GPS – Zenith Total Delay assimilation in different resolution simulations of a heavy precipitation event over southern France

Alberto Caldas-Álvarez,Olivier Bock,Samiro Khodayar

doi:10.5194/asr-14-157-2017

Abstract

Abstract. The aim of this study is to investigate the different pathways of the interaction between an improved atmospheric moisture distribution by Data Assimilation (DA) of Global Positioning System Zenith Total Delays (GPS-ZTD) on the simulation of a selected Heavy Precipitation Event (HPE) across different model horizontal resolutions (7 km, 2.8 km and 500 m). The initiation and evolution of deep moist convection and heavy precipitation taking place on the 24 September 2012, which had a dedicated Intensive Observation Period (IOP6) during the Hydrological cycle in the Mediterranean eXperiment (HyMeX) Special Observation period 1, are analysed. The results show an improvement in the representation of the Integrated Water Vapour (IWV) spatial distribution and temporal evolution when the data assimilation is applied as well as through the refinement of the model grids. However, important discrepancies between the simulated and the observed vertical profiles of humidity still remain after the DA, thus affecting the representation of convection and heavy precipitation. For the presented case study, the model simulations exhibited a wet bias. The assimilation entailed a drying of the low to middle troposphere over the study region during the 6 h prior to the storm initiation for every horizontal resolution. This reduced the instability present at the moment of storm initiation, weakening in return the intensity of convection and the number of cells triggered. The improvement observed in the atmospheric moisture content and distribution was not followed by an improved precipitation representation closer to observations. This highlights the relevance of correctly distributing the assimilated IWV in the vertical direction in the models.

Highlights

The prediction of Heavy Precipitation Events (HPEs) that typically strike the western Mediterranean region by late summer (Lee et al, 2016; Davolio et al, 2016) is still a challenge for current Numerical Weather Prediction (NWP) models
The Integrated Water Vapour (IWV) data set used for comparison of our model results against observations is provided by the IGN and was derived from the ZTD data, employing surface pressure operational analysis from the AROME model in its west-Mediterranean configuration (AWMED) and the mean temperature computed from ERA-Interim pressure-level data following the algorithm described in Bevis et al (1992)
The nearest COnsortium for Small-scale MOdelling (COSMO) grid points to the stations location are chosen for the differences

Summary

Introduction

The prediction of Heavy Precipitation Events (HPEs) that typically strike the western Mediterranean region by late summer (Lee et al, 2016; Davolio et al, 2016) is still a challenge for current Numerical Weather Prediction (NWP) models. Under a weak synoptic forcing, the location and time of the triggered convective cells is determined by mesoscale temperature and humidity inhomogeneities that define unstable regions prone to lifting. Sufficient moisture and a triggering mechanism are present, convection will take place. This major role of water vapour in convection was addressed in modelling studies where variations of 1 g kg−1 in specific humidity were able to make a difference between intense convection and its complete suppression (Crook, 1996).

Objectives

Methods

Results

Conclusion