Land-surface controls on afternoon precipitation diagnosed from observatorial data: uncertainties and confounding factors
Guillod, B.P. ; Orlowsky, B. ; Miralles, D. ; Teuling, A.J. ; Blanken, P.D. ; Buchmann, N. - \ 2014
Atmospheric Chemistry and Physics 14 (2014). - ISSN 1680-7316 - p. 8343 - 8367.
carbon-dioxide exchange - boundary-layer interactions - soil-moisture feedbacks - american regional reanalysis - low-level parameters - diurnal time scales - energy-balance - atmospheric controls - northern wisconsin - heat fluxes
The feedback between soil moisture and precipitation has long been a topic of interest due to its potential for improving weather and seasonal forecasts. The generally proposed mechanism assumes a control of soil moisture on precipitation via the partitioning of the surface turbulent heat fluxes, as assessed via the evaporative fraction (EF), i.e., the ratio of latent heat to the sum of latent and sensible heat, in particular under convective conditions. Our study investigates the poorly understood link between EF and precipitation by relating the before-noon EF to the frequency of afternoon precipitation over the contiguous US, through statistical analyses of multiple EF and precipitation data sets. We analyze remote-sensing data products (Global Land Evaporation: the Amsterdam Methodology (GLEAM) for EF, and radar precipitation from the NEXt generation weather RADar system (NEXRAD)), FLUXNET station data, and the North American Regional Reanalysis (NARR). Data sets agree on a region of positive relationship between EF and precipitation occurrence in the southwestern US. However, a region of strong positive relationship over the eastern US in NARR cannot be confirmed with observation-derived estimates (GLEAM, NEXRAD and FLUXNET). The GLEAM–NEXRAD data set combination indicates a region of positive EF–precipitation relationship in the central US. These disagreements emphasize large uncertainties in the EF data. Further analyses highlight that much of these EF–precipitation relationships could be explained by precipitation persistence alone, and it is unclear whether EF has an additional role in triggering afternoon precipitation. This also highlights the difficulties in isolating a land impact on precipitation. Regional analyses point to contrasting mechanisms over different regions. Over the eastern US, our analyses suggest that the EF–precipitation relationship in NARR is either atmospherically controlled (from precipitation persistence and potential evaporation) or driven by vegetation interception rather than soil moisture. Although this aligns well with the high forest cover and the wet regime of that region, the role of interception evaporation is likely overestimated because of low nighttime evaporation in NARR. Over the central and southwestern US, the EF–precipitation relationship is additionally linked to soil moisture variations, owing to the soil-moisture-limited climate regime.
Some observational evidence for dry soils supporting enhanced relative humidity at the convective boundary layer top
Westra, D. ; Steeneveld, G.J. ; Holtslag, A.A.M. - \ 2012
Journal of Hydrometeorology 13 (2012). - ISSN 1525-755X - p. 1347 - 1358.
diurnal time scales - land-surface - cumulus onset - vertical diffusion - model - precipitation - entrainment - prediction - moisture - column
The tendency of the relative humidity at the top of a clear convective boundary layer (RHtop) is studied as an indicator of cloud formation is studied over a semi-arid region within the conceptual framework introduced by Ek and Holtslag (2004). Typically the tendency of RHtop increases if the evaporative fraction at the land surface increases, which supports boundary layer moistening but only when boundary-layer growth is limited by atmospheric factors. This regime was supported by Cabauw observations in the original study. Here we provide new observational evidence that the tendency of RHtop can also increase as the surface becomes more dry as is consistent with another regime of the conceptual framework. The observations used are from the AMMA intensive observational campaign near Niamey, Niger, June 20-25, 2006. In addition, we evaluate whether various versions of the WRF single-column model confirm the different regimes of the conceptual framework for a typical day in the AMMA campaign. It appears that the model confirms that dryer soils can support cloud formation.
Understanding the daily cycle of evapotranspiration: a method to quantify the influence of forcings and feedbacks
Heerwaarden, C.C. van; Vilà-Guerau de Arellano, J. ; Gounou, A. ; Guichard, F. ; Couvreux, F. - \ 2010
Journal of Hydrometeorology 11 (2010)6. - ISSN 1525-755X - p. 1405 - 1422.
convective boundary-layer - land-atmosphere interaction - diurnal time scales - soil-moisture - surface - model - evaporation - parameterization - entrainment - inversion
A method to analyze the daily cycle of evapotranspiration over land is presented. It quantifies the influence of external forcings, such as radiation and advection, and of internal feedbacks induced by boundary layer, surface layer, and land surface processes on evapotranspiration. It consists of a budget equation for evapotranspiration that is derived by combining a time derivative of the Penman–Monteith equation with a mixed-layer model for the convective boundary layer. Measurements and model results for days at two contrasting locations are analyzed using the method: midlatitudes (Cabauw, Netherlands) and semiarid (Niamey, Niger). The analysis shows that the time evolution of evapotranspiration is a complex interplay of forcings and feedbacks. Although evapotranspiration is initiated by radiation, it is significantly regulated by the atmospheric boundary layer and the land surface throughout the day. In both cases boundary layer feedbacks enhance the evapotranspiration up to 20 W m-2 h-1. However, in the case of Niamey this is offset by the land surface feedbacks since the soil drying reaches -30 W m-2 h-1. Remarkably, surface layer feedbacks are of negligible importance in a fully coupled system. Analysis of the boundary layer feedbacks hints at the existence of two regimes in this feedback depending on atmospheric temperature, with a gradual transition region in between the two. In the low-temperature regime specific humidity variations induced by evapotranspiration and dry-air entrainment have a strong impact on the evapotranspiration. In the high-temperature regime the impact of humidity variations is less pronounced and the effects of boundary layer feedbacks are mostly determined by temperature variations