The impact of time‐varying sea surface temperature on UK regional atmosphere forecasts

Abstract

A new approach to improve the ocean surface boundary condition used in regional numerical weather prediction is proposed. Typically, regional atmosphere forecast systems assume a fixed sea surface temperature during a simulation. The study assesses the use of ocean temperature from an operational regional ocean model as an evolving lower boundary in a kilometrescale regional atmosphere configuration centred on the UK. Simulations of a winter and two five day duration summer case studies associated with anomalously warm temperatures are considered. The largest impact is found in summer, when a growing cold bias in mean temperature over land compared with observations is apparent when using a fixed global-scale analysis lower boundary condition. The mean error is improved by 0.1 K when using a fixed temperature boundary condition from a kilometre-scale regional ocean model initial condition. When using hourly surface temperature data from the same regional ocean model, the error is improved by 0.5 K for this case. Prediction of daytime maximum air temperature is also improved during the summer heat wave cases. A winter case study shows marginal improvement over the ocean and negligible changes over land.These results are confirmed in longer duration experiments using an hourly cycling regional forecast system with data assimilation for summer and winter periods. A systematic and statistically significant improvement of near-surface temperature verification relative to the observations over land is demonstrated for both summer and winter using the new approach. This study supports future operational implementation of a time-varying lower boundary for regional numerical weather prediction.

Simulations of Bay of Bengal tropical cyclones in a regional convection-permitting atmosphere–ocean coupled model

“…Updating SST during simulations with SST observations has been shown to improve track and intensity predictions (Mohanty et al, 2019;Rai et al, 2019). In an operational setting, it is possible to use the forecast SST from a regional ocean model as an updating lower boundary condition (Mahmood et al, 2021), but this is not common operational practice. 85…”

Sensitivity of the Short-Range Precipitation Forecast in South China Region to Sea Surface Temperature Variations

“…They further suggest the importance of assimilated ocean model in capturing significant differences in precipitation over the sea during a cold front case. A recent study addressed a similar result in a five-days simulation, showing reduced temperature bias by using a time-varying SST from regional numerical weather prediction [5].…”

“…However, for the short-term evolution of weather systems, the importance of ocean processes other than the sea surface temperature (SST) was not well appreciated until recently. Thus, to date, it is still common to prescribe persistent SST from observation or global model data in regional simulations, especially those for a short-range forecast and at high resolutions [5,6].…”

GULF18, a high-resolution NEMO-based tidal ocean model of the Arabian/Persian Gulf

“…SST strongly influences fluxes of heat, moisture and momentum across the ocean-atmosphere interface and the importance of more accurate SST simulations is widely recognised, both for long (e.g., Minobe et al (2008)) and shorter (e.g., Mahmood et al (2021)) timescales. In this section, we assess the skill of PGM4 and GULF18-* models in predicting the SST of the Gulf in the period 2014-2017.…”