Radiation transfer

These notes aim at fulfilling two different and somewhat contradictory objectives:

1) to give a general introduction to the parametrization of radiation transfer (RT) in numerical weather prediction and climate models. This might be useful to people who wish to look at general circulation model (GCM) outputs and to understand qualitatively how radiation transfer in clear and cloudy atmospheres is linked to the other physical processes, and how it can influence the atmospheric motions. In that case, the internal behaviour of the radiation scheme might remain a black box.

2) to provide a reasonably complete description of the RT parametrizations presently used in the ECMWF model to help understand the specific properties of the ECMWF forecasts. While the former task requires only basic knowledge of physics, the latter requires much more insight in how a RT scheme works.

The following pages somehow attempt to tackle these two tasks. After an introduction, the second chapter discusses the radiation budget at the top, and within the atmosphere, both globally and more regionally in relation with the other terms of the atmospheric energy balance. The third chapter is devoted to the derivation of the RT equation from an observational and qualitative point of view and to the introduction to the basic laws of physics necessary to understand the RT. The simplifications that can be used when dealing with RT in the Earth’s atmosphere, and the various approximations needed to make the RT a problem tractable for a numerical model of the atmosphere are also presented in chapter 3. The main aspects of the various RT parametrizations presently used in the ECMWF forecasting system are presented in chapter 4, together with the specification of the cloud optical properties. Examples of validation of various aspects of the parametrization of cloudradiation interactions with observed radiation fields are presented in chapter 5. As a matter of conclusion, some comments on the future of RT calculations, and on some other radiation-related hot topics are given in chapter 6.

Contents

1. Introduction: An historical perspective
2. The Earth's radiative balance and its implications
3. The theory of radiation transfer
4. Radiation schemes in use at ECMWF
5. Comparisons with observations
6. Conclusions and perspective
7. References