Post on 10-Jan-2016
description
Clouds and radiation in a LSM
L=50km
Physics@FOM, Veldhoven, 2009.
2
Stratocumulus cloud albedo: example
cloud layer depth = 400 m
effective cloud droplet radius = 10 m
optical depth = 25
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40 50 60
Cloud albedo
Cloud optical depth
homogeneous stratocumuluscloud layer
€
=32
LWPρ liqreff
, LWP = ρ air
zbase
ztop
∫ q ldz
Physics@FOM, Veldhoven, 2009.
3
Real clouds are inhomogeneous
Stratocumulus albedo from satellite
Physics@FOM, Veldhoven, 2009.
4
Albedo for an inhomogeneous cloud layer
Redistribute liquid water:
optical depths = 5 and 45
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40 50 60
Cloud albedo
Cloud optical depth
inhomogeneous stratocumuluscloud layer
mean albedo = 0.65 < 0.79
Physics@FOM, Veldhoven, 2009.
Decrease optical thickness:
Cahalan et al (1994): = 0.7 (FIRE I observations)
5
Cloud albedo in a weather forecast or climate model
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40 50 60
Cloud albedo
Cloud optical depth
effective mean
inhomogeneous albedo homogeneous
albedo
€
effective = χτ mean
Factor depends on the optical depth variance ()
Analytical results for the inhomogeneity factor Assumption: Gaussian optical depth distribution
not smaller than ~ 0.8
isolines
Environmental Physics
8
Scattering and absorption
absorption cross section a:
effective area of the molecule for removing energy from the incident beam
shortwave longwave
scattering cross section sca
absorption cross section a