A note on the diurnal averaging of aeronomical models

Abstract

We have developed an approximate technique for diurnally time-averaging atmospheric photochemical-dynamical models which eliminates the need for a detailed numerical resolution of sunrise and sunset transitions. In its application, our scheme is equivalent to scaling certain chemical rate constants and photodissociation coefficients by appropriate aeronomical factors. To calculate the scaling factors, we parameterize diurnal variations with a step-function behavior, assuming that each species has a constant day-time and night-time concentration whose ratio we can determine by analyzing the chemical interactions occurring after sunset. Our solution accounts for the effects of night-time reactions on the 24 h average values of species abundances and on the average daily rates of the catalytic processes consuming ozone in the stratosphere. We demonstrate the accuracy of our technique by comparing its predictions to those of a full diurnal simulation; typically, the precision is better than 10%. By contrast, we show that the use of some other well-known computational schemes can result in significantly larger predictive errors—up to an order of magnitude for the N 2O 5 concentration as an extreme example.