3.3.2 Meteorological conditions during extreme events across
Cumbria
Figures 6 and 7 (and Figures Supp. 1 & 2) show time series of
meteorological variables during the 4 extreme Cumbrian rainfall events
that occurred since 2005. Each event was associated with a frontal
system, the 2005, 2009 and 2015 events being classified as atmospheric
rivers, where enhanced horizontal water vapour transport from the
Atlantic Ocean occurs (Lavers et al ., 2011, 2013; Matthews,
Murphy, McCarthy, Broderick, & Wilby, 2018). The meteorological time
series show that, at many of the locations, during the main periods of
rainfall (indicated by the grey shaded area in Figures 6b, 7b, Supp. 1b
& 2b) relatively low RH (in the context of the results presented
above) occurs along with high wind speeds (Figures 6a, 7a, Supp. 1a &
2a) and an increase in air temperature (Figures 6, 7c, Supp. 1a & 2a).
A notable exception to these general patterns are the differences inRH between sites for the 2009 event where Keswick and Walney were
at approximately 90-95% RH during the main rainfall period but
the Shap and Walney sites were at, or close to, saturation. This was
also the case at the higher elevation site of Great Dun Fell where the
wind speeds were very high but the RH remained at 100%
throughout the entire period of rainfall (Figures 7a and 7b
respectively). Similarly, but for the 2015 flood event, in a clearing in
Gisburn Forest, Lancashire the RH remained very high or at
saturation for a large part of the storm (Figure 7b). There is also
consistency in that Keswick, which is a less exposed site at relatively
low elevation on the lee side of one of the highest regions of
mountains, tends to have lower wind speeds, higher temperatures and
lower RH than the other locations during all extreme events
considered (Figures 6, 7 and Supp. 1 & 2). It is worth highlighting the
caveat that the Keswick, Shap and Warcop sites are located on the
leeward side (relative to the dominant south westerly flows) of mountain
ridges such that the observed favourable conditions for Ewc loss
shown are not necessarily representative across the region as a whole:
as shown by the less favourable conditions for Ewc loss at
Gisburn Forest and Great Dun Fell. These results show that even during
the 4 most extreme events in Cumbria over the last 15 years, high
windspeeds and surprisingly low RH provide favourable conditions
for significant Ewc loss at some locations. Consequently the need
to estimate Ewc losses across large catchments, particularly in
complex mountainous terrain, requires a representation of the spatial
variability of meteorological controls which will be challenging where
only sparse meteorological observations are available.