Moist static energy
Encyclopedia
The moist static energy is a thermodynamic variable that describes the state of an air parcel
, and is similar to the equivalent potential temperature
. The moist static energy is a combination of a parcels kinetic energy
due to an air parcel's temperature, its potential energy
due to its height above the surface, and the latent energy due to water vapor present in the air parcel. It is a useful variable for researching the atmosphere because, like several other similar variables, it is conserved during adiabatic
ascent and descent.
The moist static energy, S, can be described mathematically as:
where Cp is the specific heat at constant pressure, T is the absolute air temperature
, g is the gravitational constant
, z is the height above the surface, Lv is the latent heat of vaporization, and q is water vapor specific humidity. Note that many texts use mixing ratio r in place of specific humidity q because these values tend to be close (within a few percent) under normal atmospheric conditions, but this is an approximation and not strictly correct.
Through the study of moist static energy profiles, Herbert Riehl
and Joanne Malkus determined in 1958 that hot tower
s, small cores of convection
approximately 5 kilometres (3.1 mi) wide that extend from the planetary boundary layer
to the tropopause
, were the primary mechanism that transported energy out of the tropics to the middle latitudes. More recently, idealized model simulations of the tropics indicate that the moist static energy budget is dominated by advection
, with shallow inflow in the lowest 2 kilometres (6,561.7 ft) of the atmosphere with outflow concentrated about 10 kilometres (32,808.4 ft) above the surface. Moist static energy has also been used to study the Madden–Julian oscillation (MJO). As with the tropics as a whole, the budget of moist static energy in the MJO is dominated by advection, but also is influenced by the wind-driven component of the surface latent heat flux. The relationship between the advection component and the latent heat component influence the timing of the MJO.
Air parcel
In fluid dynamics, within the framework of continuum mechanics, a fluid parcel is a very small amount of fluid, identifiable throughout its dynamic history while moving with the fluid flow. As it moves, the mass of a fluid parcel remains constant, while—in a compressible flow—its volume may...
, and is similar to the equivalent potential temperature
Equivalent potential temperature
Equivalent potential temperature, commonly referred to as theta-e \left, is a quantity related to the stability of a column of air in the atmosphere....
. The moist static energy is a combination of a parcels kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
due to an air parcel's temperature, its potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
due to its height above the surface, and the latent energy due to water vapor present in the air parcel. It is a useful variable for researching the atmosphere because, like several other similar variables, it is conserved during adiabatic
Adiabatic process
In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
ascent and descent.
The moist static energy, S, can be described mathematically as:
where Cp is the specific heat at constant pressure, T is the absolute air temperature
Thermodynamic temperature
Thermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics. Thermodynamic temperature is an "absolute" scale because it is the measure of the fundamental property underlying temperature: its null or zero point, absolute zero, is the...
, g is the gravitational constant
Earth's gravity
The gravity of Earth, denoted g, refers to the acceleration that the Earth imparts to objects on or near its surface. In SI units this acceleration is measured in metres per second per second or equivalently in newtons per kilogram...
, z is the height above the surface, Lv is the latent heat of vaporization, and q is water vapor specific humidity. Note that many texts use mixing ratio r in place of specific humidity q because these values tend to be close (within a few percent) under normal atmospheric conditions, but this is an approximation and not strictly correct.
Through the study of moist static energy profiles, Herbert Riehl
Herbert Riehl
Herbert Riehl was a German-born American meteorologist who is widely regarded as the father of tropical meteorology...
and Joanne Malkus determined in 1958 that hot tower
Hot tower
A hot tower is a tropical cumulonimbus cloud that penetrates the tropopause, i.e. it reaches out of the lowest layer of the atmosphere, the troposphere, into the stratosphere. In the tropics, the tropopause typically lies at least 15 km above sea level...
s, small cores of convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
approximately 5 kilometres (3.1 mi) wide that extend from the planetary boundary layer
Planetary boundary layer
The planetary boundary layer , also known as the atmospheric boundary layer , is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface forcing in an hour or less...
to the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...
, were the primary mechanism that transported energy out of the tropics to the middle latitudes. More recently, idealized model simulations of the tropics indicate that the moist static energy budget is dominated by advection
Advection
Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...
, with shallow inflow in the lowest 2 kilometres (6,561.7 ft) of the atmosphere with outflow concentrated about 10 kilometres (32,808.4 ft) above the surface. Moist static energy has also been used to study the Madden–Julian oscillation (MJO). As with the tropics as a whole, the budget of moist static energy in the MJO is dominated by advection, but also is influenced by the wind-driven component of the surface latent heat flux. The relationship between the advection component and the latent heat component influence the timing of the MJO.