In:
Natural Hazards and Earth System Sciences, Copernicus GmbH, Vol. 20, No. 10 ( 2020-10-21), p. 2753-2776
Abstract:
Abstract. Gaining insight into the interaction between atmospheric moisture and convection is determinant for improving the model representation of heavy
precipitation, a weather phenomenon that causes casualties and monetary losses in the western Mediterranean region every year. Given the large
variability of atmospheric moisture, an accurate representation of its distribution is expected to reduce the errors related to the representation
of moist convective processes. In this study, we use a diagnostic approach to assess the sensitivity of convective precipitation and underlying
mechanisms during a heavy precipitation event (Hydrological cycle in the Mediterranean eXperiment Intensive Observation Period; HyMeX IOP16) to
variations of the atmospheric moisture spatio-temporal distribution. Sensitivity experiments are carried out by nudging a homogenized data set of the
Global Positioning System-derived zenith total delay (GPS-ZTD) with sub-hourly temporal resolution (10 min) in 7 and 2.8 km simulations
with the COnsortium for Small-scale MOdeling in CLimate Mode (COSMO-CLM) model over the western Mediterranean region. The analysis shows that
(a) large atmospheric moisture amounts (integrated water vapour; IWV ∼ 40 mm) precede heavy precipitation in the affected areas. This
occurs 12 h prior to initiation over southern France and 4 h over Sardinia, north-eastern Italy and Corsica, which is our main study
area. (b) We found that the moisture is swept from the Atlantic by a westerly large-scale front associated with an upper level low on the one hand
and evaporated from the Mediterranean Sea and north Africa on the other. The latter moisture transport occurs in the 1 to 4 km
layer. (c) COSMO-CLM overestimated the atmospheric humidity over the study region (Corsica), and this was, to a good extent, corrected by the GPS-ZTD
nudging. This reduced maximum precipitation (−49 % for 7 km and −16 % for 2.8 km) drastically, considerably improving the
precipitation representation in the 7 km simulation. The convection-permitting simulation (2.8 km) without the GPS-ZTD nudging already did a
good job in representing the precipitation amount. (d) The two processes that exerted the largest control on precipitation reduction were the
decrease of atmospheric instability over Corsica (convective available potential energy; CAPE −35 %) and the drying of the lower free
troposphere bringing additional dry air entrainment. In addition, the 7 km simulation showed a weakening of the represented low-pressure system
and the associated cyclonic wind circulation. This ultimately reduced the intensity and number of convective updrafts represented over the island.
These results highlight the large impact exerted by moisture corrections on precipitating convection and the chain of processes leading to it across
scales.
Type of Medium:
Online Resource
ISSN:
1684-9981
DOI:
10.5194/nhess-20-2753-2020
DOI:
10.5194/nhess-20-2753-2020-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2020
detail.hit.zdb_id:
2069216-X
detail.hit.zdb_id:
2064587-9