HyARC Seminar (HyARC Seminar#166)
Date: February 21 (Friday) 16:00Room: The meeting space (#617) of Research Institutes Building.
Speaker: Prof. Gabriel Katul (Duke University)(http://nicholas.duke.edu/people/faculty/katul/homepage.html)<http://nicholas.duke.edu/people/faculty/katul/homepage.html>
Title: Soil moisture feedbacks on convection triggers and the role of soil-planthydrodynamics
Abstract:
It is now accepted that increases in greenhouse gas emissions lead to increases in air
temperature. However, the effects of such increases on the hydrologic cycle are far moredifficult to predict, especially precipitation. Nineteenth century equations presented
by John Dalton, Rudolf Clausius, and Benoit Paul Emile Clapeyron can provide
theoretical constraints on the so-called global hydrologic cycle acceleration following an
increase in global air temperature as predicted by state-of-the science generalcirculation models. However, at finer spatial and temporal scales over land, where thesoil-plant system exerts control on the atmospheric boundary-layer (ABL) dynamics andrainfall triggering mechanism, the problem becomes virtually intractable. The reason
this problem remains difficult stems from the fact that the coupling between soil
moisture and convective rainfall involves a large number of interacting processes
occurring within the soil-plant-atmosphere system that vary over a wide range of space
and time scales. Belowground and surface processes involve the dynamics of water
movement from the soil into the atmosphere (rooting system, plant hydrodynamics, andstomatal regulation dictating water movement from the roots and out of the stomata as
water vapor after the phase transition), the canopy aerodynamics (affecting thetransport of heat and water vapor from the canopy into the mixed layer), and thepartitioning of net radiation into latent, sensible, and soil heat fluxes [thereby
influencing skin temperature and directly affecting the dynamics of mean air
temperature and water vapor concentration in the ABL. On the other hand, the ABL,
with its unique coexistence of mechanically and thermally generated turbulence, acts as
an integrator of these surface processes with larger length and slowly evolving synoptic

scale processes affecting entrainment. The dynamics of these land surface fluxes andsoil-plant-atmosphere state variables control the simultaneous growth of the convective
boundary layer and lifting condensation level (LCL) and thus their crossing―anecessary but not sufficient condition for triggering convection. Even in the mostidealized cases, any exploration of the feedback mechanisms between soil moisture andconvection triggers must account, at minimum, for all these pathways at theappropriate scales. To progress on this problem, a soil-plant hydrodynamics model wascoupled to a simplified atmospheric boundary layer (ABL) budget to explore thefeedback of soil moisture on convection triggers. The soil-plant hydraulics formulationaccounted mechanistically for features such as root water uptake, root water
redistribution, and midday stomatal closure, all known to affect diurnal cycles ofsurface fluxes and, consequently, ABL growth. The ABL model considered theconvective boundary layer as a slab with a discontinuity at the inversion layer. A 30-day
dry-down simulation was used to investigate the possible feedback mechanisms
between soil moisture and convective rainfall triggers. Previous studies, which madeuse of surface flux measurements to drive an ABL model, have postulated that anegative feedback was possible, which could award the ecosystem with some degree ofself-regulation of its water status. According to model simulation results here, thisnegative feedback is unlikely. However, drastic changes in external water sources to theABL are needed for triggering convection when soil moisture is depleted. The apparent
negative feedback originated from a decoupling between the water vapor sources neededto produce convection triggers and surface water vapor fluxes.
(given in English)