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LC-15 Abstract

Mapping Global Aerodynamic Roughness Length of Land Surface at 1 km Scale (Aerodynamic Roughness and Vegetation Structure of Amazon Basin)

Regina Célia dos Santos Alvalá — Centro de Previsão de Tempo e Estudos Climáticos/INPE (SA-PI)
Sassan Sepehri Saatchi — JPL/CALTECH (US-PI)



atmospheric boundary layer problems over various landscapes, the fluxes of

sensible heat, latent heat and carbon dioxide are determined in terms of surface

characteristics.   One of the

main parameters used in Global Circulation Models (GCM) or in

soil-vegetation-atmosphere transfer (SVAT) models is the aerodynamic roughness

length (Z0).  This

parameter is important to determine the vertical gradients of mean wind speed

and the conditions for momentum transfer over a vegetated or bare rough surface.

Over vegetated surfaces, the aerodynamic roughness length has a simple

one-to-one relationship with the rms  height

variation of the vegetation at the top of the canopy. 

Once this roughness length is determined for a surface, it does not

change with  wind speed, stability

or stress.  Because the ground

measurement of this parameter is difficult, time consuming and expensive,

reliable data on its distribution and quantity over global land cover types,

even at the coarse resolution of GCM models, do not exist. 

As a result, even the spatially distributed models use values from very

limited and sparse field measurements reported in the literature.  

In this work, we propose to use the SRTM data in conjunction with limited

calibration data from the VCL system to generate a global aerodynamic roughness

parameter at 1 km spatial scale to be used in regional and global scale climate

and land-atmospheric models.  The

phase and amplitude of the cross correlation of the two complex images obtained

by SRTM is known to be sensitive to surface height which is defined as the

distance between the scattering phase centers and a reference line. 

Even though the estimation of absolute height of vegetation from the data

can be erroneous due to surface variability and the limited penetration of the

signal into the canopy, but the variation of the height can be obtained

accurately. By averaging the estimated values from 30 meter resolution of SRTM

to 1 km pixels, the estimation of the rms height of the surface can be improved

to a centimeter accuracy. An algorithm has been developed to estimate this

parameter from SRTM data at the Jet Propulsion Laboratory and has been tested

using the airborne interferometric data (AIRSAR). To improve this algorithm and

validate the estimation of the parameter from the SRTM imagery, the Amazon basin

has been chosen as the primary test site.  This

will allow the use of field measurements, satellite data, and process models to

test the application of the algorithm.   The

LBA data sets used for this are: (1) vegetation structure data from various

primary, secondary and woodland sites, (2) tower measurements of aerodynamic

roughness, (3) VCL (Vegetation Canopy Lidar) data of vegetation height from

limited orbital data processed immediately after the mission launch, (4)

airborne laser altimeter data and possible AIRSAR data acquired by the LBA project,

(5) ecosystem/atmospheric process models for sensitivity analysis and

integration of spatial distribution of roughness data over the basin.

Project Goals:


To produce a global map of aerodynamic roughness at 1km scale.


To produce a validated map of the aerodynamic roughness over the Amazon



To augment the vegetation map of the Amazon basin with structural

information derived from SRTM data.


To incorporate the aerodynamic roughness distribution in regional

atmospheric modeling system (RAMS).

The work consists of (1) physically based modeling of SRTM interferometric data over the

Amazon basin in order to enhance the application of the algorithm to various

vegetation types; (2) SRTM data analysis

and the estimation of aerodynamic roughness over the entire Amazon; (3)  laser data analysis and the estimation of vegetation

height and roughness will be performed as soon as the data from VCL or from

airborne laser altimetry becomes available. 

(3) ground studies at various

sites within the LBA project to determine the relationship between the

aerodynamic roughness and the physical roughness of the vegetation and to

produce a data set for validation; (4) modeling

to integrate the aerodynamic roughness distribution derived from SRTM in

regional atmospheric processes and to assess the sensitivity of processes to

variations of roughness length over different vegetation types.

Last Updated: August 30, 2000

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