| ABSTRACT | As a consequence of the increasing interest in the change of natural resources available due to climate change, many studies properly invest their efforts to address the management of such resources. Among others, the water management plays a key role since water scarcity will be one of the main issues to be addressed by human beings, especially because of its subsequent effects on, but not limited to the agricultural sector. To tackle this challenge, the Drought Observatory, of CNR-IBE, developed an operational chain to forecast, at seasonal time scale, the Standardized Precipitation Index (SPI) to support drought and fire risks management over the Mediterranean area. The forecast tool is based on the most recent and evolute version of the ECMWF numerical seasonal forecast system, named SEAS5. Each month, from 1993 to the present, SEAS5 provides an ensemble of daily simulations, lasting 7 months each; these simulations are free accessible from the Copernicus Data Store. The SEAS5 system is used to derive the seasonal predictions of the SPI index to evaluate drought conditions a few months in advance starting from the daily precipitation ensemble simulations. This effort is made for increasing the amount of forecast information available for decision making processes. The SEAS5 daily precipitation seasonal forecasts, with a horizontal resolution of 1°x1°, are bias adjusted using the Multi-Source Weighted-Ensemble Precipitation (MSWEP) dataset (version 2.8). MSWEP is a global precipitation product with an original 3-hourly, 0.1° resolution available from 1979 to the present; it merges gauge, satellite, and reanalysis data to obtain a high quality precipitation estimates at every location. The bias adjustment is performed by using the CSTools R Package (CSTools: Assessing Skill of Climate Forecasts on Seasonal-to-Decadal Timescales) applying a quantile-quantile mapping algorithm. This algorithm adjusts/corrects the quantiles of the modelled distribution (the raw SEAS5 daily precipitation distribution) by using an observed distribution set as reference (the MSWEP daily precipitation distribution). Thus each SEAS5 grid-points of each ensemble member is 1) reprojected onto the highest resolution MSWEP dataset, and then 2) the resulting high resolution daily time-series precipitation distribution is adjusted using a quantile transformation. A 1993 – 2016 period is used for the adjustment. The resulting high resolution and bias-adjusted daily rainfall forecast dataset are then used to compute the SPI index for a series of timescales: 1, 2, 3, 4, 5 and 6 months, for the period 1993 to the present. These new bias adjusted forecasts, along with the empirical seasonal forecasts and other monitoring drought and vegetation indices, are free accessible through the Drought Observatory Climate Service (DO - https://drought.climateservices.it). |