Fully coupled high-resolution medium-range forecasts: evaluation of the
hydrometeorological impact in an ensemble framework
Abstract
Fully coupled atmospheric-hydrological models allow a more realistic
representation of the land surface–boundary layer continuum,
representing both high-resolution land-surface/subsurface water lateral
redistribution and the related feedback towards the atmosphere. This
study evaluates the potential contribution of the fully coupled approach
in extended-range mesoscale hydrometeorological ensemble forecasts.
Previous studies have shown, for deterministic simulations, that the
effect of fully coupling for short-range forecasts is minor compared to
other sources of uncertainty, however, it becomes not negligible when
increasing the forecast period. Through a proof-of-concept consisting of
an ensemble (50 members from the ECMWF Ensemble Prediction System)
seven-days-in-advance forecast of a high impact event affecting the
Calabrian peninsula (southern Italy, Mediterranean basin) on November
2019, the paper elucidates the extent to which the improved
representation of the terrestrial water lateral transport in the Weather
Research and Forecasting (WRF) – Hydro modeling system affects the
ensemble water balance, focusing on the precipitation and the
hydrological response, in terms of both soil moisture dynamics and
streamflow in 14 catchments spanning over 42% of the region. The fully
coupled approach caused an increase of surface soil moisture and latent
heat flux from land in the days preceding the event, partially affecting
the lower Planetary Boundary Layer. However, when shoreward moisture
transport from surrounding sea rapidly increased becoming the dominant
process, only a weak signature of soil moisture contribution could be
detected, resulting in only slightly higher precipitation forecast and
not clear variation trend of peak flow, even though the latter variable
increased up to 10% in some catchments. Overall, this study highlighted
a remarkable performance of the medium-range ensemble forecasts,
suggesting a profitable use of the fully coupled approach for
forecasting purposes in circumstances in which soil moisture dynamics is
more relevant and needs to be better addressed.