The Standardized Precipitation Index (SPI), a well-reviewed meteorological drought index recommended by the World Meteorological Organization (WMO), and its more recent climatic water balance variant, the Standardized Precipitation-Evapotranspiration Index (SPEI), both rely on selection of a univariate probability distribution to normalize the index, allowing for comparisons across climates. Choice of an improper probability distribution may impart bias to the index values, exaggerating or minimizing drought severity. This study compares a suite of candidate probability distributions for use in SPI and SPEI normalization using the 0.5° × 0.5° gridded Watch Forcing Dataset (WFD) at the continental scale, focusing on Europe. Several modifications to the SPI and SPEI methodology are proposed, as well as an updated procedure for evaluating SPI/SPEI goodness of fit based on the Shapiro-Wilk test. Candidate distributions for SPI organize into two groups based on their ability to model short-term accumulation (1-2 months) or long-term accumulation (>3 months). The two-parameter gamma distribution is recommended for general use when calculating SPI across all accumulation periods and regions within Europe, in agreement with previous studies. The generalized extreme value distribution is recommended when computing the SPEI, in disagreement with previous recommendations.

https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/joc.4267

Candidate Distributions for Climatological Drought Indices (SPI and SPEI)

Pan-European comparison of candidate distributions for climatological drought indices, SPI and SPEI

Monthly rainfall data from June to October for 39 years were used to compute Standardized Precipitation Index (SPI) values based on two parameter gamma distribution for a low rainfall and a high rainfall districts of Andhra Pradesh state, India. Comparison of SPI with actual rainfall and rainfall deviation from the mean indicated that SPI values under-estimate the intensity of dryness/wetness when the rainfall is very low/very high, respectively. As a result, the SPI in the worst drought years of 2002 and 2006 in the low rainfall district indicated only moderate dryness instead of extreme dryness. SPI values of the high rainfall district showed slightly better stretching in both positive and negative directions, compared to that of the low rainfall district. Further, the SPI values of longer time scales (2, 3 and 4 months) showed an extended range compared to that of 1 month, but the sensitivity in drought years has not improved significantly.



Normality tests were conducted based on Shapiro-Wilk statistic, p-values and absolute value of the median to ascertain whether non-normality of SPI is a possible reason. Although the results confirmed normal distribution, the scatter plot indicated deviation of the cumulative probability distribution of SPI from normal probability in the lower and upper ranges.



Therefore, it is suggested that SPI as a stand alone indicator needs to be interpreted with caution to assess the intensity of drought. Further investigations should include sensitivity of SPI to the estimated shape and scale at lower and upper bounds of the gamma distribution and use of other distributions, such as Pearson III, to standardize the computational procedures, before using SPI as a substitute to the rainfall deviations from normal, for drought intensity assessment. Copyright © 2009 Royal Meteorological Society

https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/met.136

On the use of Standardized Precipitation Index(SPI) for drought intensity assessment

. This study examined surface air temperature trends over global land from 1901–2009. It is found that the warming trend was particularly enhanced, in the boreal cold season (November to March) over semi-arid regions (with precipitation of 200–600 mm yr −1 ) showing a temperature increase of 1.53 °C as compared to the global annual mean temperature increase of 1.13 °C over land. In mid-latitude semi-arid areas of Europe, Asia, and North America, temperatures in the cold season increased by 1.41, 2.42, and 1.5 °C, respectively. The semi-arid regions contribute 44.46% to global annual-mean land-surface temperature trend. The mid-latitude semi-arid regions in the Northern Hemisphere contribute by 27.0% of the total, with the mid-latitude semi-arid areas in Europe, Asia, and North America accounting for 6.29%, 13.81%, and 6.85%, respectively. Such enhanced semi-arid warming (ESAW) imply drier and warmer trend of these regions.

/pdf/enhanced-cold-season-warming-in-semi-arid-regions-5ggq1vhwzf.pdf

Enhanced cold-season warming in semi-arid regions

Spatial patterns of annual and seasonal rainfall trends of Bangladesh over the time period 1958–2007 has been assessed using rainfall data recorded at 17 stations distributed over the country. Mann–Kendall trend test and the Sen's slope method are used to detect the significance and the magnitude of rainfall change, respectively. Historical dry and wet months are identified by using standardised precipitation index method and their trends are analysed to assess the possible change in wet and dry events in Bangladesh. The result shows a significant increase in the average annual and pre-monsoon rainfall of Bangladesh. The number of wet months is found to increase and the dry months to decrease in most parts of the country. Seasonal analysis of wet and dry months shows a significant decrease of dry months in monsoon and pre-monsoon. Copyright © 2009 Royal Meteorological Society

Rainfall variability and the trends of wet and dry periods in Bangladesh

Precipitation zone systems exist for Mexico based on seasonality, quantity of precipitation, climates and geographical divisions, but none are convenient for the study of the relation of precipitation with phenomena such as El Nino. An empirical set of seven exclusively Mexican and six shared zones was derived from three series of Standardized Precipitation Index (SPI) images, from 1940 through 1989: a whole-year series (SPI-12) of 582 monthly images, a six month series (SPI-6) of 50 images for winter months (November through April), and a six-month series (SPI-6) of 50 images for summer months (May through October). By examination of principal component and unsupervised classification images, it was found that all three series had similar zones. A set of basic training fields chosen from the principal component images was used to classify all three series. The resulting thirteen zones, presented in this article, were found to be approximately similar, varying principally at zone edges. A set of simple zones defined by just a few vertices can be used for practical operations. In general the SPI zones are homogeneous, with almost no mixture of zones and few outliers of one zone in the area of others. They are compared with a previously published map of climatic regions. Potential applications for SPI zones are discussed.

/pdf/standardized-precipitation-index-zones-for-mexico-2mvqs9a3oc.pdf

Standardized Precipitation Index Zones for México

Algunos usos de Bioclimas: un sistema especializado de información geográfica

Flora of Veracruz project: an update on database management of collections and related information

The climate of the large warm subhumid area of the Mexican state of Veracruz was studied; the area is a combination of areas classified as Aw of Koppen (Aw0, Aw1 and Aw2 of Koppen as modified by Garcia). Particular attention was placed on their exposure to, or protection from masses of cold air (in Mexico known as “nortes”) due to the barrier at 20 o N latitude of the Sierra Madre Oriental mountain range that penetrates to the coast of the Gulf of Mexico. The analysis was based on data from 86 meteorological stations with an average of 20 years of records, using the Manhattan coefficient with pairs of stations, without weighting. Mathematical calculations were made with the NTsys-pc program. It is also shown that these three climates, Aw0, Aw1 and Aw2, show significant internal variations north, south, and in the neighborhood of the barrier, emphasizing that in practice it is necessary to take into account more than just the climatic classification; the combined data of the meteorological stations furnish much more detailed information, which might be called the mesoclimate. Even though it is not necessarily appropriate to derive new classifications, these additional data are necessary for understanding bioclimatic relations.

Los Climas Cálidos Subhúmedos del Estado de Veracruz, México

Es imposible dejar de apreciar la gran importancia de los registros meteorologicos historicos para los estudios del cambio climatico global. Muchos de estos estudios se basan en datos en la GHCN, Global Historical Climatology Network, la compilacion de datos meteorologicos, disponible en Internet, que esta formada por contribuciones voluntarias de todas partes del mundo. Aunque la participacion de los diferentes paises es digna de elogio, tambien se debe reconocer que estas varian en calidad y cantidad, y sobre todo en regularidad y actualizacion Los datos de varios paises de America Latina son incompletos y no estan actualizados, lo que provoca que muchos de los estudios de nivel mundial que utilizan los datos puedan llegar a conclusiones no precisas. Se presentan Tablas y figuras que muestran la participacion de todos los paises de la region. En nombre de los investigadores de cambio climatico, los autores solicitan que las autoridades de todos los paises se esfuercen por cambiar sus politicas para engrosar y actualizar sus contribuciones a GHCN.

Margarita Soto

Papers

Standardized Precipitation Index Zones for México

Algunos usos de Bioclimas: un sistema especializado de información geográfica

Flora of Veracruz project: an update on database management of collections and related information

Los Climas Cálidos Subhúmedos del Estado de Veracruz, México

Participación de América Latina y del Caribe en los registros globales climatológicos, GHCN