(2009 - 2018)
A seasonal synoptic climatology of cut-off lows (COLs) that produced extreme precipitation in the Valencia region of Spain during 1998-2014 is presented. COLs were shown to be the main producer of extreme precipitation in the region, especially during the transition seasons. The strongest raining COL events occurred during September-November. Six-day composites of lower and upper tropospheric winds, geopotential, sea-level pressure and precipitation show that COLs that produce extreme rainfall in this region remain stationary over Spain for 2-3 days and produce rainfall over the Valencia region for at least two days. In the low levels these COLs are characterized by low pressure over the Mediterranean sea and winds with an easterly, onshore component. Another interesting aspect of the composites is that transition season COLs are characterized by the presence of a filament of moisture that extends from the tropical Atlantic to Spain suggesting a role for remote moisture transport to feed COL rainfall. Further analysis is needed to confirm this hypothesis.
This paper investigates the connection of precipitation organization to the frequency distribution of precipitation intensity, including extremes. The organization of precipitating systems, for example isolated thunderstorms and mesoscale convective systems, is an expression of the influence of the large-scale environment on precipitation. A recent climatology of precipitation organization in the southeastern United States (Rickenbach et al., 2015, QJRMS) demonstrated that a simple framework of identifying the scale of precipitation organization from a radar precipitation dataset was able to capture important differences of the seasonal evolution in precipitation organization. The present study will focus on the question of whether the heaviest daily precipitation values are associated with isolated or mesoscale precipitation organization, and whether this association changes seasonally.
The analysis employs a four-year dataset of daily precipitation values across the southeastern United States to examine the association of heavy precipitation extremes with isolated versus mesoscale organization. Daily precipitation data covering the four-year period 2009-2012 is derived from the National Mosaic and Multi-sensor Quantitative Precipitation Estimation (NMQ) radar-based dataset on a 1 km x 1 km grid that extends 100 km offshore (see Rickenbach et al. 2015 for details). Each pixel is associated with either mesoscale precipitation features (> 100 km in spatial scale) or isolated precipitation features (< 100 km in spatial scale). Preliminary results will be presented at the online conference.
Reference: Rickenbach, T. M., Nieto-Ferreira, R., Zarzar, C. and Nelson, B. (2015), A seasonal and diurnal climatology of precipitation organization in the southeastern United States. Q.J.R. Meteorol. Soc., 141: 1938–1956. doi:10.1002/qj.2500