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Virtual temperature

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Virtual temperature

In atmospheric thermodynamics, the virtual temperature T_v of a moist air parcel is the temperature at which a theoretical dry air parcel would have a total pressure and density equal to the moist parcel of air.[1]

Introduction

Description

In atmospheric thermodynamic processes, it is often useful to assume air parcels behave approximately adiabatic, and thus approximately ideally. The gas constant for the standardized mass of one kilogram of a particular gas is dynamic, and described mathematically as:

{R_x}=1000\frac{R^{*}}{M_x} \, ,

where R^{*} is the universal gas constant and M_x is the apparent molecular weight of gas x. The apparent molecular weight of a theoretical moist parcel in Earth's atmosphere can be defined in components of dry and moist air as:

{M_{air}}=\frac{e}{p}M_v+\frac{p_d}{p}M_d \, ,

with e partial pressure of water, p_d dry air pressure, and M_v and M_d representing the molecular weight of water and dry air respectively. The total pressure p is described by Dalton's Law of Partial Pressures:

{p}={p_d}+{e} \, .

Purpose

Rather than carry out these calculations, it is convenient to scale another quantity within the ideal gas law to equate the pressure and density of a dry parcel to a moist parcel. The only variable quantity of the ideal gas law independent of density and pressure is temperature. This scaled quantity is known as virtual temperature, and it allows for the use of the dry-air equation of state for moist air.[2] Temperature has an inverse proportionality to density. Thus, analytically, a higher vapor pressure would yield a lower density, which should yield a higher virtual temperature in turn.

Derivation

Consider an air parcel containing masses m_d and m_v of water vapor in a given volume V. The density is given by:

{\rho}=\frac{m_d+m_v}{V}=\rho_d+\rho_v \, ,

where \rho_d and \rho_v are the densities of dry air and water vapor would respectively have when occupying the volume of the air parcel. Rearranging the standard ideal gas equation with these variables gives:

{e}=\rho_vR_vT \, and {p_d}=\rho_dR_dT \, .

Solving for the densities in each equation and combining with the law of partial pressures yields:

{\rho}=\frac{p-e}{R_dT}+\frac{e}{R_vT}\, .

Then, solving for p and using \textstyle\epsilon=\frac{R_d}{R_v}=\frac{M_v}{M_d} is approximately 0.622 in Earth's atmosphere:

{p}={\rho}R_dT_v \, ,

where the virtual temperature T_v is:

{T_v}=\frac{T}{1-\frac{e}{p}(1-{\epsilon})}\, .

We now have a non-linear scalar for temperature dependent purely on the unitless value \scriptstyle\frac{e}{p}\, , allowing for varying amounts of water vapor in an air parcel. This virtual temperature T_v in units of Kelvin can be used seamlessly in any thermodynamic equation necessitating it.

Variations

Often the more easily accessible atmospheric parameter is the mixing ratio w. Through expansion upon the definition of vapor pressure in the law of partial pressures as presented above and the definition of mixing ratio:

\frac{e}{p}=\frac{w}{w+{\epsilon}}\, ,

which allows:

{T_v}=T\frac{w+\epsilon}{\epsilon(1+w)}\, .

Algebraic expansion of that equation, ignoring higher orders of w due to its typical order in Earth's atmosphere of 10^{-3}, and substituting \epsilon with its constant value yields the linear approximation:

{T_v} \approx T(1+0.61w)\, .

An approximate conversion using T in degrees Celsius and mixing ratio w in g/kg is:

{T_v} \approx T+\frac{w}{6}\, .[3]

Uses

Virtual temperature is used in adjusting CAPE soundings for assessing available convective potential energy from Skew-T log-P diagrams. The errors associated with ignoring virtual temperature correction for smaller CAPE values can be quite significant.[4] Thus, in the early stages of convective storm formation, a virtual temperature correction is significant in identifying the potential intensity in tropical cyclogenesis.[5]

Further reading

  • Wallace, John M.; Hobbs, Peter V. (2006). Atmospheric Science.  

References

  1. ^ Bailey, Desmond T. (February 2000) [June 1987]. "Upper-air Monitoring". Meteorological Monitoring Guidance for Regulatory Modeling Applications. John Irwin. Research Triangle Park, NC:  
  2. ^ "AMS Glossary". American Meteorological Society. Retrieved 2014-06-30. 
  3. ^ U.S. Air Force (1990). The Use of the Skew-T Log p Diagram in Analysis and Forecasting.  
  4. ^ "The Effect of Neglecting the Virtual Temperature Correction on CAPE calculations, Weather and Forecasting 1994; 9: 625-629". American Meteorological Society. Retrieved 2010-06-02. 
  5. ^ "Tropical cyclone genesis potential index in climate models". International Research Institute for Climate and Society. Retrieved 2009-12-10. 
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