Missing orographic drag leads to climate model biases in jet streams and storm tracks - Met Office Unified Model (UM) results

• Main Author: Pithan, Felix
• Format: Text
• Language: English
• Published: University of Reading 2016
• Subjects: North Atlantic
• Online Access: https://researchdata.reading.ac.uk/49/; https://researchdata.reading.ac.uk/49/1/xlgta_mslp_all_daily.nc; https://researchdata.reading.ac.uk/49/6/xlgtj_mslp_all_daily.nc; https://researchdata.reading.ac.uk/49/7/xlgta_z500all_daily.nc; https://researchdata.reading.ac.uk/49/9/xlgtj_z500all_daily.nc; https://researchdata.reading.ac.uk/49/14/xlgta_u850all_6h.nc; https://researchdata.reading.ac.uk/49/5/xlgtj_u850all_6h.nc; https://researchdata.reading.ac.uk/49/3/xlgta_v850all_6h.nc; https://researchdata.reading.ac.uk/49/8/xlgtj_v850all_6h.nc; https://researchdata.reading.ac.uk/49/2/xlgta_u_mean.nc; https://researchdata.reading.ac.uk/49/4/xlgtj_u_mean.nc; https://researchdata.reading.ac.uk/49/12/readme.txt

This dataset contains the UM experiments described and used in the paper 'Missing orographic drag leads to climate model biases in jet streams and storm tracks'. State-of-the art climate models generally struggle to represent important features of the large-scale circulation. Common model deficiencies include an equatorward bias in the location of the mid-latitude westerlies and an overly zonal orientation of the North Atlantic storm track. Orography is known to strongly affect the atmospheric circulation and is notoriously difficult to represent in coarse-resolution climate models. How the representation of orography affects circulation biases in current climate models is not understood. Here, we show that the effects of switching off the parameterisation of drag from low-level orographic blocking in one climate model resemble the biases of the CMIP5 ensemble: An equatorward shift in the Southern Hemispheric jet, an overly zonal wintertime North Atlantic storm track, and an increase in the Southern Annular Mode timescale. This suggests that the representation of the extratropical circulation in coarse-resolution climate models can be improved considerably by improving the parameterisation of low-level drag. In the file listing, 'UM std' refers to a 30-year AMIP-II run of the UM at N96 resolution with standard parameters. 'UM noblock' refers to an experiment in which the low-level drag scheme is effectively switched off by setting a tuning parameter to zero, all else being equal to 'UM std'.