A portrayal of an orographic Warm Conveyor Belt using observations from aircraft, lidar and radar

Warm conveyor belts (WCBs) are important airstreams in extratropical
cyclones, leading to the formation of intense precipitation
and the transport of substantial amounts of water vapour upward and
poleward. This study presents a scenario of a WCB that ascended from
western Europe towards the Baltic Sea using aircraft, lidar and
radar observations from the field experiments HyMeX and
T-NAWDEX-Falcon in October 2012.
Trajectories based on the ensemble data assimilation
system of the ECMWF are used to quantify probabilistically
the occurrence of the WCB and Lagrangian matches
between different observations. Despite severe limitations
for research flights over Europe, the DLR Falcon successfully
sampled WCB air masses during different phases of
the ascent. The overall picture of the WCB trajectories revealed
measurements in several WCB branches: trajectories
that ascended from the East Atlantic over northern France
while others had their inflow in the western Mediterranean
region and passed across the Alps. For the latter ones, Lagrangian
matches coincidentally occurred between lidar water
vapour measurements in the inflow of the WCB south,
radar measurements during the ascent at and its outflow
north of the Alps during a mid-tropospheric flight leg over
Germany.
The comparison of observations and ensemble analyses
reveals a moist bias of the analyses in parts of the WCB inflow
and an underestimation of cloud water species in the
WCB during ascent. In between, the radar instrument measured
strongly precipitating WCB air mass with embedded
linking trajectories directly above the melting layer while
orographically ascending at the southern slops of the Alps.
An inert tracer air mass could confirm the long pathway
of WCB air from the inflow in the marine boundary layer
until the outflow in the upper troposhpere near the Baltic
sea several hours later. This case study illustrates the complexity
of the interaction of WCBs with the Alpine topography,
which leads to (i) various pathways over and around
the Alpine crest and (ii) locally steep WCB ascent with increased
cloud content that might result in enhancement
of precipitation where the WCB flows over the Alps. The
combination of observational data and detailed ensemble-based
trajectory calculations reveals important aspects of
orographically-modified WCBs.