Reproducing an extreme precipitation event over Crete using high-resolution climate model simulations

The HARMONIE model, when forced with realistic boundary conditions, is capable of simulating extreme precipitation in line with observations.

The ECLISE Project

Rossby Centre is a participant in the EU project ECLISE. The main objective in this project is to take the first step towards the realisation of a European Climate Service. Rossby Centre is involved mainly in  providing regional climate model simulations over Europe to be subsequently used in impact studies. It not only involves pan-European simulations, but also European sub-regions that have been selected for specific case studies. In these latter experiments, the goal is to use very high-resolution climate models, i.e. 2-4 km horizontal resolutions. Here we present results from an experiment that has been conducted in collaboration with the Technical University of Crete and three other climate modelling groups at KNMI, UniRes and HZG.

High Resolution Modelling

At the Rossby Centre, we are working towards the development of a new regional climate model (RCM), based on the non-hydrostatic numerical weather prediction model HARMONIE (Bénard et al., 2010; Bubnová et al., 1995). HARMONIE has physical parametrization packages specifically developed to allow simulations at horizontal resolutions ranging from 10-20 km per grid box (standard in current RCMs), down to 1-2 km (often referred to as "convective-resolving" or "cloud-resolving" models) (Seity et al. , 2011; Bubnová et al., 1995). The highest resolution model setting is expensive to run and therefore mostly applied for smaller geographical domains, and thus perfect for use in case studies. These simulations, due to the fine scale grid, better represents the spatio-temporal characteristics of the simulated precipitation patterns (i.e. frequency and intensity) and most importantly rare (extreme) events.

In order to run a regional climate model, something needs to provide boundary conditions (what comes in through the domain boundaries). This something is usually a global climate model or observations, where observations are almost exclusively taken from reanalysis. However, the resolution of these data is relatively low, O(100km), and thus there is a too big step going from this data to a high-resolution model at 2 km resolution. Therefore, one apply what is called a "double-nesting" experiment; Reanalysis data provide boundary conditions to an RCM at an intermediate resolution, which in turn provide boundary information to the inner (high-resolution) model. The reanalysis data used is ERA-Interim (Dee et al., 2011) and the model at intermediate resolution is the Rossby Centre Atmosphere model, RCA (Samuelsson et al., 2011).

Crete water resources and management

The island of Crete has suffered from numerous severe floods and flash flood events in the past decades. These are mainly the result of extreme weather events, such as extreme precipitation. In ECLISE, one of the case studies is to investigate further this issue by using high-resolution climate models. The objective is to assess the ability of models to reproduce these extreme events and further explore possible changes in the intensity of such events in a changing climate. The results will provide information to the water resource management in Crete and support the development of water management scenarios and risk assessment for future water availability.

RC News Fig 1 Crete observations
Figure 1: Map of observation stations over the island of Crete.

Model results

In a case study, you want to simulate the event as close as possible to the observations in both time and space. However, in a double-nesting experiment this could be an issue, because if the intermediate model does not capture the event correctly then the inner model won’t either. A technique to (hopefully) circumvent this problem is to constantly steer the intermediate model towards observations. This is only applied on the large-scale features of the model climate (i.e. for example winds and temperature in the free atmosphere, 1-2 km above the surface). This technique is called "nudging", and applied sensibly will increase the chance to capture events (e.g. Radu et al., 2008; von Storch et al., 2000).

A case to be investigated in ECLISE occurred 16-18 October 2006 over the island of Crete when there was a flash flood event in the Almirida basin caused by heavy precipitation (Figure 1). The Rossby Centre has performed a number of initial model simulations with HARMONIE at 2 km resolution to investigate whether the model captures accurately the event in space, time and intensity.

Top panel in figure 2 shows that the models capture the extreme event in time but the intensity is lower than observed. This is mainly due to an integrated value over the west of Crete (cf. Fig. 3). A grid box value from HARMONIE near the location of the flash flood yields 170mm/day which is close to the observed value 200mm/day (not shown). Although the RCA4 and HARMONIE simulations show more or less identical daily precipitation on the 17th of October, the high-resolution HARMONIE simulation has larger maximum intensity (bottom panel in Figure 2). Figure 3 clearly shows how the nudged simulation constrains the large scale flow from ERA-I and the increase in resolution and local features further enhances precipitation.

These preliminary results indicate that the HARMONIE model, when forced with realistic boundary conditions, is capable of simulating extreme precipitation in line with what is observed. Future work involves more detailed scrutiny of this and other cases as well as investigation of precipitation extremes under changing climate conditions

RC News Fig 2 Precipitation intensity
Figure 2: (Top/Bottom) Daily/Hourly values of precipitation intensities from models (west of Crete) and the observational station closest to the flash flood location. Yellow line is observations from the Askyfou station where observed times are marked with an asterisk. The green line is ERA-Interim(reanalysis) at 80km. RCA is the intermediate model forced by ERA-I and the blue line is HARMONIE at 2km. The 95 pctl from 30 years (October) of RCA is drawn in black.
RC News Fig 3 Daily mean precipitation Crete
Figure 3: Daily mean precipitation for three consecutive days over the Crete domain. The leftmost column shows HARMONIE, the model with highest resolution and intensities. Next two columns shows RCA, with and without spectral nudging. The rightmost column is ERA-Interim.


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