Resolution and Athena some introductory comments Tim Palmer

Resolution and Athena – some introductory comments Tim Palmer ECMWF and Oxford

General Comments • Better resolution = better representation of the laws of physics and of relevant boundary forcings (eg topography) • There are two dominant modes of instability in the atmosphere: baroclinic and convective. We should aspire to be able to simulate them both. Shame they occur on such disparate scales!

Goals • 1. To get to the minimum scale that will still allow parametrised convection. Benefits include: – Better topography – Better representation of nonlinear stages of baroclinic instability (important for blocking) • 2. To get to convectively resolved scales Benefits include: – MJO, ENSO, monsoons etc…. ?

Possible Nomenclature • Low resolution (everything so far) • Intermediate resolution (highest resolutions that will still allow parametrised deep convection) • High resolution (resolved convection)

ECMWF • Increasing resolution has been an important element in ECMWF’s strategy for improving the skill of medium range forecasts. • By extrapolation, there should also be an important element for monthly, seasonal and decadal forecasts. • Increasing resolution in seasonal forecast systems is a key element of the strategic goals of some of ECMWF’s “competitors”.

Oxford • Impact on climate change signals – Global mean temperature depends critically on feedbacks with cloud. The better we can represent cloud systems, the more confidence we can have in the representation of these cloud feedbacks – Simulating regional climate change accurately is also essential for guiding mitigation, adaptation and geoengineering strategies

Impact of intermediate resolution

JJA Precip Climate Change. From AR 4 …increased probability of warm dry summers over Europe. A robust signal from CMIP 3 multimodel ensemble. Reliable?

Timeslice estimates of JJA precip climate change using MRI climate model at three resolutions (Matsueda and Palmer, in progress) Much weaker drying signal at high resolution 180 km (climate) 60 km 20 km (NWP)

observed Climate change, blocking frequency and model resolution 180 km (climate) 20 km (NWP)

Surface pressure Potential Vorticity

Impact of high resolution

Velocity Potential Spectra 0. 625 1. 0 0. 875 0. 375 0. 25 0. 1 In preparation by Tompkins, Jung and Vitart motivated by Tompkins and Jung (2003)

Large-scale vs Convective Precipitation (DJF)

Conclusions • The Athena Project is an amazing outcome of the Summit (please tell Guy Brasseur) • Resolution is a key issue for monthly/seasonal/decadal and for the climate change problem (given its widespread implications for mitigation, adaptation and geoengineering policy) • Resolutions within which convection (a dominant mode of instability) is parametrised, should be considered “intermediate”, not “high”.