Enhancing Afternoon Precipitation: The Role of Evaporation and Heat Fluxes in the Eastern United States and Mexico

1. Probability of afternoon precipitation in eastern United States and Mexico enhanced by high evaporationby Kirsten L. Findell et al.2011, Nature Geoscience Andrew Badger GGS 656 February 14, 2012

2. “Moisture and heat fluxes from the land surface to the atmosphere form a critical nexus between surface hydrology and atmospheric processes, particularly relevant to precipitation.” How can we link them?

3. Focus • Determine the roles of latent and sensible heat fluxes in summertime convective precipitation • Where do these fluxes play large roles in summertime convective precipitation • Develop a metric for quantifying the impacts of these fluxes on the frequency and intensity of convective precipitation events

4. What is needed for SM to effect summertime precipitation? • Initial SM anomaly must be large • Evaporation must be strongly sensitive to SM • Precipitation must be strongly sensitive to evaporation This paper emphasizes linking strong precipitation sensitivity to these surface fluxes, a very uncertain step in the SM-precipitation feedback loop

5. Evaporative Fraction • The ratio of latent heat flux to the sum of sensible and latent heat fluxes • Higher EF is usually linked to higher SM and increased vegetation • Does higher EF mean a greater likelihood of afternoon precipitation?

6. EF and Afternoon Precipitation

7. Filtering Data • No morning rainfall or negative CTP days • Filters out precipitation from synopitc-scale systems • Filters out days that are too stable to support convection

8. Triggering Feedback Strength • Measure of the probability of afternoon precipitation changes with EF • Basically it means how sensitive convective precipitation is to EF

9. Amplification Feedback Strength • AFS is a measure of precipitation changes with EF once convection is “triggered” • Relatively small values where TFS and precipitation probability were largest

10. Does the EF and TFS relationship extend past the afternoon? • The link between them drops off dramatically after 6 pm • Has virtually no influence on the following days rainfall

11. Does ENSO effect this relationship? ENSO changes the precipitation pattern, but the EF and TFS framework still has the same pattern

12. Conclusion • In high EF regimes, typically areas with higher SM and more dense vegetation, the land surface and atmosphere are tightly coupled • Higher EF locations are likely to persist due to a high likelihood of afternoon precipitation • TFS indicates substantial surface flux partitioning controls frequency of afternoon precipitation • These metrics can be applied to atmospheric models to better simulate convective rainfall events

13. Criticisms • Simplified the atmosphere to a 1-D column • No account for the advection of sensible and latent heat fluxes • Two fundamentals of cloud formation are neglected, PBL growth and the lifted condensation level are neglected • Sensible heat flux provides the PBL growth to reach the LCL • Latent heat flux provides the moisture