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Raining from the Ground Up

Key Findings

  • In the eastern US and in Mexico, the probability of afternoon rain is impacted by the fluxes of heat and moisture from the land surface: increased evaporation before noon leads to increased probability of afternoon rain.
  • Afternoon rainfall probability in the rest of the US shows little sensitivity to surface fluxes.
  • The depth of rain from a given event is largely insensitive to contributions of heat and moisture from the land surface.
  • Surface evaporation can influence the triggering of convective rainfall but not the rainfall intensity.

Kirsten L. Findell (GFDL), Pierre Gentine (Columbia University), Benjamin R. Lintner (Rutgers University), and Christopher Kerr (GFDL/UCAR). “Probability of afternoon precipitation in eastern United States and Mexico enhanced by high evaporation.” Nature Geoscience – Advance online release June 5 2011, doi: 10.1038/NGEO1174.

Figure 1: Triggering Feedback Strength (units of probability of afternoon rain): a measure of the sensitivity of afternoon rainfall frequency to changes in evaporation.

Though it is obvious that rainfall moistens the land surface, our scientific understanding of how land surface moisture may interact with the atmosphere to encourage or suppress subsequent rainfall is limited. Using data from the North American Regional Reanalysis (NARR, Mesinger et al., 2006), this study shows that the likelihood of afternoon rainfall in the eastern United States and Mexico is strongly linked to evaporation from the land surface earlier in the day. In these regions, typical variations in surface flux partitioning – how available energy is divided between moisture flux (evapotranspiration) and sensible heat flux (heating up the air) – lead to a 10-25% change in the probability of afternoon rain, with higher evaporation increasing the probability of afternoon rainfall. The intensity of rain events is not sensitive to surface flux partitioning anywhere in the North American domain. These results demonstrate the role of local surface fluxes in triggering summertime continental convective rainfall and the minor impact of these fluxes on rainfall intensity.

Impact of surface fluxes on frequency and intensity of afternoon rainfall

Land-atmosphere interactions are critical for understanding Earth’s climate, with the feedback between soil moisture and precipitation among the most important elements. This feedback can be decomposed into soil moisture’s influence on evapotranspiration and evapotranspiration’s influence on precipitation. The latter process is the least well understood. While current research suggests that soil moisture likely has a positive impact on subsequent precipitation, assessment of this interaction has been limited by scarce large-scale observations. Here we develop and apply physically-based, objective metrics for quantifying the impacts of surface evaporative and sensible heat fluxes on the frequency and intensity of subsequent convective rainfall using summertime North American reanalysis data.

Figure 2: Amplification Feedback Strength (units of mm of afternoon rain): a measure of the sensitivity of afternoon rainfall
to changes in evaporation.

In this analysis, we look for regions where the probability of afternoon rainfall is sensitive to before-noon evaporative fraction (EF), or the fraction of available energy consumed by evaporation. The dependence of afternoon rainfall on before-noon evaporation is assessed through two measures: a triggering feedback strength (TFS, Figure 1), a measure of the sensitivity of afternoon rainfall frequency to changes in evaporation, and an amplification feedback strength (AFS, Figure 2), a measure of the sensitivity of afternoon rainfall  intensity to changes in evaporation. Figure 1 indicates that the variability in surface evaporative fraction (EF), or the fraction of available energy consumed by evaporation, explains 10-20% of the observed variability in the probability of afternoon convection over most of the eastern US and Mexico, with peaks over 25% in Florida. Higher evaporation, generally associated with higher soil moisture, enhances the probability of afternoon rainfall by up to 25%. This positive land-atmosphere feedback can perpetuate wet or dry extremes by modulating the frequency of afternoon convection.

In contrast to Figure 1, Figure 2 indicates that once triggered, afternoon rainfall intensity is rather insensitive to evaporation. Where the triggering feedback is highest, higher EF may lead to depth increases of < 1 mm (typically < 10% of mean afternoon rainfall).

Figures 1 and 2 underscore the importance of land surface and atmospheric boundary layer processes for convective triggering in some regions while indicating that rainfall amounts are largely independent of local surface moisture conditions in all but the wettest regions.

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