to create land use and deforestation

maps for the tropical areas for the 1970s, the 1980s, and the 1990s. For

the deforestation maps, the project has further aggregated the fine scale

maps into 16km grid maps. Using these land cover/land cover change maps,

we will investigate the methodology to implement land cover change detection

product and how does the land cover change affect the regional hydrometeorological

conditions in South America.


4). Sensitivity of model simulations to parameterizations in the

hydrologic processes

In this project, several important parameterizations crucial to the

hydrologic cycle will be tested, which include soil hydrology, stomatal

resistance, and convective scheme. The observational data from LBA will

be used for these studies. The goal is to elucidate the mechanisms of climate

change in South America and evaluate the sensitivity and uncertainty of

these interactions in model simulations.


4.1. Soil properties

In the Amazon area, there is a wide variety of soil types. The Radambrasil

project (1973-1986) has surveyed and mapped soil types, and produced 1162

soil profiles to a depth of 2m. Tomasella & Hodnett (1998) derived

and tested a PTF using soil data of the Brazilian Amazonia and related

the estimations to the Brooks & Corey (1964) retention parameters.

In this project, we will create an Amazon soil map for the Eta and the

GCM based on Radambrasil's soil tables up to 2 meters. This soil map will

be used for model integration and its results will be compared with those

from current SSiB soil map in the regional model. We will conduct another

experiment, in which the Tomasella and Hodnett parameterizations are used

for the soil. These three tests will provide information regarding the

sensitivity of land-atmosphere interaction to soil processes and improve

the specification of soil condition in the models.


4.2 Parameterizations of stomatal resistance

Stomatal resistance affects the energy partitioning between the latent

heat flux and the sensible heat flux. It is a crucial component of biosphere

models. We plan to introduce the physiological control of photosynthesis

on stomatal resistance to the model. This component of the photosynthetical

control in stomatal resistance model not only adds more constrains on the

fluxes through the stomata but also produce CO2 flux estimates. Because

the current physiological approach requests substantial amount of computer

time and is too complex for a GCM or a regional model, some simplification

will be made.

In the current physiological approach of stomatal resistance modeling,

the gross CO2 assimilation rate is limited by three rates: the efficiency

of the photosynthetic enzyme system, the reproduction rate of


and the leaf capacity to export or utilize the products of photosynthesis.

we plan to simplify the computation in the co-limitation regime, which

will allow us to solve the equations analytically. Therefore, it will substantially

simplify the computation.


4.3 Convective schemes

The convective activity is a crucial hydrometeorological process in

the Amazon Basin. However, the convective processes and its influence on

model simulation are uncertain. We will conduct investigation to study

the role of moist convection in transferring the energy and to identify

the uncertainty in tropical land-atmosphere interactions caused by the

cloud scheme. Two cloud schemes: a modified Betts and Miller scheme and

a prognostic cloud scheme called McRAS (Microphysics of clouds with Relaxed

Arakawa-Schubert Scheme) will be used in the Eta model for comparison.

The cloud scheme will also be evaluated using the LBA data. LBA will couple

with the TRMM program to provide polarimetric and Doppler radars during

the wet season in Rondonia. Besides the precipitation estimated from the

radars, the identification of hydrometers, their shape and size, and the

circulations within the convective cells will help in the validation of

the cloud schemes.


4.4 LBA data applications

The parameterizations discussed above will be investigated using observational

data from LBA field measurements. During the LBA, measurements will be

taken at different scales including about 11 instrumented towers. In the

application of LBA field data, we will collaborate with Dr. D. Fitzjarrald

of the State University of New York (SUNY) at Albany for the LBA research.




Zhou and Lau, 1989: Does a monsoon climate exist over South America?

J. Climate.

Xue, Y., H.G. Bastable, P. A. Dirmeyer, and P.J. Sellers, 1996a: Sensitivity

of simulated surface fluxes to changes in land surface parameterization

-- a study using ABRACOS data. J. Appl. Meteor., 35, 386-400.

Xue, Y., F.J. Zeng, K. Mitchell, and Z. Janjic, 1996f: The impact of

land surface processes on the prediction of the hydrological cycle over

the U.S.- A study using a coupled ETA/SSiB model. Preprint of Second International

Scientific Conference on the Global Energy and Water Cycle, 73-74.


Figure 1.

Figure 2.


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