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This study presents a climatology and trend analysis of reference crop evapotranspiration (ETo) over continental Spain and the Balearic Islands. Geographic features of the study region play a substantial role in the climatology of ETo. The highest values (in excess of 1,200 mm y−1) are found at lower elevations in the south, while the lowest values (less than 900 mm y−1) are found in the highest elevations in the north. A deep analysis reveals: (a) a low interannual variability; (b) summer accumulates more than 50% of annual values; and (c) the radiative component contribution is higher than 50%. A positive long‐term trend (1961–2014) has been detected for most of the study area, but showing contrasting situations when shorter periods (20–30 years) are analysed. A short initial period of negative trend was followed by a longer period of a positive trend. We argue that global dimming/brightening played a role in this process, as these two contrasting periods are clearly guided by the radiative component. A seasonal analysis of the trends reveals that spring and summer are the seasons showing the long‐term positive trends. Interestingly, a monthly analysis shows that spring trends are guided by March and April and summer trends are mostly guided by June, which is the month showing the highest relative changes. This could have important consequences for agriculture and natural ecosystems since this month represents the start of the summer (dry) period. [ABSTRACT FROM AUTHOR]

Flash droughts are characterized by rapid onset and intensification, as well asmajor environmental and agricultural impacts. In this study, we developed an objective method for identifying flash droughts using the standardized evaporation precipitation index (SPEI) based on a short time scale (1-month) and high-frequency data (weekly). The identification of flash droughts was focused on the development phase, anomalous decreases in index values in a short time period (4 weeks), and the magnitude of the events. The method was applied to mainland Spain and the Balearic Islands using a high spatial resolution gridded dataset for the period 1961-2018. For this period of 58 years, we characterized the occurrence of flash droughts and showed that for Spain, there was a large spatial and temporal variability in their frequency, with more occurring in the northwest than in the central and southern regions. The northern regions, where a higher frequency of flash droughts was found, showed negative trends in the frequency of flash droughts, while the regions subject to fewer flash drought events showed generally positive trends. We investigated the relative frequency of flash droughts affecting the study regions and found that they are a common phenomenon, as 40% of all droughts were characterized by rapid development. The findings of this study have important implications for drought assessment, monitoring, and mitigation. [ABSTRACT FROM AUTHOR]

This study describes a newly developed high-resolution (1.1 km) Normalized Difference Vegetation Index dataset for the peninsular Spain and the Balearic Islands (Sp_1km_NDVI). This dataset is developed based on National Oceanic and Atmospheric Administration–Advanced Very High Resolution Radiometer (NOAA–AVHRR) afternoon images, spanning the past three decades (1981–2015). After a careful pre-processing procedure, including calibration with post-launch calibration coefficients, geometric and topographic corrections, cloud removal, temporal filtering, and bi-weekly composites by maximum NDVI-value, we assessed changes in vegetation greening over the study domain using Mann-Kendall and Theil-Sen statistics. Our trend results were compared with those derived from some widely recognized global NDVI datasets [e.g. the Global Inventory Modelling and Mapping Studies 3rd generation (GIMMS3g), Smoothed NDVI (SMN) and Moderate-Resolution Imaging Spectroradiometer (MODIS)]. Results demonstrate that there is a good agreement between the annual trends based on Sp_1km_NDVI product and other datasets. Nonetheless, we found some differences in the spatial patterns of the NDVI trends at the seasonal scale. Overall, in comparison to the available global NDVI datasets, Sp_1km_NDVI allows for characterizing changes in vegetation greening at a more-detailed spatial and temporal scale. In specific, our dataset provides relatively long-term corrected satellite time series (>30 years), which are crucial to understand the response of vegetation to climate change and human-induced activities. Also, given the complex spatial structure of NDVI changes over the study domain, particularly due to the rapid land intensification processes, the spatial resolution (1.1 km) of our dataset can provide detailed spatial information on the inter-annual variability of vegetation greening in this Mediterranean region and assess its links to climate change and variability. [ABSTRACT FROM AUTHOR]

This study characterizes the climatology of drought events over the mainland Spain and the Balearic Islands using high‐resolution (1.21 km2) meteorological data from 1961 to 2014. The climatology of drought was assessed based on two widely‐recognized drought indices: the Standardized Precipitation Index (SPI) and the Standardized Precipitation‐Evapotranspiration Index (SPEI), considering four different timescales (1‐, 3‐, 6‐ and 12‐months). Drought events were simply defined as sequences of months with negative values of the indices. We analysed the spatial and temporal variability of the frequency, duration and magnitude of the drought events. In general, the frequency of drought events is higher in the northern than in the southern regions. Conversely, the average duration and magnitude of the drought events in central and southwestern regions duplicate those recorded in northern areas. Although drought characteristics exhibit a general north–south gradient irrespective of the drought timescale and the drought index analysed, we found important spatial differences in terms of both drought duration and severity. As opposed to the SPI, the SPEI shows, on average, higher drought durations and magnitudes at 1‐, 3‐ and 6‐months timescales. Albeit of the absence of significant temporal changes in drought duration or magnitude at the regional scale, a nonsignificant tendency toward higher drought duration and magnitude is observed over the majority of Spain. Our result provide valuable guidance to stakeholders and decision‐makers on detecting, monitoring and adapting to drought impacts at local, regional and national scale in Spain. [ABSTRACT FROM AUTHOR]

Obtaining climate grids describing distinct variables is important for developing better climate studies. These grids are also useful products for other researchers and end users. The atmospheric evaporative demand (AED) may be measured in terms of the reference evapotranspiration (ETo), a key variable for understanding water and energy terrestrial balances and an important variable in climatology, hydrology and agronomy. Despite its importance, the calculation of ETo is not commonly undertaken, mainly because datasets consisting of a high number of climate variables are required and some of the required variables are not commonly available. To address this problem, a strategy based on the spatial interpolation of climate variables prior to the calculation of ETo using FAO-56 Penman–Monteith equation was followed to obtain an ETo database for continental Spain and the Balearic Islands, covering the 1961–2014 period at a spatial resolution of 1.1 km and at a weekly temporal resolution. In this database, values for the radiative and aerodynamic components as well as the estimated uncertainty related to ETo were also provided. This database is available for download in the Network Common Data Form (netCDF) at 10.20350/digitalCSIC/8615. A map visualization tool (http://speto.csic.es , last access: 10 December 2019) is available to help users download the data corresponding to one specific point in comma-separated values (csv) format. A relevant number of research areas could take advantage of this database. For example, (i) studies of the Budyko curve, which relates rainfall data to the evapotranspiration and AED at the watershed scale, (ii) calculations of drought indices using AED data, such as the Standardized Precipitation–Evapotranspiration Index (SPEI) or Palmer Drought Severity Index (PDSI), (iii) agroclimatic studies related to irrigation requirements, (iv) validation of climate models' water and energy balance, and (v) studies of the impacts of climate change in terms of the AED. [ABSTRACT FROM AUTHOR]

ABSTRACT In this study we retrieved the spatial distribution of mid-afternoon clouds under various synoptic regimes across the Iberian Peninsula and the Balearic Islands for the warm/convective-season, from May to October. Accurate daily cloud masks were derived by applying a daytime over land multispectral convective cloud detection algorithm spanning 15 years (1997-2011) of Advanced Very High Resolution Radiometer ( AVHRR) HRPT data. We processed a total of 2094 afternoon overpasses (between 1230 and 1720 UTC) corresponding to the NOAA-14, NOAA-16 and NOAA-18 spacecrafts, and stratified daily cloud masks as a function of: (1) the automated circulation-typing scheme of Jenkinson and Collinson and (2) the prevailing wind field at 850 hPa. The AVHRR warm-season cloud climatology with high spatial resolution (1.1-km) identified six representative areas (regions of interest; ROIs) with intensified cloud activity (hot spots). The results also revealed the typical spatial distribution of clouds for each synoptic regime across the whole region, identified the synoptic patterns and wind regimes under which high amounts of clouds occur for each ROIs, and showed that strong boundary layer winds in general increase the frequency of clouds. The regional cloud climatology presented here could be useful, e.g. to improve convective short-term forecasting by identifying active cloud areas for each atmospheric type. [ABSTRACT FROM AUTHOR]

ABSTRACT A daytime over land multispectral cloud detection algorithm is presented to derive accurate convective cloud climatologies with high spatial resolution (1.1 km) over the Iberian Peninsula (IP) and the Balearic Islands (BI). The cloud detection scheme was designed to process Advanced Very High Resolution Radiometer (AVHRR) HRPT data and is tested here on NOAA-17 morning (0900-1200 UTC) and NOAA-16 afternoon (1200-1500 UTC) overpasses for the warm 6-month study period May-October. The algorithm consists of four spectral threshold tests applied to each pixel. Test 1 corresponds to the snow-ice removal, test 2 is the thermal infrared test, test 3 is the albedo or visible test and test 4 is the ratio between near-infrared and visible channels. The algorithm discretizes all AVHRR data into four groups called cloud-filled, cloud-free, snow-ice and snow-free radiances. The high-resolution convective cloud masks are obtained by subtracting snow-ice pixels from cloudy ones. In this article, a detailed description of the convective cloud detection scheme and the sources of error detected for each test are given, and the first seasonal and monthly regional convective cloud frequency composites are presented. Future applications of the newly proposed threshold algorithm in climate and meteorology are also discussed in this article, particularly the production of convective cloud composites for climate monitoring of storms over the IP and BI. [ABSTRACT FROM AUTHOR]

Flash drought is the result of strong precipitation deficits and/or anomalous increases in atmospheric evaporative demand (AED), which triggers a rapid decline in soil moisture and stresses vegetation over short periods of time. However, little is known about the role of precipitation and AED in the development of flash droughts. For this paper, we compared the standardized precipitation index (SPI) based on precipitation, the evaporative demand drought index (EDDI) based on AED, and the standardized evaporation precipitation index (SPEI) based on the differences between precipitation and AED as flash drought indicators for mainland Spain and the Balearic Islands for 1961–2018. The results show large differences in the spatial and temporal patterns of flash droughts between indices. In general, there was a high degree of consistency between the flash drought patterns identified by the SPI and SPEI, with the exception of southern Spain in the summer. The EDDI showed notable spatial and temporal differences from the SPI in winter and summer, while it exhibited great coherence with the SPEI in summer. We also examined the sensitivity of the SPEI to AED in each month of the year to explain its contribution to the possible development of flash droughts. Our findings showed that precipitation is the main driver of flash droughts in Spain, although AED can play a key role in the development of these during periods of low precipitation, especially in the driest areas and in summer. [ABSTRACT FROM AUTHOR]