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

Self Cite

Land‐atmosphere coupling can amplify heat extremes under declining soil moisture. Here we evaluate this coupling in 25 Coupled Model Intercomparison Project Phase 5 models using flux tower observations over Europe and North America. We compared heat extremes (2.5% of the hottest days of the year) and the evaporative fraction (EF; a measure of land surface dryness) on the day the heat extremes occurred. We found a negative relationship between the magnitude of heat extremes and EF in both models and observations in transitional regions, with the hottest temperatures occurring during the driest days, with a similar but less certain relationship in dry regions. Surprisingly, many models also showed an amplification of heat extremes by low EF in wet regions, a finding not supported by observations. Many models may therefore overamplify heat extremes over wet regions by overestimating the strength of land‐atmosphere coupling, with consequences for future projections of heat extremes.

Section: Resultsmentioning

confidence: 99%

Evaluating the Contribution of Land‐Atmosphere Coupling to Heat Extremes in CMIP5 Models

Ukkola

Pitman

Donat

et al. 2018

Self Cite

“…Predicted responses of transpiration to eCO 2 are highly variable among process-based models depending on what processes are accounted for and their parameterization, highlighting the current uncertainty about plant water use in response to eCO 2 (De Kauwe et al, 2013). Recent findings further suggest that land surface models show systematic bias in simulating water and energy fluxes under water-stressed conditions (Ukkola et al, 2016). Due to the large uncertainty in the estimation of evapotranspiration that takes into account the effect of eCO 2 and the consequent warming, we build our analysis on reference evapotranspiration (ETo), which is a supply-independent measure of the evaporative demand of a terrestrial climate and represents the rate at which a given climate is trying to evaporate water from the soil-vegetation system (Scheff & Frierson, 2014).…”

Section: Naumann Et Al 3285mentioning

confidence: 99%

Global Changes in Drought Conditions Under Different Levels of Warming

Naumann

Alfieri

Wyser

et al. 2018

545

298

Higher evaporative demands and more frequent and persistent dry spells associated with rising temperatures suggest that drought conditions could worsen in many regions of the world. In this study, we assess how drought conditions may develop across the globe for 1.5, 2, and 3°C warming compared to preindustrial temperatures. Results show that two thirds of global population will experience a progressive increase in drought conditions with warming. For drying areas, drought durations are projected to rise at rapidly increasing rates with warming, averaged globally from 2.0 month/°C below 1.5°C to 4.2 month/°C when approaching 3°C. Drought magnitudes could double for 30% of global landmass under stringent mitigation. If contemporary warming rates continue, water supply‐demand deficits could become fivefold in size for most of Africa, Australia, southern Europe, southern and central states of the United States, Central America, the Caribbean, north‐west China, and parts of Southern America. In approximately 20% of the global land surface, drought magnitude will halve with warming of 1.5°C and higher levels, mainly most land areas north of latitude 55°N, but also parts of South America and Eastern and South‐eastern Asia. A progressive and significant increase in frequency of droughts is projected with warming in the Mediterranean basin, most of Africa, West and Southern Asia, Central America, and Oceania, where droughts are projected to happen 5 to 10 times more frequent even under ambitious mitigation targets and current 100‐year events could occur every two to five years under 3°C of warming.

“…Finally, our methodology is currently restricted to 10‐day dry spells out of a pragmatic choice of sampling sufficient events globally. Land schemes may perform well at this time scale but poorly at longer time scales relevant for seasonal drought (Ukkola et al, ).…”

Section: Summary and Discussionmentioning

confidence: 99%

Evaluation of Regional‐Scale Soil Moisture‐Surface Flux Dynamics in Earth System Models Based on Satellite Observations of Land Surface Temperature

Gallego-Elvira

Taylor

Harris

et al. 2019

There is a lack of high‐quality global observations to evaluate soil drying impacts on surface fluxes in Earth system models (ESMs). Here we use a novel diagnostic based on the observed warming of the land relative to the atmosphere during dry spells (relative warming rate, RWR) to assess ESMs. The ESMs show that RWR is well correlated with changes in the partition of surface energy between sensible and latent heat across dry spells. Therefore, comparisons between observed and simulated RWR reveal where models are unable to capture a realistic soil moisture‐heat flux relationship. The results show that in general, models simulate dry spell ET dynamics well in arid zones while decreases in evaporative fraction appear excessive in some models in continental climate zones. Our approach can help guide land model development in aspects that are key in simulating extreme events like heat waves.