Geoscience Reference
In-Depth Information
16
Equations of
Atmospheric
Flow in the ABL
Introduction
This chapter introduces the set of equations that are used to describe the
movement and evolution of the atmosphere and its constituents at any point in
time. Later these equations are developed to provide a description of mean
atmospheric flow and atmospheric turbulence by separating the variables used
into mean and fluctuating components and then applying the Reynolds averaging
described in the chapter 15.
One of the equations meteorologists use to describe the atmosphere is the
ideal gas law introduced in Chapter 1. Otherwise, the set of equations used are
simply the conservation laws for mass, momentum, energy and atmospheric
constituents, including moisture. However, to those unfamiliar with them, these
conservation equations may appear complex because they typically involve
many terms. The need to include several terms arises because the conservation
laws are being applied in a complex situation, i.e., in a moving fluid which has
viscosity, which is subject to a gravitational force and constrained to the surface of
a rotating Earth, and which has constituents some of which can undergo phase
changes.
Nonetheless, it is important to understand that the equations are fundamen-
tally just conservation laws, and the approach used to define them in this chapter
reflects this. In each case, the rate of change with time of the local concentration
of each conserved entity is first defined, recognizing that it is the rate of change
appropriate in a moving fluid that is required. Then the several physical processes
that can give rise to changes in local concentration of the conserved entity are
each separately identified and expressed algebraically. The required conservation
equation follows immediately by setting the rate of change in local concentration
equal to the sum of the terms that describe how it might be altered.
