Meteorological drought lacunarity around the world and its classification

Robert Monjo,Javier Martin-Vide,Dominic Royé

doi:10.5194/essd-12-741-2020

Abstract

Abstract. The measure of drought duration strongly depends on the definition considered. In meteorology, dryness is habitually measured by means of fixed thresholds (e.g. 0.1 or 1 mm usually define dry spells) or climatic mean values (as is the case of the standardised precipitation index), but this also depends on the aggregation time interval considered. However, robust measurements of drought duration are required for analysing the statistical significance of possible changes. Herein we climatically classified the drought duration around the world according to its similarity to the voids of the Cantor set. Dryness time structure can be concisely measured by the n index (from the regular or irregular alternation of dry or wet spells), which is closely related to the Gini index and to a Cantor-based exponent. This enables the world’s climates to be classified into six large types based on a new measure of drought duration. To conclude, outcomes provide the ability to determine when droughts start and finish. We performed the dry-spell analysis using the full global gridded daily Multi-Source Weighted-Ensemble Precipitation (MSWEP) dataset. The MSWEP combines gauge-, satellite-, and reanalysis-based data to provide reliable precipitation estimates. The study period comprises the years 1979–2016 (total of 45 165 d), and a spatial resolution of 0.5∘, with a total of 259 197 grid points. The dataset is publicly available at https://doi.org/10.5281/zenodo.3247041 (Monjo et al., 2019).

Highlights

Drought depends mainly on the sector affected and the timescale considered (Wilhite and Glantz, 1985; Crausbay et al, 2017)
Rainforests like those in the Amazon, the Congo, or southeastern Asia present values of n < 0.3. This is consistent with the low values found for other rainforest zones such as those in Madagascar, Central America, and South America. This is due to the high degree of persistence of very short dry spells, alternating with very frequent wet days
The study demonstrates that drought lacunarity can be analysed with the use of selfsimilarity features obtained from the dry-spell spell (DSS) n index

Summary

Introduction

Drought depends mainly on the sector affected and the timescale considered (Wilhite and Glantz, 1985; Crausbay et al, 2017). We usually distinguish between dry spells (daily timescale) and negative anomalies, commonly represented by monthly or yearly indices such as the standardised precipitation index, the standardised evapotranspiration index, or the Palmer drought severity index, among others (Vicente-Serrano et al, 2010, 2015). Alternation between dry and wet events presents selfsimilarity (characteristic of fractal objects) in the same manner that the Cantor set alternates points with gaps (Martínez et al, 2007; Feng et al, 2015; Dayeen and Hassan, 2016; Lucena et al, 2018). According to Mandelbrot, fractality can be found by measuring. The more accurate the measurement ruler, the more infinite the British coastline appears to be, since the immeasurable curves of the coast situate it between a line (one dimension) and a surface (two dimensions), i.e. with a fractal or fractional dimension (Mandelbrot, 1967).

Methods

Results

Discussion

Conclusion