Design of Global Models to Adhere to Thermodynamic Relationships (Invited Presentation)

Global weather prediction models endeavor to improve the realism (correspondence to observations) of their predictions by making their formulations more universal. In recent years the universality of model formulation has increased as a result of increases in available computing power, but these must be driven by the most important physical laws, such as conservation of energy, mass and momentum, and adherence to thermodynamic laws. Professor Johnson's contributions are many, but a very important one is the body of work related to thermodynamic formulation. These concepts have motivated a number of approaches to properly incorporate thermodynamic roles in model integrations. The use of isentropic coordinates have the advantages in that vertical motions do not cross coordinate surfaces, and numerically required dissipation does not cause false increases in entropy. The Flow-following Icosahedral Model (FIM), a hydrostatic model developed at ESRL, uses a hybrid coordinate that has many of the advantages of isentropic coordinates. The use of higher order finite volume and vertically Lagrangian coordinates are another method of increasing fidelity of thermodynamic treatment. The design of ESRL's Non Hydrostatic Model (NIM) was based on a third approach that should have advantages for clouds, as well as improving thermodynamic formulation. Specifically, the finite volume advection used in NIM is fully three dimensional and highly conservative for vertical motions.

Some numerical results will be presented to show that the model maintains thermodynamic relationships through extensive vertical motions trajectories. In addition, one year integrations of the FIM will be presented that show the importance of vertical motions in maintaining realistic clouds. NIM results will show how high vertical resolution and three- dimensional finite volume advection improve cloud realism.