Methodological approach to parameter perturbations

in GEM-LAM simulations

Leo Separovic, Ramon de Elia and Rene Laprise

MOTIVATION• Sub-grid parameterization schemes describe:

- well-known processes that can be exactly represented (e.g. radiation transfer) but need to be approximated so that they do not take excessive computational time;

- less-well understood processes (e.g. turbulent energy transfer) that are situation dependent; parameters in such parameterizations rely on mixture of theoretical understanding and empirical fitting;

• Parameters’ values are uncertain due to measurement errors and problems with their “representativity”; hence the tuning cannot completely eliminate the model error.

• We develop methodological approach to quantify the parametric uncertainty in RCM seasonal simulations. To this end we study the response of GEM-LAM seasonal climate to multiple perturbations of parameters. The perturbations’ size are within a range of uncertainty specified by the experts that participated in the model tuning. GEM-LAM (0.5deg, ERA40) is ran over 1 year and the response is studied for each of the 4 seasons.

• For each combination of parameters a small ensemble with perturbed initial conditions is run in order to estimate the statistical significance of the response.

- the following slide shows the changes in 2m temperature (signal) induced by two singleton perturbations of the threshold vertical velocity in Kain-Fritch convection;

- the second slide shows the rejection level of the 0 th hypothesis that the difference is only due to sensitivity to initial conditions.

2m-Temperature (signal)

DJF MAM

JJA SON

DJF MAM

JJA SON

KFCTRIG=0.020 (-) KFCTRIG=0.048 (+)

K

KFCTRIG(ref)=0.034

2m TEMP: rejection level

DJF MAM

JJA SON90.0

95.0

97.5

99.0

99.5

99.9

99.9

99.5

99.0

97.5

95.0

90.0

DJF MAM

JJA SON

KFCTRIG=0.020 (-) KFCTRIG=0.048 (+)

%

KFCTRIG(ref)=0.034