Contextualization

of extreme rainfall in Jordan

Introduction

Every extreme rainfall event in Jordan and elsewhere have a larger-scale atmosphere context described by the transport of air masses across different geographical regions. By using ERA5 global reanalysis data we retrospectively identified critical circulation pattern associated with local extreme rainfall in Amman (nearest grid cell) for past (1961-1990) and present (1991-2020) climate periods. A standard atmosphere field in synoptic meteorology is the geopotential height at 500 hPa (Z500) representing the circulation conditions in the middle troposphere at 5 km. The curvature of the contour lines are good indicators for the origin and transport pathways of the air masses. A better understanding of the dynamical drivers of local extreme weather events can enhance existing early warning systems and long-term climate risks assessments.

Method

The basic approach is given in this scheme combining local daily time series (PR_d) with the respective atmospheric fields (Z500_x,y,d). For climate periods of 30 years we filter out days (t), where the daily precipitation exceeding the 99^th percentile (N=110d). For these days we normalize Z500 and calculating composite patterns by averaging. This approach is applied to the past (1961-1990) and present (1991-2020) climate periods. Comparison of the patterns indicate possible changes in dynamical drivers. By looking at the respective values of precipitation above the 99^th percentile also changes in the intensity can be identified and attributed. Changes in frequency are identified by applying the threshold for the 99_th percentile of the past period also for the present period!

  graph LR;
  A(<b>Z500<sub>x,y,d</sub></b><hr>Atmospheric Fields) == 1961-1990 ==> C(<b>Filtering of Dates</b><hr>t = where PR>99<sup>th</sup><hr>avg Z500<sub>a,y,t</sub>);
  B(<b>PR<sub>d</sub></b><hr>Precipitation Amman) == 1961-1990 ==> C;
  C ===> D(Context<hr><img src='http://www.pik-potsdam.de/~peterh/img/context.png' width='70' height='70' />)
  classDef a fill:orange,stroke:black,color:black;
  class A,B,C,D,E,F a;

Results

The resulting maps of the contextualization of extreme rainfall in Jordan result in critical circulation/weather patterns associated with heavy rainfall in Jordan (Amman). The prominent feature in the maps show a trough-like curvature of the contour lines over eastern Europe down to the eastern Mediterranean. The transport of cold air masses is directed from north to south and often associated with instabilities over the eastern Mediterranean. This applies for both reanalysis (ERA5) and climate simulations (CMIP6). The difference pattern in the center indicate long-term changes in the large-scale context associated with the local extremes.

➩ weakening of the trough-like pattern (brown color)
➩ intensification of heavy rainfall (values in box)