Campbell CSAT3 Manual de usuario descargar pdf (Pagina 2)

D:\Shared on Tuch\SOP\JHU_SOPsonic_v03.doc - 2 -

Measurement of wind speed and air temperature and

computation of sensible heat and momentum fluxes using a

3-D sonic anemometer

1. Introduction

This SOP describes the basic working principle of the Campbell Scientific CSAT3 three

dimensional sonic anemometer used for wind speed and air temperature measurements

during the Baltimore PM Supersite project intensives. In addition, theoretical background

is given on the computation of momentum and sensible heat fluxes.

2. Method

A Campbell Scientific CSAT3 three-dimensional (3-D) sonic anemometer is used for this

study. It measures wind speed and the speed of sound on three nonorthogonal axes. The

wind speeds are then transformed into the orthogonal wind components u, v and w and

are referenced to the anemometer head. The speed of sound, c, is the average between the

three nonorthogonal sonic axes. From the sonically determined speed of sound the

ambient air temperature is deduced. Errors caused by wind blowing normal to the sonic

path are corrected before the wind speed is transformed into orthogonal coordinates. The

CSAT3 can be used to measure average horizontal wind speeds and direction and the

turbulent fluctuations of horizontal and vertical wind. From turbulent wind fluctuations,

momentum flux is calculated. As an example, the vertical flux of horizontal x-momentum

is defined as

><−= ''wu

ρτ

(1)

where < . > denotes the averaging operation, ρ is the density of air and u' (= <u> - u) and

w' (= <w> - w) are the fluctuating wind components in the x- and z-direction,

respectively. Similarly, the vertical flux of y-momentum τ

can be computed. Combining

this information furnishes the friction velocity u

. By finding the covariance between the

vertical wind speed fluctuations and temperature the sensible heat flux H can be

computed. The sensible heat flux is given as

><= '' TwcH

(2)

where c

is the specific heat of air at constant pressure and T' (= <T> - T) is the

fluctuating temperature. In addition to the fluxes of momentum and heat, the information

acquired with the sonic anemometer allows to characterize atmospheric stability.

Furthermore, from the horizontal wind components u and v, the wind direction can be

determined.

1 2 3 4 5 6 7 8 9

Sin comentarios

Campbell CSAT3 Manual de usuario Pagina 2