Predictability of weather and climate

Abstract:

The mean state of the MLT (mesosphere – lower thermosphere) is key in the exchange of energy, momentum, and trace species between the middle and upper atmosphere. Knowledge of the mean state wind and temperature is endangered by an upcoming gap in measurements. Needed actions include continued operation of existing space-borne instruments and rapid development of replacement options.

Abstract:

The impact of the quasi-biennial oscillation (QBO) on tropical convection and precipitation is investigated through nudging experiments using the UK Met Office Hadley Center Unified Model. The model control simulations show robust links between the internally generated QBO and tropical precipitation and circulation. The model zonal wind in the tropical stratosphere was nudged above 90 hPa in atmosphere-only and coupled ocean-atmosphere configurations. The convection and precipitation in the atmosphere-only simulations do not differ between the experiments with and without nudging, which may indicate that SST-convection coupling is needed for any QBO influence on the tropical lower troposphere and surface. In the coupled experiments, the precipitation and sea-surface temperature relationships with the QBO phase disappear when nudging is applied. Imposing a realistic QBO-driven static stability anomaly in the upper-troposphere lower-stratosphere is not sufficient to simulate tropical surface impacts. The nudging reduced the influence of the lower troposphere on the upper branch of the Walker circulation, irrespective of the QBO, indicating that the upper tropospheric zonal circulation has been decoupled from the surface by the nudging. These results suggest that grid-point nudging mutes relevant feedback processes occurring at the tropopause level, including high cloud radiative effects and wave mean flow interactions, which may play a key role in stratospheric-tropospheric coupling.

Abstract:

Land-cover change (LCC) is an important driver of climate change through carbon emissions (biochemical effects), but also through changes in the surface energy balance (biophysical effects). Quantifying magnitude and sign of surface temperature responses to biophysical effects is still challenging and under debate. We develop a new semi-empirical model based on a linearized surface energy balance for biophysical and an empirical model for the biochemical responses to LCC. Neglecting indirect effects, we find average global direct biophysical and biochemical warmings in response to a stylized deforestation scenario (1.22 K and 0.50 K) and historical LCC (0.42 K and 0.15 K), whereas an afforestation experiment leads to cooling (−1.95 K and −0.96 K). Our results underline the non-negligible impact of biophysical effects, especially non-radiative effects, and stress the importance of including these in the assessment of climate change mitigation and adaptation policies

Abstract:

The ‘signal-to-noise paradox’ for seasonal forecasts of the winter NAO is often described as an ‘underconfident’ forecast and measured using the ratio-of-predictable components metric (RPC). However, comparison of RPC with other measures of forecast confidence, such as spread-error ratios, can give conflicting impressions, challenging this informal description. We show, using a linear statistical model, that the ‘paradox’ is equivalent to a situation where the reliability diagram of any percentile forecast has a slope exceeding 1. The relationship with spread-error ratios is shown to be far less direct. We furthermore compute reliability diagrams of winter NAO forecasts using seasonal hindcasts from the European Centre for Medium-range Weather Forecasts and the UK Meteoro logical Office. While these broadly exhibit slopes exceeding 1, there is evidence of asymmetry between upper and lower terciles, indicating a potential violation of linearity/Gaussianity. The limitations and benefits of reliability diagrams as a diagnostic tool are discussed.