Is SBDART on Target?: An Analysis of the Radiative Transfer Model to Observations

1. Is SBDART on Target?: An Analysis of the Radiative Transfer Model to Observations Daniel P. Tyndall Department of Marine and Environmental Systems Florida Institute of Technology 20 July 2005

2. Overview • What is radiative transfer? • What is SBDART? • How did we evaluate SBDART? • How well does SBDART do? • What can we conclude?

3. What is radiative transfer? • Transfer of radiant (or electromagnetic) energy through a medium • The four types of basic radiative transfer processes: • Transmission • Absorption • Reflection • Scattering • All radiative transfer processes are based on these four processes

4. Components of Radiative Transfer • Solar radiation strikes cloud droplet • Reflection • Scattering • Absorption • Transmission as infrared radiation     

5. SBDART • Why do we care about radiative transfer? • Radiative transfer drives the weather • Computing all interactions for the entire thickness of atmosphere is impossible • Santa Barbara DISORT Atmospheric Radiative Transfer model • Written by P. Ricchiazzi et al. at the Institute of Computational Earth Systems Science, University of California, Santa Barbara • FORTRAN code first compiled in 1998 • Code continuously improved

6. SBDART Input Parameters • 75 input parameters • Atmospheric profile sounding input • Changes in temperature, pressure, water vapor and ozone concentrations with height • Cloud layer input • Particulate pollution input • Aerosol profile input • Ground albedo parameters • Geographical location, date, and time input

7. SBDART Output

8. Verification of the Model • Goal: Verification of SBDART • Clear sky • Cloudy sky (well developed cumulus) • Methods of verification • Measuring total downward flux using a radiometer • Measuring the effective temperature of an object in a specific wavelength

9. Cloud and Sky Temperature Measurements • Heitronics KT15.85 IIP infrared pyrometer (supported by 2005 ACITC faculty grant) • Sensitive to radiation between 9.6 and 11.5 micrometers • Pyrometer pointed to clouds and clear sky • Temperatures recorded every second Source: Heitronics

10. Pyrometer Calibration

11. Calculating Temperature from SBDART

12. Changing Incoming Flux to Temperature • Plank’s Equation • Plank’s equation used to change flux to temperature • Integrating over range of pyrometer • Solved for the term T, temperature, using iterative approach

13. Simulating the Melbourne Atmosphere • How do we simulate the Melbourne atmosphere? • XMR 1500Z (Cape Canaveral) sounding used • Temperature • Pressure • Humidity • Tropical ozone profile built in to SBDART also used • Clouds • Droplet size • Optical depth • Cloud height and thickness

14. Estimating Cloud Parameters STID = XMR STNM = 74794 TIME = 050608/1500 PRES HGHT TMPC VAPR 1014.00 3.00 27.20 31.67 1000.00 131.00 25.40 30.93 980.47 305.00 24.04 29.39 977.00 336.29 23.80 29.12 974.00 363.32 23.40 27.92 946.94 610.00 22.32 22.50 944.00 637.20 22.20 21.96 914.42 914.00 20.05 22.08 911.00 946.56 19.80 22.10 882.61 1219.00 18.34 19.64 850.00 1543.00 16.60 17.04 • Cloud bases estimated from the pyrometer temperatures and temperature profile from soundings • Clouds treated as blackbodies (e.g. M. Griggs 1968) • Maximum droplet radius • Maximum optical depth • Cloud thickness set at 1 kilometer Output from SNLIST, showing sounding information. If we were evaluating a cloud that measured 22°C on the pyrometer, we would estimate its base to be at 637.20 m.

15. Two SBDART Model Runs   Clear Sky Temperature Cloud Base Temperature

16. Clear Sky Temperature Verification

17. Cloud Base Temperature Verification

18. Why the difference in temperatures? • Possible flaws in approximating atmosphere • Impact of intervening water vapor • Model uses plane parallel approximations • Approximation of clouds as perfect blackbodies • Differences in temperature are not large in both cases • At 20°C, error of 2% causes a temperature variance of 1°C

19. Conclusions • Difference between SBDART and measured temperatures of low level cumulus clouds within a few degrees • Model-observation clear sky comparisons are much greater • Differences in model and observed temperatures • Problem with observed measurement, model, model input, or a combination of these? • More analysis on SBDART encouraged

20. Acknowledgements • Rebecca Davis for cloud base estimates • Melissa Martin for pyrometer calibration data • ACITC for providing funding for pyrometer

21. References Aestheimer, Robert W. Handbook of Infrared Radiation Measurement. Barnes Engineering Company, Stamford, Connecticut, 82 pp., 1983. Hottel, H.C. and A.F. Sarofim. Radiative Transfer. McGraw-Hill, New York/St. Louis/San Francisco/Toronto/London/Sydney, 520 pp., 1967. Griggs, M. Emissivities of Natural Surfaces in the 8- to 14-Micron Spectral Region. J. Geophys. Res., 73(24): 1968. Ricchiazzi, Paul et al. Santa Barbara DISORT Atmospheric Radiative Transfer. <http://arm.mrcsb.com/sbdart/> 2001. Ricchiazzi, Paul, Shiren Yang, and Catherine Gautier. SBDART: A Practical Tool for Plane Parallel Radiative Transfer in the Earth’s Atmosphere. Earth Space Research Group, Santa Barbara, CA <http://www.crseo.ucsb.edu/esrg/pauls_dir/>, 2005. Ricchiazzi, Paul, Shiren Yang, Catherine Gautier, and David Sowle. SBDART: A Research and Teaching Software Tool for Plane-Parallel Radiative Transfer in the Earth’s Atmosphere. Bull. Am. Meteorol. Soc., 85(1): 2004. Wallace, John M. and Peter V. Hobbs. Atmospheric Science: An Introductory Survey. Academic Press, San Diego/New York/Boston/London/Sydney/Tokyo/Toronto, 467 pp., 1977. And now, Wanda Reeves…