Pdf changes in rainfall seasonality in the tropics scinapse. Get daily timeseries of us station data and others. In an attempt to quantify these contrasts, the seasonality index st was developed, which is simply the sum of the absolute deviations of mean monthly rainfalls from the overall monthly mean, divided by the mean annual rainfall. Rainfall seasonality in the niger delta belt, nigeria. Annual rainfall seasonality is an index derived from two ratios. S is shown for stations in the ghcn monthly data set as a function of normalized mean annual rainfall.
Oct 17, 2019 the normalized seasonality index is defined as the product of the entropy of the monthly rainfall distribution relative to a uniform distribution and the annual rainfall normalized by the maximum annual rainfall across all sites feng et al. Sep 01, 20 this paper examined rainfall seasonality in the niger delta, nigeria, using both monthly and annual rainfall data from 1931 to 1997. How calculate the seasonal index, deasonalise data, convert deseasonalised and actual data back. Data required for the calculation of the rainfall seasonality index are. The bigger the difference the more the seasonality. Pdf rainfall seasonality in the niger delta belt, nigeria. Gabriels, r hartmann ghent summary the concentration of the rainfall in a year is an important aspect of the climate. Relationships between mean monthly temperature and mean monthly rainfall in modern environments are quantified to generate a seasonality index associated with summer or. The variable r i is the total annual precipitation in a particular year, and x in is the actual monthly rainfall in month n. We are examining the rainfall seasonality of a monthly time series of rainfall, which measures how one month in a given year rainfall differs than if each month received 112 of the annual rainfall. Rainfall behavior seasonality and time trends in the average monthly rainfall in major cities of nigeria olaoluwa s. In the case of using an index some transformations are needed. The predictability index for the highest total precipitation within the area. Monthly rainfall analysis national weather service.
Although a few studies on rainfall spatial and temporal variability in the uae have been carried out, evidence of the impact of climate change on rainfall trends has not been reported. Jun 01, 2015 two new indicators of rainfall seasonality based on information entropy, the relative entropy re and the dimensionless seasonality index dsi, together with the mean annual rainfall, are evaluated on a global scale for recently updated precipitation gridded datasets and for historical simulations from coupled atmosphereocean general circulation models. Using the seasonal and temporal precipitation concentration index. Rainfall seasonality index is a critical environmental factor affecting the evolution of natural vegetation. Other methods are based on estimation of linear trends in precipi. The probabilistic interpretation of the monthly rainfall fractions also allows us to estimate circular statics moments associated describing the annual distribution of rainfall and, in particular, the center of mass centroid of the annual precipitation, which for seasonal rainfall regimes is a measure of the. Using the seasonal and temporal precipitation concentration index for characterizing the monthly rainfall distribution in spain. Nonstationarity in seasonality of extreme precipitation. Precipitation seasonality and variability over the tibetan. Peac seasonal rainfall forecast national weather service.
This plot shows ndvi values for the selected year and for a medium term average 12 years, 200220. Highresolution monthly rainfall database for ethiopia. When monthly figures are added vectorially, the resultant can be used to compute a seasonality index, which is the ratio of the magnitude of the. Rainfall seasonality captured in micromammalian fauna in late. It was found that the length of the dry period is lower at the northeastern part of northern bangladesh. The variation in seasonality in rainfall over the indian region is examined using monthly rainfall values for the period 1951 to 2015 of 34 meteorological subdivisions excluding two sea islands. The data was collected on 9 synoptic stations in the region. How to use microsoft excel to calculate seasonal indexes. Rainfall seasonality is related to the temporal distribution of rainfall on a monthly. Rappaport, 2000, particularly in coastal regions xu et al. The assumption is made that mean monthly rainfall totals for a location are vector quantities, magnitude being the amount of rain, and direction being the month expressed in units of arc. Liana abundance and diversity increase with rainfall. Analysis of rainfall seasonality index in iraq hasanain k. The quality of data set was firmly controlled before its release and was organized by attributes such as date, latitude, longitude, tc number, landfalling region, tc center pressure, and intensity the maximum sustained wind from 1949 zhang et al.
Us dept of commerce national oceanic and atmospheric administration national weather service wakefield, va 9 general mahone highway wakefield, va 23888. Rainfall seasonality index and individual seasonality index values were calculated, and trend analysis was performed on yearly data for the individual seasonality index, using the mannkendall test, the sequential mannkendall test and sens slope method also known as the theilsen estimator. For example, the pages can plot variables for years where the pna index was high. Changes in seasonality index over subdivisions of india. Detecting changes in rainfall pattern and seasonality index. Fossil proxies which can be related directly to rainfall seasonality have the potential to assist in quantifying these shifts. If season rainfall total rainfall % or monthly rainfall total rainfall % are used then calculations are very simple. Rainfall seasonality captured in micromammalian fauna in.
In the past, several methods have been used to analyze trends in seasonality of precipitation characteristics. Seasonality of precipitation in the united states jstor. Trends and variability of monthly, seasonal and annual. This paper examined rainfall seasonality in the niger delta, nigeria, using both monthly and annual rainfall data from 1931 to 1997. Some of these methods are based on estimation of seasonality index from mean monthly rainfall and mean annual rainfall walsh and lawler, 1981. The insets show qualitative differences in the monthly rainfall distribution between. Individual seasonality index of rainfall regimes in greece. May 18, 2016 the mean individual seasonality index 0. Mar 14, 2019 despite the importance of seasonal distribution and interannual variability of rainfall to the ecosystem and society, there is a lack of regionallevel studies on rainfall seasonality, and teleconnections between global climate events and rainfall seasonality are not well understood. Rainfall seasonality can be estimated by the walsh and lawler 1981 index. Ukmo, nasa gmao, ecmwf, iri, ncep constructed analog, ncep coupled model and. The peac seasonal rainfall outlook is an experimental consenus forecast, produced monthly for 14 stations within hawaii and the usapi. Analysis of rainfall seasonality from observations and.
The rainfall seasonality index is used to study the spatial and temporal change in rainfall behavior in that contribute in improvement of water and management plans of water resources systems and agriculture in a certain region especially during dry seasons. The strong response of lianas to the normalized seasonality index indicates that, in addition to. Analysis of rainfall seasonality from observations and climate models. Changes in rainfall seasonality pattern over india rai. Insignificant trends in monthly precipitation were also described by. The ratio of cool to warm season rainfall totals plus 1 are assigned negative values when rainfall in the cool. Jul 25, 2019 where r i is the total annual rainfall for the particular year under study. May 14, 20 because of the scarcity of meteorological observations, the precipitation climate on the tibetan plateau and surrounding regions tp has been insufficiently documented so far. Rainfall seasonality over india is studied using an individual seasonality index. Precipitation regimes and associated seasonality index values are documented by walsh and lawler 1981 are provided in table 2. The assumption is made that mean monthly rainfall totals for a location. Pdf individual seasonality index of rainfall regimes in greece. Pdf analysis of rainfall seasonality from observations and. Relationships between mean monthly temperature and mean monthly rainfall in modern environments are quantified to generate a seasonality index associated with summer or winter rainfall.
Rainfall variability and seasonality in northern bangladesh. Pdf individual seasonality index of rainfall regimes in. Spatial and trend analyses of rainfall seasonality and. If season rainfalltotal rainfall % or monthly rainfalltotal rainfall % are used then calculations are very simple. Assessment of seasonal and annual rainfall trends and. Individual seasonality index of rainfall regimes in greece article pdf available in climate research 282. An equal distribution would assign approximately 8% to each month. Though this index can only provide a very simple description of the seasonality pattern, it can be a very useful tool with which to study spatial and temporal seasonality variations sumner et al.
The statistical theoretic normal value for each season is 25%. This study aims at assessing the significance of longterm rainfall trends and temporal variability at sharjah city, uae. To address these issues, this study analyzed the spatial and temporal characteristics of rainfall seasonality. The index amount represents a decimal fraction indicating the ratio of a period amount to the average of all periods.
To delineate southern africa into regions of rainfall seasonality, the all year rainfall region was first identified, by analysing the median i. Recent citations impact of rainfall seasonality on intraspecific trait variation in a shrub from a. Seasonal historical rainfall south florida water management. Changes in rainfall pattern and seasonality index 641 table 2. The rift valley and adjoining highlands receive much of the rainfall during jjas, which accounts for 30%60% of rainfall variability, and secondary rainfall during mam, which accounts for 20. Ukmo, nasa gmao, ecmwf, iri, ncep constructed analog, ncep coupled model and apcc mme. In this study, the characteristics and basic features of precipitation on the tp during an 11yr period 200111 are described on monthlytoannual time scales. Our data has the below structure and continues monthly through water year 2014. Walsh and lawler 1981 linked rainfall seasonality in africa, to latitude. Map and the variability of monthly rainfall throughout the year, was a significant predictor of both liana density and species richness, whereas map, rainfall seasonality and the mean dry season precipitation mdp were far weaker predictors. Long memory, seasonality and time trends in the average monthly rainfall in major cities of nigeria olaoluwa s.
Changes in the seasonality of precipitation over the contiguous usa s. Additional analyses of the seasonality index this section presents additional analyses of the seasonality index and its trends over the past century using monthly rainfall data from stations in highly seasonal regions supplementary figure s1 as well as monthly gridded data over the entire tropics supplementary figures s2 and s3. Spatial trends in rainfall seasonality royal meteorological society. Monthly seasonal composites monthly seasonal correlation daily timeseries daily teleconnection plots and forecasts. The statistical tools employed for the study are the cumulative index analysis and the percentage of mean. Seasonality index s d rrmax figure 1 jseasonality index in the tropics. The remaining variability of the monthly rainfall is due to rainfalls in the other months as these regions get rainfall throughout nearly the whole year. Effects of global warming and solar geoengineering on. Detecting changes in rainfall pattern and seasonality. Changes in seasonality index over subdivisions of india during. Data and methodology monthly rainfall data of around 335 raingauge stations of the state maharashtra for the period 1901 to 2006 are collected from national data centre of india meteorological department, pune. Monthly climate timeseries noaa earth system research. The impact of tcs on extreme rainfall has been investigated by dare et al. The present study is based on data from monthly rainfall recorded in the period 1945 to 2005 in a total of 225 raingauge stations spread all over region.
Kanellopoulou 2002, which is a function of mean monthly and annual rainfall, is computed using the following formula. Trend analysis results show no significant change in the seasonality of rainfall in this. Changes in the seasonality of precipitation over the. For each disease, a monthly seasonality index was applied to describe both overall and regional variations in seasonal patterns. Spatial and trend analyses of rainfall seasonality and erosi. Long memory, seasonality and time trends in the average. Trends, variability, and seasonality of maximum annual. Jan 25, 2019 the index amount represents a decimal fraction indicating the ratio of a period amount to the average of all periods. As with rainfall concentration, the seasonality of rainfall is best determined by the socalled markham technique. The spatial distribution of the individual seasonality index and the mean individual seasonality index over two periods, that is, early 19011970 and recent 19712015, is analysed. Trend analysis using linear regression during these. Akomolafe3 several features may be present in rainfall data, and sophisticated time series procedures are needed for the analysis. This means that january is about 76 percent of the average. Storm surge, heavy rainfall, and flooding resulting from tropical cyclones tcs can cause tremendous damages e.
The tc data were collected from the shanghai typhoon institute of the china meteorological administration. They discovered that seasonality is low in equatorial areas but increases rapidly with latitude, particularly towards the sahara where the highest seasonality index s. For locations with very low rainfall or during dry season periods, anomalies can take extreme values. The data was retrieved from the archives of the nigerian meteorological agency in lagos. S is shown for stations in the ghcnmonthly data set as a function of normalized mean annual rainfall.
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