Monday 12 December 2016

Precipitation past and future for Ireland.

A  quick overview from Met Éireann's rainfall climate page:

Most of the eastern half of the country gets between 750 and 1000 (mm) of rainfall in the year. Rainfall in the west generally averages between 1000 and 1400 mm. In many mountainous districts rainfall exceeds 2000mm per year.

 Rainfall shows great year to year variability. A 30 year running mean of the national annual rainfall indicated an increase in average national rainfall of approximately 70mm over the last two decades.

 The average number of wet days (days 1mm or more of rain) ranges from about 150 days a year along the east and south east coasts, to about 225 days a year in parts of the west.
Annual rainfall map:
Average Annual Rainfall:

What do I need to look at to define a precipitation climate?

Precipitation is quite a noisy variable to deal with, and there are lots of ways to look at it. Climate Change Indices have been defined by the Expert Team on Climate Change Detection, Monitoring and Indices (ETCCDMI):
http://etccdi.pacificclimate.org/list_27_indices.shtml

 Ones that I've used here are:
  • PRCPTOT: Total precipitation in wet (>1mm) days.
  • SDII: Simple Daily Intensity Index. Mean precipitation amount for wet days (mm/wet day)
  • R10mm: Heavy Precipitation Days. No. days >10mm
  • CDD: Max no. Consecutive Dry (<1mm) Days
  • CWD: Max no. Consecutive Wet (>=1mm) Days

Future Projections:

The Climate Indices are available for CMIP3 and CMIP5 data from the Canadian Centre for Climate Modelling and Analysis:
http://www.cccma.ec.gc.ca/data/climdex/
ftp://ftp.cccma.ec.gc.ca/data/climdex

 This website from KNMI has a nice interface to the data:
http://climexp.knmi.nl/start.cgi?id=someone@somewhere
For each of these, I have downloaded data defined by the following:
  • CMIP5 mean ( of all models and members)
  • Latitude: 51 TO 56 N
  • Longitude: -5 TO -11 E
  • Boundaries: halfway grid points

CMPI5 Plots:


The annual precipitation amounts for the current climate are lower in the CMIP5 mean than observations. This could be because the climate model resolution is quite low, around one grid point every 200 km, so the increased precipitation due to mountains would not be seen in the models, nor would precipitation driven by convection (thunderstorms) be well-represented. Nevertheless, I would consider the trends to be indicative of future projections for Ireland.

The simple daily intensity index is the average when wet day precipitation is averaged across all wet days. This shows, as expected, an increase in this averaged daily precipitation:
However, the plot of the number of wet days (>1mm) is noisy, and doesn't seem to show any large increase in the number of wet days in the future:
Therefore we may expect the increase in precipitation amounts to be due to an increase in days with heavier rain, as is shown by the plot of heavy precipitation days (>10mm):
Dry and wet spells, show by CDD and CWD plots, are also noisy and show no large changes:

Notes:

This paper is relevant:
http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50188/full

 More patterns may emerge if I looked at high-resolution data, and separated events by season.

 R code for CLIMDEX is available here: https://bitbucket.org/climdex/r/src

 ECA&D E-OBS gridded data can be downloaded here:

There is a heading called "Indices of extremes", but it seems to only deal with station data, not gridded data.

I could download the data from CLIMDEX, but resolution is >2.5 degrees, so not many gridpoints represent Ireland, but it could be comparable to climate models.: