Predictability of weather and climate

Abstract:

Posit numbers, a recently proposed alternative to floating-point numbers, claim to have smaller arithmetic rounding errors in many applications. By studying weather and climate models of low and medium complexity (the Lorenz system and a shallow water model) we present benefits of posits compared to floats at 16 bit. As a standardised posit processor does not exist yet, we emulate posit arithmetic on a conventional CPU. Using a shallow water model, forecasts based on 16-bit posits with 1 or 2 exponent bits are clearly more accurate than half precision floats. We therefore propose 16 bit with 2 exponent bits as a standard posit format, as its wide dynamic range of 32 orders of magnitude provides a great potential for many weather and climate models. Although the focus is on geophysical fluid simulations, the results are also meaningful and promising for reduced precision posit arithmetic in the wider field of computational fluid dynamics.

Abstract:

The Quasi-Biennial Oscillation (QBO), the dominant mode of interannual variability of monthly-mean zonal winds and temperatures in the equatorial stratosphere, has a significant influence on the circulation in other regions of the stratosphere and troposphere in boreal winter. To better understand these teleconnections, results from twelve models are analysed for three complementary experiments in which the QBO is either internally generated by the models (free-running), nudged towards the observed QBO, or nudged towards the observed climatology. For the multi-model ensemble mean we assess how well the free-running experiment captures the observed teleconnections, and whether the simulated teleconnections improve when the models are nudged to have a more realistic QBO. The observed relationship between the QBO phase and the strength of the boreal winter Polar Night Jet (PNJ) is weakly captured by the free-running experiment. The PNJ response is larger when the models are nudged towards the observed QBO, primarily because nudging reduces the bias in the QBO amplitude, resulting in a larger proportion of the models exhibiting a stronger QBO-PNJ relationship. Both free-running and nudged-QBO experiments capture a small and marginally significant increase in Sudden Stratospheric Warming (SSW) frequency during the QBO-easterly phase relative to the QBO-westerly phase. Neither experiment captures the observed QBO influence on the timings of SSWs during winter nor the QBO teleconnection to the North Atlantic Oscillation. Nudging the QBO does, however, improve its influence on the latitudinal profile of the North Pacific sub-tropical jet and strength of tropical precipitation. We find that a more realistic representation of the QBO is beneficial for certain teleconnections, but their effective simulation likely depends on how well the underlying model processes respond to the QBO.