www.nr.no Evaluation of a dynamic downscaling of precipitation over the Norwegian mainland Orskaug E. a , Scheel I. b , Frigessi A. c,a , Guttorp P. d,a , Haugen J. E. e , Tveito O. E. e , Haug O. a a Norwegian Computing Center, Oslo, Norway b Department of Mathematics, University of Oslo, Oslo, Norway c Department of Biostatistics, University of Oslo, Oslo, Norway d University of Washington, Seattle, USA e The Norwegian Meteorological Institute, Oslo, Norway
21
Embed
Www.nr.no Evaluation of a dynamic downscaling of precipitation over the Norwegian mainland Orskaug E. a, Scheel I. b, Frigessi A. c,a, Guttorp P. d,a,
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
www.nr.no
Evaluation of a dynamic downscaling of precipitation over the Norwegian mainland
Orskaug E.a, Scheel I.b, Frigessi A.c,a, Guttorp P.d,a, Haugen J. E.e, Tveito O. E.e, Haug O.a
a Norwegian Computing Center, Oslo, Norway
b Department of Mathematics, University of Oslo, Oslo, Norway
c Department of Biostatistics, University of Oslo, Oslo, Norway
d University of Washington, Seattle, USA
e The Norwegian Meteorological Institute, Oslo, Norway
Motivation
► Climate research produces an increasing number of data sets combining different GCMs, CO2 emission scenarios and downscaling techniques.
► For impact studies, but also as an issue of separate interest, the quality of these data need to be verified.
Goal
► We want to compare downscaled ERA-40 reanalysis data (RCM) against observations of Norwegian precipitation.▪ How good are the RCM data?▪ Where (in the distribution) does the RCM differ from
the observations?▪ Where (geographically) does the RCM perform
best/worst?
Why is this work important?
► It assesses the quality of a dynamic downscaled data and highlights which areas these data capture reality and where there are deviations from the truth.
► Another aim is to show how standard methods of statistical testing may be used to assess dynamic downscaling.
Data
Model data► RCM model, dynamically
downscaled HIRHAM model, forced by ERA-40 reanalysis data from the ENSEMBLES project.
► Spatial resolution of 25 x 25 km2.
► Reliant on the downscaling, still supposed to possess properties similar to real weather locally over longer time periods.
Observations
► Interpolations (1 x 1 km2) from a triangulation of the official measurement stations operated by The Norwegian Meteorological Institute.
► Aggregated to 25 x 25 km2 scale by collecting 1 x 1 km2 grid cells with centre points within the RCM cell, the mean is representing the precipitation within that grid cell.
Data – The RCM
► The RCM from the ENSEMBLES project
Data – properties for both data sets
► Climate variable: precipitation
► Time period: 1961 – 2000
► Time scale: Daily, seasonal
► Resolution: 25 x 25 km2
► Number of grid cells: 777 grid cells covering Norway
Methods for comparison
► Evaluate the distributions1. Global measure:
Kolmogorov Smirnov test
2. Local measures:
Comments
► Drizzle effect avoided: conditioned on wet days; i.e. days with precipitation below a small, positive threshold (0.5 mm/day) are discarded.
► Day-to-day correlation in the RCM is partly lost due to downscaling, hence the distributions have to be compared instead of comparing day by day.
► Separate tests for each grid cell and each season.
Kolmogorov-Smirnov test
► K-S two sample test is used to check whether the empirical distributions from the RCM and the observations are equal.
► To avoid the problem of tied data, a small, random normally distributed number, N(0, σ2), is added to each data point.
σ = 1e-7
Kolmogorov-Smirnov test – Results
► The null hypothesis of equality of the distributions are rejected for almost all grid cells for all the four seasons.
► Global picture: the RCM does not have the same distribution as the observations.
► Next: want to find out where the distributions differ; local measures.
Methods for comparison
► Evaluate the distributions1. Global measure:
Kolmogorov Smirnov test
2. Local measures:
Test equality of quantiles
Construction of the 2 x 2 contigency table
0.05-quantile – Results
► Hardly any rejections of null hypothesis of equality.
► For low quantiles: the RCM reproduces the observations well both season- and nationwide.
0.95-quantile – Results
► Mainly rejections of the null hypothesis of equality.
► Overall picture: the RCM underestimates high precipitation.
Generalized Pareto Distribution (GPD)
►
GPD – Results
► One-year return levels from GPD are more similar than expressed through the Kolmogorov-Smirnov test.
► But still: tendency that the RCM underestimates high precipitation.
Wet day frequency
► Wet day frequency = Proportion of wet days (among all days in the data)
► A wet day is defined to be above 0.5 mm/day for both data sets.
► The equality of the wet day frequency is tested by permutation testing.
Wet day frequency – Results
► Mainly rejections of the null hypothesis of equality.
► Total picture: Wet day frequency of the RCM is greater than for the observations.
Summary
► Small amounts of rainfall: the RCM shows good agreement with the observations.
► When rainfall amounts is beyond the first quartile, the agreement disappear.
► The RCM has too many and too small rain events for all seasons.
► This work is accepted for publication in Tellus A.
An improvement/correction of the RCM is needed.
What to do next?
► We want to add a statistical correction method to the output of the RCM, especially improve the right tail.
► Simple linear regression was tried out, but did not improve the results.
► We are currently working on a more complex transformation with spatial corrections.