The Global Atmospheric Circulation in Moist Isentropic Coordinates

Abstract

Differential heating of the earth's atmosphere drives a global circulation that transports energy from the tropical regions to higher latitudes. Because of the turbulent nature of the flow, any description of a ''mean circulation'' or ''mean parcel trajectories'' is tied to the specific averaging method and coordinate system. In this paper, the NCEP-NCAR reanalysis data spanning 1970-2004 are used to compare the mean circulation obtained by averaging the flow on surfaces of constant liquid water potential temperature, or dry isentropes, and on surfaces of constant equivalent potential temperature, or moist isentropes. While the two circulations are qualitatively similar, they differ in intensity. In the tropics, the total mass transport on dry isentropes is larger than the circulation on moist isentropes. In contrast, in midlatitudes, the total mass transport on moist isentropes is between 1.5 and 3 times larger than the mass transport on dry isentropes.It is shown here that the differences between the two circulations can be explained by the atmospheric transport of water vapor. In particular, the enhanced mass transport on moist isentropes corresponds to a poleward flow of warm moist air near the earth's surface in midlatitudes. This low-level poleward flow does not appear in the zonally averaged circulation on dry isentropes, as it is hidden by the presence of a larger equatorward flow of drier air at same potential temperature. However, as the equivalent potential temperature in this low-level poleward flow is close to the potential temperature of the air near the tropopause, it is included in the total circulation on moist isentropes. In the tropics, the situation is reversed: the Hadley circulation transports warm moist air toward the equator, and in the opposite direction to the flow at upper levels, and the circulation on dry isentropes is larger than that on moist isentropes.The relationship between circulation and entropy transport is also analyzed. A gross stratification is defined as the ratio of the entropy transport to the net transport on isentropic surfaces. It is found that in midlatitudes the gross stability for moist entropy is approximately the same as that for dry entropy. The gross stratification in the midlatitude circulation differs from what one would expect for either an overturning circulation or horizontal mixing; rather, it confirms that warm moist subtropical air ascends into the upper troposphere within the storm tracks.

A ‘warm path’ for Gulf Stream–troposphere interactions

“…Everything else being equal, this will increase the likelihood of connecting upper and lower levels' high θ e surfaces into a single frontal surface, leading to vigorous ascent throughout the troposphere (e.g. Pauluis et al, 2010). In contrast, in the SMTH experiment (Fig.…”

Comments on “MSE minus CAPE is the True Conserved Variable for an Adiabatically Lifted Parcel”

“…These assumptions are similar to those made in Emanuel (1994) and Pauluis et al (2010), but they all contradict the third law of thermodynamics, which states that the entropy of any substance is equal to a universal constant (set to zero) for the most stable crystalline form of the substance and at absolute zero temperature (namely for T = 0 K different from T trip and independently for all substances). More on this important issue will be discussed in the conclusion.…”

“…It is worth highlighting the advice of Richardson: the third law must not be applied to liquids or gases, only to the more stable solid state at 0 K. This explains why the criticisms about the third law published in Appendix A of Pauluis et al (2010) are not valid, since they wrongly argued that the term ln(T ) would be infinite at 0 K for a perfect gas. In fact, Debye's law is well defined and can indeed be considered for all solids, leading to finite values of entropy for all atmospheric species (N 2 , O 2 , H 2 O, Ar, CO 2 , ...).…”

“…Indeed, Tiselius clearly explains that the existence or nonexistence of chemical reactions depends on the difference in free enthalpy (or Gibbs' function), with differences in entropy to be computed with values obtained from the third law, and without any other arbitrary choices such as those chosen in Romps (2008) and retained in R15 or those previously chosen in Emanuel (1994) or Pauluis et al (2010).…”

Understanding Atmospheric Behaviour in Terms of Entropy: A Review of Applications of the Second Law of Thermodynamics to Meteorology

“…After that some alternative coordinate schemes are suggested, including the (transformed) isentropic coordinates [64][65][66][67][68][69][70][71], (hybrid) isentropic-sigma coordinates [72][73][74][75][76], combining terrain-following and isentropic coordinates, e.g., [77], and, isentropic semi-geostrophic coordinates [78] as well as hybrid isentropic-terrain-following coordinates, e.g., [79], employed in either regional or global models. Recently, in addition to the moist isentropic coordinates, an adaptive, hybrid-isentropic vertical coordinate was used in global atmospheric modeling 80,81].…”