Equation (103) leads to the following equation for the nocturnal urban boundary layer height

due to convective effects alone:

68

where

ziuois the boundary layer height corresponding toPo. Based on lidar measurements takenin Indianapolis (1991), and estimates of

ziufound by Bornstein (1968) in a study conducted inNew York city,

ziuois set to 400 m in AERMOD.In addition, since effects from urban heating should not cause

ziuto be less than themechanical mixing height,

ziuis restricted from being less thanzim. Therefore, the mixed layerheight for the nighttime urban boundary layer is computed as:

Once the urban mixing height has been estimated, a surrogate convective velocity scale

(appropriate for the magnitude of convective turbulence present) is computed by substituting

ziuand

Huinto the definitional equation forw*(Deardorff 1970). That is,where

w*uis the urban nighttime convective velocity scale andTis the near-surface airtemperature.

Having estimated

w*uthe turbulence in the nighttime urban can be enhanced using theexpressions found in Section 4.1.5. However, since for low level sources F

wTdepends primarilyon

u*(see eqs. (34) and (35)) it is not possible to directly enhance FwTfor these sources usingw*u.Therefore, an effective friction velocity (

u*eff) is developed as a surrogate forw*uin the lowerportion of urban PBL. We define

u*effas the friction velocity that is consistent with Fwm= Fwcatz= 7

zo. Assuming thatz= 7zois always less 0.1ziuc,u*effis estimated by equating Fwc(eq. (35)) withF

wm(eq. (37)) and solving foru*. Onceu*effis found, the urban friction velocity for nighttimeconditions (

u*u) is calculated as the maximum ofu*effandu*(the rural and daytime urban frictionvelocity).

Then using the enhanced velocity scales

u*uandw*u, the nighttime convective portion of theturbulence in the urban boundary layer is computed using the expressions turbulence found in

Section 4.1.5. That is, F

wcand Fwm are calculated from eqs. (35) and (37), respectively, withw*uused in place of the daytime convective velocity scale (

w*) andu*usubstituted for the ruralu*.Furthermore, for consistency purposes, a urban nighttime Monin-Obukhov length is calculated

using eq. (8) with substitutions

u*uforu*andHu(eq. (102)) forH.Finally, the total nighttime turbulence in the urban boundary layer is calculated as the sum (in

quadrature) of the convective and mechanical portions. With these enhanced levels, vertical

dispersion due to ambient turbulence (F

za) in the urban boundary layer is calculated from eq. (

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