Role of radiation parametrization update frequency in Tropical Cyclone simulation

Question

I am using a mesoscale model to simulate a tropical cyclone. The model input file has an option of 'Frequency of Radiation Tendencies update time' for Longwave and Shortwave radiation. According to me, all the parametrization schemes are updated at every time step,then why is it mentioned separately and how will it work? My question might be vary basic but instead of having the codes, I am unable to visualize what is happening with the radiation parametrization in my model. Is there any tutorial where I can get its clear description?

I am also searching for a tutorial for TC simulation where the basic steps to simulate a TC are given for beginners. It will be of great help if I can get it here.

Answer 1

Parameterizations are not usually updated at every time step, generally speaking. Radiation parameterization, in particular, is the most computationally expensive parameterization, since it calculates the integral of an exponential of an integral.

So what happens is you set a frequency to update the radiation parameterization, say every 30 minutes. So your model wiil find the diabatic heating tendencies from the initial state and use that for the first 30 minutes of model-time. After the first 30 minutes of model time, the parameterizations recalculate the diabatic tendencies based on the model values (such as clouds, etc). The shorter the timestep, the more you will account for changing model conditions. In short, it is how frequently the radiation parameterization is invoked.

So for example, if you are modeling your tropical cyclone, and you have the update frequency for too long of a time, you could be heating a place like the eye is passing over it, but the eye has already moved.

The shorter the frequency, the more the radiation will adapt to the model state, but at the cost of computational time.