In this article, we investigate the differences between two different quantities ([theta] and [[theta].sub.e]) and three functions (magnitude of the horizontal gradient, thermal front parameter, and frontogenesis) based on global reanalyses using a gridpoint-based method of identifying fronts described below.

To help visualize the importance of the moisture in the diagnostic fields involving [[theta].sub.e] without the influence of the temperature field, we compute the same three functions (magnitude of the gradient, thermal front parameter, and frontogenesis) using the specific humidity q at 850 hPa (Fig.

Climatologies are constructed for the six different frontal diagnostics discussed earlier: magnitude of the gradient, thermal front parameter, and frontogenesis for 850-hPa potential temperature and equivalent potential temperature (Figs.

Another way to show that the moisture field is affecting the climatologies involving [[theta].sub.e] is to compute the same three functions (magnitude of the gradient, thermal front parameter, and frontogenesis) using the specific humidity q at 850 hPa (Fig.

* The advantages and disadvantages of two thermodynamic quantities ([theta] and [[theta].sub.e]) and three functions (magnitude of the horizontal gradient, thermal front parameter, and frontogenesis) were reviewed in the text and summarized in Table 2.

* The Petterssen frontogenesis function using [theta] was effective for identifying midlatitude fronts, although may have quite a bit of alongfront variability in individual cases.

Given that Petterssen frontogenesis represents the process of frontal formation (i.e., isotherms being concentrated in the horizontal by the horizontal wind), it can be a useful tool for the analysis of fronts in operational forecasting and climatological analysis, as well as for research where its usage is already common (e.g., Keyser et al.

If the community requires fronts to have temperature gradients as the synoptic-dynamic literature and synoptic experience imply, then fronts should be a subset of airmass boundaries with a requisite thermal gradient and possibly also Petterssen frontogenesis along it.

Schultz, 2005: Contraction rate and its relationship to frontogenesis, the Lyapunov exponent, fluid trapping, and airstream boundaries.

Bretherton, 1972: Atmospheric frontogenesis models: Mathematical formulation and solutions.

Shapiro, 1986: Diagnosis of the role of vertical deformation in a two-dimensional primitive equation model of upper-level frontogenesis. J.