PDF《数值模式内物理参数化》

数值模式内物理参数化 个人的看法 潘华陆 The purpose of these lectures ? Give you a flavor of how complicated the phenomena we try to parameterize are and that the only way to make the forecasts better is to build physically based schemes ? Give you a personalized view of the problems and the solutions.

What is the purpose of a parameterization package? ? Because of the limited computer resource, we can not hope to simulate the atmosphere to the molecular level. Some important physics (e.g. heating) are needed to make the model simulation closer to the real atmosphere. ? Convection and turbulence are two mechanism that have been considered necessary for the maintenance of the atmospheric circulation. Over the years we learned that complications of the earth surface and the complexities of the land also are important. In addition, cloud and radiation also make important contributions. While the model resolution increase seemed to give us better handle of the circulation of the large scale, errors made in the parameterization schemes continue to hamper our ability to simulate the atmosphere. Parameterization for weather and climate models ? Traditionally, climate modelers worry about time mean of the simulation. The idea is that, if we can get the time mean right, we don'

t need to know how it gets there. Over time, we are learning that how the model gets there is still important. In that, we mean the weather. Climate is still the average of the weather. It is important to get the weather statistics right for climate models as well. ? Weather modelers, on the other hand, worry more about getting the timing and intensity of the weather events and less about the time mean. In that sense, weather modelers traditionally worry less about cloud-radiation feedbacks and more about the evolution of the diurnal boundary layer (as an example). ? We need to learn to do both. Numerical Modeling Laboratory Numerical modeling Dynamics Physics Precipitation Solar Radiation PGF, Gravity, Coriolis force, advection Terrestrial radiation Turbulence Fluxes Surface friction Vegetation Air-sea interaction Clouds Air pollution Gases O3 What is physical parameterization? ? Moist and dry turbulent mixing in the atmosphere act mostly in scales smaller than all climate models and most of the weather models. ? Representing sub-grid scale effects of turbulence is commonly referred as physical parameterization. ? Other form of heating (clear sky radiation, microphysics for saturated grid) are not strictly parameterized. But the effect due to fractional clouds need to be parameterized. What is parameterization? ? Using a few parameters to represent the effect. ? Selecting and tuning of the parameters. ? The end? ? The challenge is to build a simple set of equations (the scheme) to represent the sub-grid scale phenomenon as closely as we can. ? The better the scheme, the fewer the tunable parameters should matter. ? Convergence issue: when grid size is small enough, can the scheme behave properly?

8 Schematic configuration 1) concept O O Concept Engineer vs physicist ? Engineers have to solve real life problems which are complicated and not-well-understood. Practical solutions are necessary evils. Use simple equations with a few parameters to adjust to the current solution is a practical way to move forward. ? In modeling the atmosphere, using a few local observations to form a parameterization tends not to work because the same turbulence acts differently over different regions: land-ocean, tropics-midlatitudes, etc. A new consensus is forming that we need to formulate the schemes based on better understanding of the phenomenon. So physical understanding becomes more important in formulating the scheme.