Large-scale surface responses during European dry spells diagnosed from land surface temperatureJ. Hydrometeor


Sonja S. Folwell, Phil P. Harris, Christopher M. Taylor
Atmospheric Science


Journal of Hydrometeorology


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The DOI for this manuscript is doi: 10.1175/JHM-D-15-0064.1

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Folwell, S., P. Harris, and C. Taylor, 2015: Large-scale surface responses during

European dry spells diagnosed from land surface temperature. J. Hydrometeor. doi:10.1175/JHM-D-15-0064.1, in press. © 2015 American Meteorological Society




Large-scale surface responses during European dry spells 1 diagnosed from land surface temperature 2 3

Sonja S. Folwell 1, Phil P. Harris 1, 2 and Christopher M. Taylor 1, 2 4 [1] Centre for Ecology and Hydrology, Crowmarsh Gifford, Oxfordshire, United Kingdom 5 [2] National Centre for Earth Observation, Wallingford, UK 6

Correspondence to: S. S. Folwell ( 7 8

Abstract 9

Soil moisture plays a fundamental role in regulating the summertime surface energy balance 10 across Europe. Understanding the spatial and temporal behaviour in soil moisture and its 11 control on evapotranspiration (ET) is critically important, and influences heat wave events. 12

Global climate models (GCMs) exhibit a broad range of land responses to soil moisture in 13 regions which lie between wet and dry soil regimes. In situ observations of soil moisture and 14 evaporation are limited in space, and given the spatial heterogeneity of the landscape, are 15 unrepresentative of the GCM grid box scale. On the other hand, satellite-borne observations of 16 land surface temperature (LST) can provide important information at the larger scale. As a key 17 component of the surface energy balance, LST is used to provide an indirect measure of surface 18 drying across the landscape. In order to isolate soil moisture constraints on evaporation, time 19 series of clear sky LST are analysed during dry spells lasting at least 10 days from March to 20

October. Averaged over thousands of dry spell events across Europe, and accounting for 21 atmospheric temperature variations, regional surface warming of between 0.5 and 0.8 K is 22

Manuscript (non-LaTeX)

Click here to download Manuscript (non-LaTeX): Folwell_LST_warming_Europe_20150729_JHM_figs_revised.docx 2 observed over the first 10 days of a dry spell. Land surface temperatures are found to be 1 sensitive to antecedent rainfall; stronger dry spell warming rates are observed following 2 relatively wet months, indicative of soil moisture memory effects on the monthly time scale. 3

Furthermore, clear differences in surface warming rate are found between cropland and forest, 4 consistent with contrasting hydrological and aerodynamic properties. 5 6 1 Introduction 7 8

Soil moisture plays a fundamental role in controlling the surface energy budget through its 9 constraint on evapotranspiration (ET). In regions of high soil moisture seasonality, such as the 10

European mid-latitudes, soil moisture deficits develop during spring and summer. This shifts 11 the surface energy budget towards greater sensible heat production as latent heat flux is 12 reduced, which warms and dries the overlying air. This in turn can establish feedbacks on soil 13 moisture through increased evaporative demand and impacts on cloud cover and precipitation. 14

Several authors have linked historic summer heat wave and drought events in Europe to 15 summer soil moisture state (Chiriaco et al. 2014; Weisheimer et al. 2011) and more specifically 16 to precursor spring soil moisture deficits (Bisselink et al. 2011; Fischer et al. 2007b). Miralles 17 et al. (2014) showed that under certain conditions soil moisture-induced atmospheric heating 18 can persist above the nocturnal boundary layer and accumulate over several days to produce 19 “mega-heat waves”. They attribute the strength of the 2003 European and 2010 Russian heat 20 wave events to this mechanism. Similarly, non-local effects may play a role, with anomalously 21 low winter and spring soil moisture patterns propagating northwards from the Mediterranean 22 to Central and Northern Europe, through transportation of warm dry air (Quesada et al. 2012; 23

Vautard et al. 2007b; Zampieri et al. 2009). Established soil moisture deficits can then interact 24 3 with the large-scale circulation to amplify the summertime temperature variability (Fischer et 1 al. 2007a). These feedbacks can lead to increased air temperatures and drought conditions over 2 wide areas through cloud suppression, increased short wave radiation, reduced precipitation 3 and the import of warmer, drier air masses. Land cover also plays an important role particularly 4 in a well-watered regime, with forests contributing higher sensible heat fluxes than grasslands 5 during heatwaves in response to developing daytime vapour pressure deficits (Stap et al. 2014). 6

Beyond the meteorological domain, summer heat wave events, such as in 2003, have important 7 effects on human health (Garcia-Herrera et al. 2010), air quality (Vautard et al. 2007a) and the 8 regional carbon cycle (Ciais et al. 2005). 9

Capturing these land-atmosphere feedbacks in global climate models (GCMs) is problematic. 10

Analysis of GCM simulations in the third Coupled Model Intercomparison Project (CMIP3) 11 shows wide disagreement in the extent to which soil moisture availability constrains summer 12 evapotranspiration in central and Eastern Europe (Boé and Terray 2008). This feature is also 13 present in a more recent group of regional climate models run under the ENSEMBLES project 14 (Boé and Terray 2014), and a multi-model analysis of regional climate simulations under the 15