A Regional Method for the Estimation of Sensible and Latent Heat Fluxes and Convective Boundary Layer Thickness

A novel method for estimating the surface fluxes of energy and the evolution of the Convective Boundary Layer (CBL) height at a monthly scale is presented. It is based on the hypothesis that the monthly mean evolution of air temperature and specific humidity is controlled solely by the convergence of the surface fluxes of sensible and latent heat. Such assumption is often valid on a monthly scale, in subtropical and mid-latitude regions away from coastal zones, because advective contributions tend to balance out between pre- and post-frontal situations. Under these constraints, the tendency equations of mean temperature and specific humidity are integrated, and the combinations of surface fluxes that provide a best fit for the average observed daily evolutions of these variables is taken as best estimate of the regional fluxes. Mean observed vertical profiles of these quantities are used to provide the background environment against which the CBL grows.

In this study, the method is applied for Santa Maria, Rio Grande do Sul, southern Brazil (29.72°S, 53.72°W; elevation: 103 m), where advective terms are neglectable on the monthly scale. The period studied is from November 2013 to August 2015. The estimates of the surface fluxes of energy are compared to the observations from a micrometeorological tower in which the energy balance closure is assumed.

In spite of the simplicity of the method, which uses only simple meteorological data such as mean hourly observations of air temperature and specific humidity and vertical profiles of these variables at 12 UTC, it provides estimates of the surface sensible and latent heat fluxes that are very similar to the monthly-averaged eddy covariance observations. Generally, the method was very efficient for the whole period studied, except for the winter months of 2014, where it was slightly less efficient. Also, the method displayed a slight overestimation of the latent heat fluxes for some of the spring and summer months and subtle underestimations for the remaining months. Conversely, the estimates of the sensible heat fluxes showed an opposite trend: In November and December of 2013 there was a small underestimation followed by an overestimation until August 2014, while in October and December of 2014 there was an overestimation of the fluxes followed by underestimations in the remaining months.

Despite the method still being in its initial stages of development, the estimates of surface fluxes of energy and CBL height are very promising. As long as the method has a universal physical foundation, its application is potentially efficient for other subtropical and mid-latitude regions.