3.1

Energy Balance in the PBLThe fluxes of heat and momentum drive the growth and structure of the PBL. To properly

characterize the PBL, one first needs a good estimate of the surface sensible heat flux (

H) whichdepends on the net radiation (

Rn) and surface characteristics such as the available surfacemoisture (described in the form of the Bowen ratio (

Bo)). In the CBL, a simple energy balanceapproach, as in Oke (1978), is used to derive the expression, used in AERMET, to calculate the

sensible heat flux,

H. We begin with the following simple characterization of the energy balancein the PBL:

where

His the sensible heat flux, 8Eis the latent heat flux,Gis the soil heat flux, andRnis thenet radiation. To arrive at an estimate of

Hsimple parameterizations are made for the soil andlatent heat flux terms; that is,

G R, and , respectively. Substituting thesen= 01. λE H Bo=expressions into eq. (1) the expression for surface heat flux becomes,

3.1.1 NET RADIATION

If measured values for

Rnare not available, the net radiation is estimated from theinsolation and the thermal radiation balance at the ground following the method of Holtslag and

van Ulden (1983) as

where

c1= 5.31x10-13 W m-2 K-6,c2= 60 W m-2,c3= 0.12, FSBis the Stefin Boltzman Constant(5.67x10-8 Wm-2K-4),

Trefis the ambient air temperature at the reference height for temperatureand

Rnis the net radiation. The albedo is calculated asr{ϕ} =r′ + (1−r′) exp(aϕ +b) , where

a= −0.1 ,b= − 05(1−r′) , and Note, braces, {}, are used throughout this 2 .r′ =r{ϕ = 90o}.report to denote the functional form of variables.

Solar radiation,

R, corrected for cloud cover, is taken from Kasten and Czeplak (1980) aswhere

nis the fractional cloud cover andRois the clear sky insolation which is calculated as

R( ) , and N is the solar elevation angle (tpandtare theo= 990 sinϕ − 30 =⎛ +

⎝ ⎜

⎞

⎠ ⎟

ϕ{

t} ϕ{t}p2

previous and present hours, respectively) (1975) . Note that when observations of cloud cover

are unavailable a value of 0.5 is assumed in eq. (3) and measurements of solar radiation are

required.

3.1.2 TRANSITION BETWEEN THE CBL AND SBL

When the PBL transitions from convective to stable conditions the heat flux changes sign

from a positive to a negative value. At the point of transition the heat flux must therefore vanish,

implying that the net radiation is equal to zero. By setting

Roequal to zero in eq. (3), and solvingfor sin N, the critical solar elevation angle N

crit,corresponding to the transition point between theCBL and the SBL can be determined from

n951 - n952 - n953 - n954 - n955 - n956 - n957 - n958 - n959 - n960 - n961 - n962 - n963 - n964 - n965 - n966 - n967 - n968 - n969 - n970 - n971 - n972 - n973 - n974 - n975 - n976 - n977 - n978 - n979 - n980 - n881 - n982 - n983 - n984 - n985 - n986 - n987 - n988 - n989 - n990 - n991 - n992 - n993 - n994 - n995 - n996 - n997 - n998 - n999 - n1000

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