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