Abstract
The tendency of convective rainfall to initiate over a wetter or drier land surface is a critical feedback process in the climate system, influencing the hydrological cycle on a variety of spatial scales, especially in parts of the world where water is limited. A simple algebraic solution is derived from fundamental physical equations, to predict the sign of this convective rainfall feedback with the surface. The tendency for convection to occur is evaluated by the rate at which the convective boundary‐layer top approaches the level of free convection. Well‐known integral models predict the rate of ascent of the boundary‐layer top, which tends to be faster over a dry surface. The associated changes in equivalent potential temperature in the boundary layer determine the rate at which the level of free convection descends, typically faster over a wet surface, as a function of the ambient profile, the thermodynamic forcing and the surface Bowen ratio. The resulting system is controlled by three parameters. Two nondimensional parameters determine whether there is wet or dry “advantage”; the Bowen ratio at the boundary‐layer top and a “convective instability parameter,” defined as the ratio of the vertical gradient of saturated equivalent potential temperature at the level of free convection to the profile stability just above the boundary layer. A dimensional function, dependent on the surface fluxes, the boundary‐layer depth, and the profile stability, provides the magnitude of the response. In comparison with previous work, the solution is both rigorously derived from physical principles and encapsulated in a simple algebraic form. A first evaluation of the theoretical framework has been made using data from a convection‐permitting numerical model simulation over India, and this indicates that the equations successfully determine the conditions under which convection is triggered over dry surfaces.
Highlights
Over recent decades, it has increasingly become clear that soil moisture conditions can control the initiation of deep convection, and control rainfall
The observational study of Guillod et al (2014) for regions of the United States found that precipitation the previous day was a better predictor of afternoon rainfall than surface state, implying that external drivers dominated the initiation in these examples
We have developed a thermodynamic model that describes the evolution of an atmospheric profile towards deep convective triggering, according to the underlying surface fluxes over wetter or drier surfaces
Summary
It has increasingly become clear that soil moisture conditions can control the initiation of deep convection, and control rainfall. Surface roughness and surface albedo both influence the boundary-layer state and influence the conditions supplying rainfall, most attention has focussed on the role of spatial patterns in soil moisture (for example, Pielke et al, 1991; Chen and Avissar, 1994; Schär et al, 1999; Findell and Eltahir, 2003a; Taylor et al, 2011; Huang and Margulis, 2012; Gentine et al, 2013; Lintner et al, 2013; Guillod et al, 2015), which affects the partitioning of the latent heat flux and sensible heat flux and the surface Bowen ratio. If afternoon convection is more likely to be triggered over such a vegetated or wet surface, there is said to be “wet advantage” (or positive feedback), whereas “dry advantage” (negative feedback) refers to conditions in which afternoon convection is more likely over a surface of high Bowen ratio, where there are conditions of low evaporation
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