Therefore, AERMET defines the point of transition between the CBL and SBL (day to

night) as the point in time when the solar elevation angle N = N

crit. On average, for clear andpartly cloudy conditions, the transition from stable to convective conditions occurs when N

17

reaches approximately 13o; for overcast conditions N

critincreases to about 23o (Holtslag and vanUlden 1983).

However, if solar radiation measurements are available AERMET determines N

critfrom anestimate of cloud cover rather than the actual observations themselves. In eq. (5) the cloud cover

(

n) is replaced with an equivalent cloud cover (neq) that is calculated from eq. (4) such that

n R R.3.2

Derived Parameters in the CBLIn this section the methods used by AERMET to calculate the PBL parameters in the

convective boundary layer are discussed. AERMET first estimates the sensible heat flux (

H),then calculates the friction velocity (

u*) and the Monin Obukhov Length (L). WithH,u*andL,AERMET can then estimate the height of the mixed layer and the convective velocity scale (

w*).3.2.1 FRICTION VELOCITY (

u*) & MONIN OBUKHOV LENGTH (L) IN THE CBLIn the CBL, AERMET computes the surface friction velocity,

u*, and the Monin-Obukhovlength,

L, using the value ofHestimated from eq. (2). Since the friction velocity and the MoninObukhov length depend on each other, an iterative method, similar to that used in CTDMPLUS

(Perry 1992), is used. AERMOD initializes

u*, andLby assuming neutral conditions (i.e., L=4).The final estimate of

u*andLis made once convergence is reached through iterativecalculations (i.e., there is less than a 1% change between successive iterations). The expression

for

u*(e.g., Panofsky and Dutton (1984)) is

where

gis the acceleration of gravity,cpis the specific heat of air at constant pressure, D is thedensity of air, and

Trefis the ambient temperature representative of the surface layer. Thenu*and

Lare iteratively recalculated using eqs. (6), (7) and (8) until the value ofLchanges by less than1%.

The reference heights for wind speed and temperature that are used in determining the

friction velocity and Monin-Obukhov length are optimally chosen to be representative of the

surface layer in which the similarity theory has been formulated and tested with experimental

data. Typically, a 10 m height for winds and a temperature within the range of 2 to 10 m is

chosen. However, for excessively rough sites (such as urban areas with

zocan be in excess of 1m), AERMET has a safeguard to accept wind speed reference data that range vertically between

7

zoand 100 m. Below 7zo(roughly, the height of obstacles or vegetation), measurements areunlikely to be representative of the general area. A similar restriction for temperature

measurements is imposed, except that temperature measurements as low as

zoare permitted.Above 100 m, the wind and temperature measurements are likely to be above the surface layer,

especially during stable conditions. Therefore, AERMET imposes an upper limit of 100 meters

for reference wind speed and temperature measurements for the purpose of computing the

similarity theory friction velocity and Monin-Obukhov length each hour. Of course, other US

EPA guidance for acceptable meteorological siting should be consulted in addition to keeping

the AERMET restrictions in mind.

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castellano: DISPER CUSTIC DESCAR RADIA italiano:

castellano: DIS CUS DES RAD english: DIS CUS DES RAD

português: DIS CUS DES RAD italiano: DIS CUS DES RAD