A Question of Adequacy of Observations in Variational Data Assimilation - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 111

Fig. 5.1 Schematic diagram
indicating the sequence of
synoptic events associated
with a typical return flow
event over the Gulf of
Mexico. Wind speeds are in
knots ( ms 1 2 1 knots)

the ocean and returns to land (a bias that was cold/dry in the late-1980s through
the early-to-mid 1990s but warm/moist since that time). The consequence of a
poor forecast in these return-flow events is serious since slight changes in the
moisture and heat content of the returning air leads to significantly different weather
regimes—a range of weather that varies from mist and low stratus to shallow
convection (without precipitation) to deep convection with thunderstorms.
From experience with return flow events during project GUFMEX ( Lewis
et al. 1989 ), we pattern our study after a typical return-flow event in the northwestern
Gulf. The typical trajectory associated with a shallow intrusion of cold air into the
Gulf is shown in Fig. 5.2 . The bathymetry of the Gulf underpins this trajectory in
Fig. 5.2 and indicates that the low-level airflow takes place over shelf water. When
we assume surface winds of ms 1 along the over-water path of 1;000

15 km, the

time of transit over water is 18 h.