A Mie Code for Homogeneous Spherical Particles Application to Optical Properties of Water Clouds

1. A Mie Code for Homogeneous Spherical ParticlesApplication to Optical Properties of Water Clouds Presented by: Xiong Liu March 19, 2001

2. Outline • Introduction • Development of Mie Code • Validation • Optical properties of water clouds (UV-MV) • Mixture Of Carbon Aerosols in Clouds • Summary

3. Introduction • The globe is covered by clouds ~50% of time. Clouds play an very important role in the earth energy budget balance. • SW Cloud forcing: ~ -50 W/m2 • LW Cloud forcing: 30 W/m2 • Net Cloud forcing: -20 W/m2 • Cess et al. [1995] and Ramanthan et al. [1995] reported significant solar absorption by clouds of >=25 W/m2 (global-mean) from observations, which is called cloud absorption anomaly and still remains unexplained. • Clouds are usually treated as pure water clouds. In reality, cloud droplets grows from CCN, which might contains some absorbing aerosols such as soot (contains element carbon and organic carbon).

4. Mie Code • Input • Wavelength(or size parameter), radius, refractive index, angle bin • Output • Qext, Qsca, Qabs, Qback, Csca, Cext, Cabs, g, w, S11, S12, S33, S34 • Mie Coefficient Truncation: nmax =  + 4 0.3333+2 • Forward recursion for computing qn() • Downward recursion for computing Pn(), Pn() • N=1.1 |z| +10. If |z| <= 10000. • N=1.01 |z| +10. If |z| > 10000.

5. Mie Code-Equation an1, an2 Qext, Qsca Qabs Qback g S1, S2 S11,S12 S33,S34

6. Validation

7. Validation-Scattering Matrix

8. Homework Plot 1

9. Homework Plot 2

10. Homework Plot 3

11. Optical Properties of Water Clouds (1) • Refractive Index (HITRAN) • UV-MW, 90 wavelengths. Resolution is very coarse. • No size distribution is used. [Hu and Stammnes, 1993]. • Cloud Radius = 10um [Han et al., 1994]. • In UV,VIS, NIR, g, w, Qext, doesn’t change much. Qabs is very small until 1.5 um. • Qabs becomes more important at Thermal Infrared, and dominates Qext at Far IR and MW. • When lamda >=100um, Qext, g, w, Qabs start to decrease exponentially with lamda. • MW is suitable for terrestrial remote sensing (penetrate clouds, all weather).

12. Optical Properties of Water Clouds (2)

13. Optical Properties of Water Clouds (3) Retrieve Optical Thickness Retrieve Droplet Radius

14. Mixture of Carbon-Containing Aerosols in Clouds (Radius =10 um) 0.99790 0.99907 0.9999996

15. Mixture of Carbon-Containing Aerosols in Clouds (Radius =10 um) • Assume a cloud of optical thickness 100. • ppbv Absorption Optical Thickness • 0 100 X 4 X10-7 = 4X10-5 • 10 100 X 1 X10-6 =0.0001 • 100 0.001 • 1000 0.01 • 10000 0.1 BRDF

16. Summary • Mie Code works pretty well except for the direct expansion to obtain the absorption extinction. • Optical properties of clouds largely dependent on wavelengths. From UV to 1.5 um, g (~1) and w (~0.86) doesn’t change much. Qext, g, w, Qabs start to drop exponentially when wavelength is larger than 100 um. In MW, g, w, and Qext, Qabs drops to essentially zero. • At 0.63um, w almost don’t change with cloud droplet radius. At 3.7um, w drops from 0.98 to 0.73. • At 0.55 um, single scattering albedo drops from 0.9999996 to 0.9990 by mixing with soot of 10000 ppbv, approximately dropping one 9 for increasing one order of carbon.

17. Thank you !