On assessing ERA5 and MERRA2 representations of cold-air outbreaks
across the Gulf Stream
Abstract
The warm Gulf Stream sea surface temperatures (SSTs) strongly impact the
evolution of winter clouds behind atmospheric cold fronts. Such cloud
evolution remains challenging to model. The Gulf Stream is too wide
within the ERA5 and MERRA2 reanalyses, affecting the turbulent surface
fluxes. Known problems within the ERA5 boundary layer (too-dry and
too-cool with too strong westerlies), ascertained primarily from
ACTIVATE 2020 campaign aircraft dropsondes and secondarily from older
buoy measurements, reinforce surface flux biases. In contrast, MERRA2
winter surface winds and air-sea temperature/humidity differences are
slightly too weak, producing surface fluxes that are too low. Reanalyses
boundary layer heights in the strongly-forced winter cold-air-outbreak
regime are realistic, whereas late-summer quiescent stable boundary
layers are too shallow. Nevertheless, the reanalysis biases are small,
and reanalyses adequately support their use for initializing
higher-resolution cloud process modeling studies of cold-air outbreaks.