Large-scale surface responses during European dry spells diagnosed from land surface temperature

Soil moisture plays a fundamental role in regulating the summertime surface energy balance across Europe. Understanding the spatial and temporal behaviour in soil moisture and its control on evapotranspiration (ET) is critically important, and influences heat wave events. Global climate models (GCMs) exhibit a broad range of land responses to soil moisture in regions which lie between wet and dry soil regimes. In situ observations of soil moisture and evaporation are limited in space, and given the spatial heterogeneity of the landscape, are unrepresentative of the GCM grid box scale. On the other hand, satellite-borne observations of land surface temperature (LST) can provide important information at the larger scale. As a key component of the surface energy balance, LST is used to provide an indirect measure of surface drying across the landscape. In order to isolate soil moisture constraints on evaporation, time series of clear sky LST are analysed during dry spells lasting at least 10 days from March to October. Averaged over thousands of dry spell events across Europe, and accounting for atmospheric temperature variations, regional surface warming of between 0.5 and 0.8 K is observed over the first 10 days of a dry spell. Land surface temperatures are found to be sensitive to antecedent rainfall; stronger dry spell warming rates are observed following relatively wet months, indicative of soil moisture memory effects on the monthly time scale. Furthermore, clear differences in surface warming rate are found between cropland and forest, consistent with contrasting hydrological and aerodynamic properties

Evaluation of Regional-Seale Soil Moisture-Surface Flux Dynamics in Earth System Models Based On satellite Observations of Land Surface Temperature

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. Plain Language Summary We present a methodology to assess how land evapotranspiration (ET) responds to soil moisture (SM) in climate models across the world. Our method is based on looking at the observed versus modeled evolution of land surface temperature and the overlying air temperature during rain free periods, when SM decreases and limits the amount of land ET. We observed a good relationship between modeled ET and the modeled relative evolution of these temperatures during dry periods. Therefore, our method can be used to evaluate how realistic are modeled SM-ET relationships when soils are drying. Our results show that in general, models capture fairly well the influence of SM on ET in arid climate zones, while they perform less well in continental climate zones. Our method provides a unique new tool to improve aspects of weather and climate models important for simulating phenomena such as heat waves.This research was funded under the U.K. NERC e-stress project (NE/K015990/1). Additional support was provided by IMPALA (NE/M017230/1) and European Commission grant agreement 282673 (EMBRACE), and through U.K. NERC support of the National Centre for Earth Observation. Observational data used in the study are available from http://reverb.echo.nasa.gov/(MODIS), http://due.esrin.esa.int/(ERA-Interim), http://disc.sci.gsfc.nasa.gov (TRMM), http://www.cpc.ncep.noaa.gov/(CMORPH), and http://chrs.web.uci.edu/(PERSIANN).Model data are available from http://pcmdi9.llnl.gov/(CMIP5) and http://www.embrace-project.eu (EMBRACE). We gratefully acknowledge the providers of climate model data in the CMIP5 archive and EMBRACE partners, in particular, Gill Martin from Met Office (United Kingdom), Frederique Cheruy from LMD (France), and Klaus Wyser from SMHI (Sweden), for providing outputs from additional simulations. The code and aggregated observations are available from GitHub website (https://github.com/ppharris/dry_spell_rwr)

Contrasting responses of vegetation productivity to intraseasonal rainfall in Earth system models

Correctly representing the response of vegetation productivity to water availability in Earth system models (ESMs) is essential for accurately modelling the terrestrial carbon cycle and the evolution of the climate system. Previous studies evaluating gross primary productivity (GPP) in ESMs have focused on annual mean GPP and interannual variability, but physical processes at shorter timescales are important for determining vegetation–climate coupling. We evaluate GPP responses at the intraseasonal timescale in five CMIP6 ESMs by analysing changes in GPP after intraseasonal rainfall events with a timescale of approximately 25 d. We compare these responses to those found in a range of observation-based products. When composited around all intraseasonal rainfall events globally, both the amplitude and the timing of the GPP response show large inter-model differences, demonstrating discrepancies between models in their representation of water–carbon coupling processes. However, the responses calculated from the observational datasets also vary considerably, making it challenging to assess the realism of the modelled GPP responses. The models correctly capture the fact that larger increases in GPP at the regional scale are associated with larger increases in surface soil moisture and larger decreases in atmospheric vapour pressure deficit. However, the sensitivity of the GPP response to these drivers varies between models. The GPP in NorESM is insufficiently sensitive to vapour pressure deficit perturbations when compared all to other models and six out of seven observational GPP products tested. Most models produce a faster GPP response where the surface soil moisture perturbation is larger, but the observational evidence for this relationship is weak. This work demonstrates the need for a better understanding of the uncertainties in the representation of water–vegetation relationships in ESMs and highlights a requirement for future daily-resolution observations of GPP to provide a tighter constraint on global water–carbon coupling processes

Contrasting responses of vegetation productivity to intraseasonal rainfall in Earth system models

Correctly representing the response of vegetation productivity to water availability in Earth System Models (ESMs) is essential for accurately modelling the terrestrial carbon cycle and the evolution of the climate system. We evaluate this response at the intraseasonal timescale in five CMIP6 ESMs by analysing changes in Gross Primary Productivity (GPP) after intraseasonal rainfall events and comparing to the responses found in a range of observation-based products. When composited around all intraseasonal rainfall events globally, both the amplitude and the timing of the GPP response show large inter-model differences, demonstrating discrepancies between models in their representation of water-carbon coupling processes. However, the responses calculated from the observational datasets also vary considerably, making it challenging to assess the realism of the modelled GPP responses. The models correctly capture that larger increases in GPP at the regional scale are associated with larger increases in surface soil moisture and larger decreases in atmospheric vapour pressure deficit. However, the sensitivity of the GPP response to these drivers varies between models. The GPP in NorESM is insufficiently sensitive to surface soil moisture perturbations when compared to any observational GPP product tested. Most models produce a faster GPP response where the surface soil moisture perturbation is larger, but the observational evidence for this relationship is weak. This work demonstrates the need for a better understanding of the uncertainties in the representation of water-vegetation relationships in ESMs, and highlights a requirement for future daily-resolution observations of GPP to provide a tighter constraint on global water-carbon coupling processes

An evaluation of modeled evaporation regimes in Europe using observed dry spell land surface temperature

Soil moisture availability exerts control over the land surface energy partition in parts of Europe. However, determining the strength and variability of this control is impeded by the lack of reliable evaporation observations at the continental scale. This makes it difficult to refine the broad range of soil moisture–evaporation behaviours across global climate models (GCMs). Previous studies show that satellite observations of land surface temperature (LST) during rain-free dry spells can be used to diagnose evaporation regimes at the GCM grid box scale. This Relative Warming Rate (RWR) diagnostic quantifies the increase in dry spell LST relative to air temperature, and is used here to evaluate a land surface model (JULES) both offline and coupled to a GCM (HadGEM3-A). It is shown that RWR can be calculated using outputs from an atmospheric GCM provided the satellite clear-sky sampling bias is incorporated. Both offline JULES and HadGEM3-A reproduce the observed seasonal and regional RWR variations, but with weak springtime RWRs in central Europe. This coincides with sustained bare soil evaporation (Ebs) during dry spells, reflecting previous site-level JULES studies in Europe. To assess whether RWR can discriminate between surface descriptions, the bare soil surface conductance and the vegetation root profile are revised to limit Ebs. This increases RWR by increasing the occurrence of soil moisture limited dry spells, yielding more realistic springtime RWRs as a function of antecedent precipitation but poorer relationships in summer. This study demonstrates the potential for using satellite LST to assess evaporation regimes in climate models

Global observational diagnosis of soil moisture control on the land surface energy balance

An understanding of where and how strongly the surface energy budget is constrained by soil moisture is hindered by a lack of large-scale observations, and this contributes to uncertainty in climate models. Here we present a new approach combining satellite observations of land surface temperature and rainfall.We derive a Relative Warming Rate (RWR) diagnostic, which is a measure of how rapidly the land warms relative to the overlying atmosphere during 10 day dry spells. In our dry spell composites, 73% of the land surface between 60°S and 60°N warms faster than the atmosphere, indicating water-stressed conditions, and increases in sensible heat. Higher RWRs are found for shorter vegetation and bare soil than for tall, deep-rooted vegetation, due to differences in aerodynamic and hydrological properties. We show how the variation of RWR with antecedent rainfall helps to identify different evaporative regimes in the major nonpolar climate zones

Superradiance by mini black holes with mirror

The superradiant scattering of massive scalar particles by a rotating mini black hole is investigated. Imposing the mirror boundary condition, the system becomes the so called black-hole bomb where the rotation energy of the black hole is transferred to the scattered particle exponentially with time. Bulk emissions as well as brane emissions are considered altogether. It is found that the largest effects are expected for the brane emission of lower angular modes with lighter mass and larger angular momentum of the black hole. Possibilities of the forming the black-hole bomb at the LHC are discussed.Comment: 20 pages, 2 figures, 7 tables. More discussions. To appear in JHE

Evaluation of regional-scale soil moisture-surface flux dynamics in Earth system models based on satellite observations of land surface temperature

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

Constraints on the mass and abundance of black holes in the Galactic halo: the high mass limit

We establish constraints on the mass and abundance of black holes in the Galactic halo by determining their impact on globular clusters which are conventionally considered to be little evolved. Using detailed Monte Carlo simulations and simple analytic estimates, we conclude that, at Galactocentric radius R~8 kpc, black holes with masses M_bh >~(1-3) x 10^6 M_sun can comprise no more than a fraction f_bh ~ 0.025-0.05 of the total halo density. This constraint significantly improves those based on disk heating and dynamical friction arguments as well as current lensing results. At smaller radius, the constraint on f_bh strengthens, while, at larger radius, an increased fraction of black holes is allowed.Comment: 13 pages, 10 figures, revised version, in press, Monthly Notice