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  • Author or Editor: Joseph A. Santanello Jr. x
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Joseph A. Santanello Jr.
and
Mark A. Friedl

Abstract

Diurnal variation in soil heat flux is a key constraint on the amount of energy available for sensible and latent heating of the lower troposphere. Many studies have demonstrated that soil heat flux G is strongly correlated with net radiation R n . However, methods to parameterize G based on this relationship typically do not account for the dependency of G on soil properties and ignore asymmetry in the diurnal variation of G relative to R n . In this paper, the diurnal behavior of G as a function of R n is examined for sparse cover and bare soil conditions, focusing on patterns of diurnal variation as well as on the effects of soil moisture and soil type. To this end, information from field data is combined with simulations from a multilayer, diffusion-based soil model over a range of soil conditions and vegetation densities. The results show that a relatively simple function can be used to capture the first-order diurnal covariation between G and R n . Within this framework, soil moisture exerts an important control on this relationship. When soils make the transition from stage-1 (atmosphere limited) to stage-2 (soil limited) evaporation, the ratio of G to R n tends to increase. Further, soils in stage-2 evaporation exhibit positive G later in the day relative to moist soils. Data from several field experiments show that the amplitude of diurnal surface temperature can be used to predict the magnitude and behavior of G/R n by integrating the effects of soil type and moisture. Based on these results, a method to estimate G/R n is proposed that provides a robust representation of G/R n on hourly timescales for varying soil conditions. This method provides improvement over previous semiempirical treatments for G for which diurnal energy balance closure is required.

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Joseph A. Santanello Jr.
,
Mark A. Friedl
, and
William P. Kustas

Abstract

Relationships among convective planetary boundary layer (PBL) evolution and land surface properties are explored using data from the Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed in the southern Great Plains. Previous attempts to infer surface fluxes from observations of the PBL have been constrained by difficulties in accurately estimating and parameterizing the conservation equation and have been limited to multiday averages or small samples of daily case studies. Using radiosonde and surface flux data for June, July, and August of 1997, 1999, and 2001, a conservation approach was applied to 132 sets of daily observations. Results highlight the limitations of using this method on daily time scales caused by the diurnal variability and complexity of entrainment. A statistical investigation of the relationship among PBL and both land surface and near-surface properties that are not explicitly included in conservation methods indicates that atmospheric stability in the layer of PBL growth is the most influential variable controlling PBL development. Significant relationships between PBL height and soil moisture, 2-m potential temperature, and 2-m specific humidity are also identified through this analysis, and it is found that 76% of the variance in PBL height can be explained by observations of stability and soil water content. Using this approach, it is also possible to use limited observations of the PBL to estimate soil moisture on daily time scales without the need for detailed land surface parameterizations. In the future, the general framework that is presented may provide a means for robust estimation of near-surface soil moisture and land surface energy balance over regional scales.

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Emilee Lachenmeier
,
Rezaul Mahmood
,
Chris Phillips
,
Udaysankar Nair
,
Eric Rappin
,
Roger A. Pielke Sr.
,
William Brown
,
Steve Oncley
,
Joshua Wurman
,
Karen Kosiba
,
Aaron Kaulfus
,
Joseph Santanello Jr.
,
Edward Kim
,
Patricia Lawston-Parker
,
Michael Hayes
, and
Trenton E. Franz

Abstract

Modification of grasslands into irrigated and nonirrigated agriculture in the Great Plains resulted in significant impacts on weather and climate. However, there has been lack of observational data–based studies solely focused on impacts of irrigation on the PBL and convective conditions. The Great Plains Irrigation Experiment (GRAINEX), conducted during the 2018 growing season, collected data over irrigated and nonirrigated land uses over Nebraska to understand these impacts. Specifically, the objective was to determine whether the impacts of irrigation are sustained throughout the growing season. The data analyzed include latent and sensible heat flux, air temperature, dewpoint temperature, equivalent temperature (moist enthalpy), PBL height, lifting condensation level (LCL), level of free convection (LFC), and PBL mixing ratio. Results show increased partitioning of energy into latent heat relative to sensible heat over irrigated areas while average maximum air temperature was decreased and dewpoint temperature was increased from the early to peak growing season. Radiosonde data suggest reduced planetary boundary layer (PBL) heights at all launch sites from the early to peak growing season. However, reduction of PBL height was much greater over irrigated areas than over nonirrigated croplands. Relative to the early growing period, LCL and LFC heights were also lower during the peak growing period over irrigated areas. Results note, for the first time, that the impacts of irrigation on PBL evolution and convective environment can be sustained throughout the growing season and regardless of background atmospheric conditions. These are important findings and applicable to other irrigated areas in the world.

Significance Statement

To meet the ever-increasing demand for food, many regions of the world have adopted widespread irrigation. The High Plains Aquifer (HPA) region, located within the Great Plains of the United States, is one of the most extensively irrigated regions. In this study, for the first time, we have conducted a detailed irrigation-focused land surface and atmospheric data collection campaign to determine irrigation impacts on the atmosphere. This research demonstrates that irrigation significantly alters lower atmospheric characteristics and creates favorable cloud and convection development conditions during the growing season. The results clearly show first-order impacts of irrigation on regional weather and climate and hence warrant further attention so that we can minimize negative impacts and achieve sustainable irrigation.

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