Impact of model upgrades on diabatic processes in extratropical cyclones and downstream forecast evolution

Abstract

Models are continuously developed at numerical weather prediction (NWP) centres to improve forecast skill, with new operational model configurations adopted every few years. The parametrizations of diabatic processes are probably the most frequently updated part of NWP models as they are crucial for accurate weather predictions and contain uncertainties in their formulation. The impact of model developments is assessed here in forecasts from the Met Office's weather forecast model initialised throughout the North Atlantic Waveguide and Downstream Impact Experiment field campaign period in autumn 2016. Planned model parametrization developments are considered, together with an 'inexpensive coupled' forecast with daily updating of the sea surface temperature and sea-ice fraction. Forecasts produced from the coupled system have, on average, indistinguishable skill from the control forecasts, suggesting the benefits of coupled atmosphere-ocean NWP systems can be small. In contrast, a reduction in forecast error (∼4%) is identified in forecasts produced using an upgraded convection scheme. Periods of low forecast skill during the study period are shown to be associated with the onset and decay of blocking events and increased diabatic heating of air masses reaching the upper troposphere. In forecasts of a specific block development case that was not accurately predicted in any of the experiments or in the operational ensemble forecast from the Met Office, the representation of diabatic heating in the warm conveyor belt of an upstream cyclone is shown to moderate the subsequent block development; forecasts in which the heating is stronger generally have a more-amplified blocking ridge and amplified heating contributions from all parametrizations as diagnosed using diabatic tracers. Hence, we demonstrate that plausible changes to the representation of several different diabatic processes in models can impact forecast block development via changes within upstream cyclones. K E Y W O R D Satmospheric blocking, numerical weather prediction, parametrized diabatic processes, warm conveyor belts This article is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.

Reply on RC1

“…These findings support the theory developed in Harvey et al (2020) and are consistent with those seen in a different cyclone's WCB (Oertel et al 2020). The case included in this study has been the subject of several recent articles (Maddison et al 2020, Blanchard et al 2020, including a recent publications by the authors, and this contribution adds useful new insights to complement the recent research, particularly with the novel observations within the WCB. I thus recommend the article be published subject to minor revisions.…”

“…Maddison et al (2020) seems another appropriate reference to add here. Added L401: "matches with the organised" → "matches the organised".…”

How Well do Models Represent the Development of Extra-Tropical Cyclones? Evaluation of Two General Circulation Models Against NAWDEX IOP 6 Observations

“…The eastward shift in the track agrees with the global weather forecasts prior to 29 September 2016 (e.g. Maddison et al, 2020). Given the rapid divergence of the forecast track from the analysis, differences in the cyclogenesis could be one aspect leading to the track occurring too far east.…”

Linking rapid forecast error growth to diabatic processes

“…(2019); Maddison et al . (2020). DIAA helps with ridge building on the westward side and its contribution is weaker in the longer forecast (Figure 6e,f).…”