Assessing spatial precipitation uncertainties in a convective‐scale ensemble. (7th October 2016)

• Record Type: Journal Article
• Authors: Dey, Seonaid R. A.; Plant, Robert S.; Roberts, Nigel M.; Migliorini, Stefano
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 142:Number 701(2016)
• Page Start: 2935
• Page End: 2948
• Publication Date: 2016-10-07
• Subjects: convective‐scale -- ensemble -- forecasting -- neighbourhood -- verification -- spatial -- spread–skill; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.2893
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract : New techniques have recently been developed to quantify the location‐dependent spatial agreement between ensemble members, and the spatial spread–skill relationship. In this article a summer of convection‐permitting ensemble forecasts are analysed to better understand the factors influencing location‐dependent spatial agreement of precipitation fields and the spatial spread–skill relationship over the UK. The aim is to further investigate the agreement scale method, and to highlight the information that could be extracted for a more long‐term routine model evaluation. Overall, for summer 2013, the UK 2.2 km grid spacing ensemble system was found to be reasonably well spread spatially, although there was a tendency for the ensemble to be overconfident in the location of precipitation. For the forecast lead times considered (up to 36 h), a diurnal cycle was seen in the spatial agreement and in the spatial spread–skill relationship: the forecast spread and error did not increase noticeably with forecast lead time. Both the spatial agreement and the spatial spread–skill were dependent on the fractional coverage and average intensity of precipitation. A poor spread–skill relationship was associated with a low fractional coverage of rain and low average rain rates. The times with a smaller fractional coverage, or lower intensity, of precipitation were found to have lower spatial agreement. The spatial agreement was found to be location dependant, with higher confidence in the location of precipitation to the northwest of the UK. Abstract : The figure shows example ensemble member rainfall forecasts at 0900 UTC on 28 th July 2013 (left) along with the corresponding observed radar derived rain rates (centre). We know that there is spatial uncertainty in these forecasts: on the right of the figure a map characterizing this uncertainty is shown, as calculated in the paper. Using a whole summer of cases the paper systematically analyzes these spatial differences between precipitation forecasts, and the spatial differences between precipitation forecasts and radar observations.