Impacts of Land–Atmosphere Feedbacks on Deep, Moist Convection on the Canadian Prairies

Abstract

The purpose of this study was to focus on how anomalies in the normalized difference vegetation index (NDVI; a proxy for soil moisture) over the Canadian Prairies can condition the convective boundary layer (CBL) so as to inhibit or facilitate thunderstorm activity while also considering the role of synoptic-scale forcing. This study focused on a census agricultural region (CAR) over central Alberta for which we had observed lightning data (proxy for thunderstorms), remotely sensed NDVI data, and in situ rawinsonde data (to quantify impacts of vegetation vigor on the CBL characteristics) for 11 summers from 1999 to 2009. The authors' data suggest that the occurrence of lightning over the study area is more likely (and is of longer duration) when storms develop in an environment in which the surface and upper-air synoptic-scale forcing are synchronized. On days when surface forcing and midtropospheric ascent are present, storms are more likely to be triggered when NDVI is much above average, compared to when NDVI is much below average. Additionally, the authors found the response of thunderstorm duration to NDVI anomalies to be asymmetric. That is, the response of lightning duration to anomalies in NDVI is marked when NDVI is below average but is not necessarily discernible when NDVI is above average. The authors propose a conceptual model, based largely on observations, that integrates all of the above findings to describe how a reduction in vegetation vigor-in response to soil moisture deficits-modulates the partitioning of available energy into sensible and latent heat fluxes at the surface, thereby modulating lifting condensation level heights, which in turn affect lightning activity.

Soil moisture–precipitation coupling: observations from the Oklahoma Mesonet and underlying physical mechanisms

“…Through its control of surface evaporative fraction (EF), anomalously wet or dry soils can induce modification of the planetary boundary layer (PBL), including changes in the height of the lifting condensation level (LCL) and the level of free convection (LFC) (Brimelow et al, 2011). Without consideration of free-tropospheric conditions, afternoon LCL and LFC heights generally decrease with sufficient moisture flux from a wet soil surface, which increases energy available for convection (i.e., CAPE).…”

Distinguishing between Unorganized and Organized Convection When Examining Land–Atmosphere Relationships

“…Previous studies examining potential soil moisture feedbacks identify mechanisms for dry soils to initiate convection (Santanello et al 2011), as well as initiation over wet soils (Brimelow et al 2011). FRQ15 found a strong preference for precipitation to occur over wet soils, when atmospheric conditions were otherwise not conducive to convection.…”

Climate of the Critical Zone

“…They demonstrated that surface energy partitioning was strongly controlled by SM in all three schemes, although the form of the relationship varied somewhat between schemes. Brimelow et al (2011) proposed that reductions in latent heat flux due to dry soils resulted in a deeper, warmer boundary layer and less convective storms during the summer of 2002 in Alberta, Canada. They found strong, linear correlations between SM and surface energy flux, representing strong land-atmosphere coupling on days that they identified as ideal (i.e., clear skies, low winds, and no recent precipitation).…”

“…One issue is the different mechanisms by which SM can influence convection. Brimelow et al (2011) compared convection-related indices with observations of normalized difference vegetation index, a proxy for evapotranspiration. Their explanation was that anomalously dry soils increase sensible heat flux and surface heating, which destabilizes the atmospheric profile in the PBL and creates an area of local convergence, possibly leading to convection and precipitation over the relatively dry soils.…”

“…Past studies have shown evidence of SM feedback to precipitation from wet soils (Brimelow et al, 2011) and dry soils (Westra et al, 2012). Mechanistically, wet soils partition incoming energy into a higher ratio of latent heat over sensible heat, which increases moist static energy in the near-surface atmosphere (Pal and Eltahir, 2001).…”