Mark Decker

Evaluation of the Reanalysis Products from GSFC, NCEP, and ECMWF Using Flux Tower Observations

AbstractReanalysis products produced at the various centers around the globe are utilized for many different scientific endeavors, including forcing land surface models and creating surface flux estimates. Here, flux tower observations of temperature, wind speed, precipitation, downward shortwave radiation, net surface radiation, and latent and sensible heat fluxes are used to evaluate the performance of various reanalysis products [NCEP–NCAR reanalysis and Climate Forecast System Reanalysis (CFSR) from NCEP; 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and ECMWF Interim Re-Analysis (ERA-Interim) from ECMWF; and Modern-Era Retrospective Analysis for Research and Applications (MERRA) and Global Land Data Assimilation System (GLDAS) from the Goddard Space Flight Center (GSFC)]. To combine the biases and standard deviation of errors from the separate stations, a ranking system is utilized. It is found that ERA-Interim has the lowest overall bias in 6-hourly air temper...

Improving the Numerical Solution of Soil Moisture-Based Richards Equation for Land Models with a Deep or Shallow Water Table

Abstract The soil moisture–based Richards equation is widely used in land models for weather and climate studies, but its numerical solution using the mass-conservative scheme in the Community Land Model is found to be deficient when the water table is within the model domain. Furthermore, these deficiencies cannot be reduced by using a smaller grid spacing. The numerical errors are much smaller when the water table is below the model domain. These deficiencies were overlooked in the past, most likely because of the more dominant influence of the free drainage bottom boundary condition used by many land models. They are fixed here by explicitly subtracting the hydrostatic equilibrium soil moisture distribution from the Richards equation. This equilibrium distribution can be derived at each time step from a constant hydraulic (i.e., capillary plus gravitational) potential above the water table, representing a steady-state solution of the Richards equation. Furthermore, because the free drainage condition h...

Groundwater Constraints on Simulated Transpiration Variability over Southeastern Australian Forests

A land surface scheme with and without groundwater‐vegetation interactions is used to explore the impact of rainfall variability on transpiration over drought-vulnerable regions of southeastern Australia. The authors demonstrate that if groundwater is included in the simulations, there is a low correlation between rainfall variability and the response of transpiration to this variability over forested regions. Groundwater reduces near-surface water variability, enabling forests to maintain transpiration through several years of low rainfall, in agreement with independent observations of vegetation greenness. If groundwater is not included, the transpiration variability matches the rainfall variability independent of land cover type. The authors’ results suggest that omitting groundwater in regions where groundwater sustains forests will 1) probably overestimate the likelihood of forest dieback during drought, 2) overestimate a positive feedback linked with declining transpiration and a drying boundary layer, and 3) underestimate the impact of land cover change due to inadequately simulating the different responses to drought for different land cover types.

Land surface models systematically overestimate the intensity, duration and magnitude of seasonal-scale evaporative droughts

Land surfacemodels (LSMs)must accurately simulate observed energy andwater fluxes during droughts in order to provide reliable estimates of futurewater resources.We evaluated 8 different LSMs (14model versions) for simulating evapotranspiration (ET) during periods of evaporative drought (Edrought) across sixflux tower sites. Using an empirically defined Edrought threshold (a decline in ET below the observed 15th percentile), we show that LSMs simulated 58 Edrought days per year, on average, across the six sites,∼3 times asmany as the observed 20 d. The simulated Edrought magnitudewas∼8 times greater than observed and twice as intense. Ourfindings point to systematic biases across LSMswhen simulating water and energy fluxes underwater-stressed conditions. The overestimation of key Edrought characteristics undermines our confidence in themodels’ capability in simulating realistic drought responses to climate change and haswider implications for phenomena sensitive to soilmoisture, including heat waves.

Influence of Leaf Area Index Prescriptions on Simulations of Heat, Moisture, and Carbon Fluxes

AbstractLeaf area index (LAI), the total one-sided surface area of leaf per ground surface area, is a key component of land surface models. The authors investigate the influence of differing, plausible LAI prescriptions on heat, moisture, and carbon fluxes simulated by the Community Atmosphere Biosphere Land Exchange version 1.4b (CABLEv1.4b) model over the Australian continent. A 15-member ensemble monthly LAI dataset is generated using the Moderate Resolution Imaging Spectroradiometer (MODIS) LAI product and gridded observations of temperature and precipitation. Offline simulations lasting 29 years (1980–2008) are carried out at 25-km resolution with the composite monthly means from the MODIS LAI product (control simulation) and compared with simulations using each of the 15-member ensemble monthly varying LAI datasets generated. The imposed changes in LAI did not strongly influence the sensible and latent fluxes, but the carbon fluxes were more strongly affected. Croplands showed the largest sensitivit...

Development and evaluation of a new soil moisture and runoff parameterization for the CABLE LSM including subgrid‐scale processes

New conceptual parameterizations of subgrid-scale soil moisture, runoff generation, and groundwater are developed and tested in the offline version of the Community Atmosphere and Biosphere Land Exchange (CABLE) model. These developments are designed to address a known bias in CABLE: the excess simulation of mean evapotranspiration over most of the Northern Hemisphere and over most vegetation types at global scales. The inclusion of subgrid-scale soil moisture parameterizations and runoff generation processes largely mitigates the overestimation of evapotranspiration, and simultaneously improves the simulation of mean runoff. There are also pronounced improvements in total water storage anomalies as compared to various observationally derived estimates. It is shown that surface and subsurface runoff generation resulting from subgrid-scale soil moisture heterogeneity is necessary to accurately simulate the mean observed runoff and evapotranspiration. Surface runoff generation is largely responsible from reducing the mean evapotranspiration bias north of 30°N. We note that the inability of previous CABLE evaluation studies to demonstrate the need for subgrid-scale heterogeneity is likely due to only testing at flux tower site scales. Evaluating land models at point scale and catchment scale across a large range of climate and vegetation types is necessary to evaluate the contributions of processes that influence the model at various spatial scales. Flux tower data are therefore a necessary but insufficient constraint on land surface models.

Applying Scaled Vegetation Greenness Metrics to Constrain Simulated Transpiration Anomalies: A Study over Australia*

AbstractThe feasibility of using vegetation greenness metrics as a proxy for transpiration variability over Australia is demonstrated. Several global evapotranspiration datasets, one of which provides transpiration data and is constructed independently of the vegetation greenness measurements, are compared to four satellite-based observations representative of the state of the vegetation over several regions in Australia. Further estimates of the transpiration are obtained by decomposing the evapotranspiration datasets using an ensemble of land surface model simulations. On monthly time scales, the greenness anomaly metrics show a near one-to-one relationship with the transpiration estimates when the time series are appropriately scaled by the mean. The authors demonstrate that anomalous vegetation greenness metrics, when properly scaled, provide a tool for evaluating transpiration variability simulated by land surface models and observation-based evapotranspiration datasets that include transpiration. Th...

Evaluating CMIP5 Model Agreement for Multiple Drought Metrics

AbstractGlobal climate models play an important role in quantifying past and projecting future changes in drought. Previous studies have pointed to shortcomings in these models for simulating droug...