The CORDEX.be initiative as a foundation for climate services in Belgium

The CORDEX.be project created the foundations for Belgian climate services by producing high-resolution Belgian climate information that (a) incorporates the expertise of the different Belgian climate modeling groups and that (b) is consistent with the outcomes of the international CORDEX ("COordinated Regional Climate Downscaling Experiment") project. The key practical tasks for the project were the coordination of activities among different Belgian climate groups, fostering the links to specific international initiatives and the creation of a stakeholder dialogue. Scientifically, the CORDEX.be project contributed to the EURO-CORDEX project, created a small ensemble of High-Resolution (H-Res) future projections over Belgium at convection-permitting resolutions and coupled these to seven Local Impact Models. Several impact studies have been carried out. The project also addressed some aspects of climate change uncertainties. The interactions and feedback from the stakeholder dialogue led to different practical applications at the Belgian national level

Evaluation framework for sub-daily rainfall extremes simulated by regional climate models

peer reviewedSub-daily precipitation extremes are high-impact events that can result in flash floods, sewer system overload, or landslides. Several studies have reported an intensification of projected short-duration extreme rainfall in a warmer future climate. Traditionally, regional climate models (RCMs) are run at a coarse resolution using deep-convection parameterization for these extreme events. As computational resources are continuously ramping up, these models are run at convection-permitting resolution, thereby partly resolving the small-scale precipitation events explicitly. To date, a comprehensive evaluation of convection-permitting models is still missing. We propose an evaluation strategy for simulated sub-daily rainfall extremes that summarizes the overall RCM performance. More specifically, the following metrics are addressed: the seasonal/diurnal cycle, temperature and humidity dependency, temporal scaling and spatio-temporal clustering. The aim of this paper is: (i) to provide a statistical modeling framework for some of the metrics, based on extreme value analysis, (ii) to apply the evaluation metrics to a micro-ensemble of convection-permitting RCM simulations over Belgium, against high-frequency observations, and (iii) to investigate the added value of convection-permitting scales with respect to coarser 12-km resolution. We find that convection-permitting models improved precipitation extremes on shorter time scales (i.e, hourly or two-hourly), but not on 6h-24h time scales. Some metrics such as the diurnal cycle or the Clausius-Clapeyron rate are improved by convection-permitting models, whereas the seasonal cycle appears robust across spatial scales. On the other hand, the spatial dependence is poorly represented at both convection-permitting scales and coarser scales. Our framework provides perspectives for improving high-resolution atmospheric numerical modeling and datasets for hydrological applications

Impacts of regional climate change upon the warm rain process and surface precipitation from deep convective storms: A numerical modeling study

Multiple studies have stated that precipitation changes resulting from climate change are reflected mainly in the heavy and extreme daily precipitation events, at the expense of more moderate events. Observations in the contiguous U.S. over the 20th century have shown that these precipitation changes have been larger in the summer months. However, the details of how precipitation production within deep convective storms will be modified as regional climates change are still not well understood, owing to the complex interactions between microphysical processes, thermodynamic aspects of the atmosphere, the cloud dynamics, and the large-scale environment. This study investigates if storms developing in a warmer, moister future environment can be expected to have a more active warm rain process, and if so, how it may affect ice processes aloft and precipitation at the surface. A detailed 1D microphysical model and the 3D Weather Research and Forecasting (WRF) model are both used to simulate continental convective storms at different sites within past (1970-1999) and future (2070-2099) environments derived from NCAR CCSM3 model output. Results suggest future storms will be dynamically stronger with shorter lifetimes, but producing more rain and less hail within the storms, and higher rainfall rates at the surface. The warm rain process is more important in the initial rain production in the future storms; but the contribution of ice processes to surface rainfall is decreased. The precipitation efficiency of the future storms show higher values owed to the increased productivity of the warm rain process

Concurrent Sensitivities of an Idealized Deep Convective Storm to Parameterization of Microphysics, Horizontal Grid Resolution, and Environmental Static Stability

This study investigated the sensitivity of idealized deep convective storm simulations to microphysics parameterization, horizontal grid spacing (Δx), and environmental static stability. Three different bulk microphysics schemes in the Weather Research and Forecasting Model were tested for Δx between 0.125 and 2 km and three different environmental soundings, modified by altering static stability above 5 km. Horizontally and temporally averaged condensation and surface precipitation rates and convective updraft mass flux were sensitive to microphysics scheme and Δx for all environmental soundings. Microphysical sensitivities were similar for 0.125 < Δx < 1 km, but they varied for different soundings. Sensitivities of these quantities to Δx were less robust and varied with microphysics scheme. Other statistical convective characteristics, such as the mean updraft width and strength, exhibited similar sensitivities to Δx for all of the microphysics schemes. Microphysical sensitivities were primarily attributed to interactions between microphysics, cold pools, and dynamics that affected the spatial coverage of convective updrafts and hence the horizontally averaged convective mass flux, condensation rate, and surface precipitation. However, these linkages were less clear for the lowest convective available potential energy (CAPE) sounding, and in this case other mechanisms compensated to give a similar spatial coverage of convective updrafts even in simulations without a cold pool. For higher CAPE, there was considerable production of rimed ice from all of the microphysics schemes and its assumed characteristics, especially the fall speed, were important in explaining sensitivity via microphysical impacts on the cold pool. These results highlight the need for continued improvement in representing the production of rimed ice and its characteristics in microphysics scheme

A multisensor investigation of rime splintering in tropical maritime cumuli

Three flights from the Ice in Clouds Experiment-Tropical (ICE-T) field campaign examined the onset of ice near the ascending cloud tops of tropical maritime cumuli as they cooled from 0� to -14�C. Careful quantitative analysis of ice number concentrations included manual scrutiny of particle images and corrections for possible particle-shattering artifacts. The novel use of the Wyoming Cloud Radar documented the stage of cloud development and tops relative to the aircraft sampling, complemented the manual estimates of graupel concentrations, and provided new clear evidence of graupel movement through the rime-splintering zone. Measurements of ice-nucleating particles (INPs) provided an estimate of primary initiated ice. The data portray a dynamically complex picture of hydrometeor transport contributing to, and likely resulting from, the rime-splintering process. Hundreds per liter of supercooled raindrops ascended within the updrafts as the cloud tops reached 0�C and contributed in part to the 0.1 L-1 graupel detected soon after the cloud tops cooled to -5�C. Rime splintering could thus be initiated upon first ascent of the cloud top through that zone and arguably contributed to the 1 L-1 or more graupel observed above it. Graupel ascending/descending into, or balanced within, the rime-splintering zone were found. In wider, less isolated clouds with dying updrafts and tops near -14�C, ice particle concentrations sometimes reached 100 L-1. Future 3D numerical modeling will be required to evaluate if rime splintering alone can explain the difference of three to four orders of magnitude in the observed INPs and the graupel observed at -5�C and colder

Evaluation and Bias Correction of Simulated Sub-daily Rainfall Extremes by Regional Climate Models

Sub-daily precipitation extremes can have a huge impact on society as they cause hazards such as flooding, erosion and landslides. For example, the July floods in Germany, Belgium and nearby countries, were one of the costliest events in Europe, with insured losses up to USD 13 billion. Climate change is expected to intensify precipitation extremes as atmospheric water content increases by 6-7% per degree of warming, underscoring the need to predict future hydrological hazards. Regional Climate Models (RCMs) typically run at a spatial resolution of 12-25 km, but they insufficiently describe the small-scale features of observed sub-daily precipitation extremes. The past decade, convection-permitting RCMs were developed which run at high resolution (1-4 km), and explicitly resolve deep convection. Confidence in future projections requires that RCMs adequately simulate the statistical features of observed sub-daily extreme precipitation and also represent the physical processes associated with convective events. We propose a diagnostic framework for simulated 1h-24h rainfall extremes that summarizes the overall RCM performance. This includes the following metrics: the seasonal/diurnal cycle, temperature and humidity dependency, temporal scaling, and spatiotemporal clustering. A substantial part of the work is devoted to the statistical modelling of the metrics with Extreme Value Theory (EVT). We illustrate the evaluation tool with convection-permitting RCM simulations over Belgium against high-frequency observations and assess the benefit of the convection-permitting RCMs with respect to coarser scales. Finally, we give some guidelines for bias correction of simulated precipitation extremes

Combining regional downscaling expertise in Belgium: CORDEX and beyond

The main objectives of the CORDEX.be project were: 1. Contribute to the international climate community by participating to EURO-CORDEX by performing regional climate simulations over Europe. 2. Provide an ensemble of High-Resolution (H-Res) climate simulations over Belgium i.e. to create a small ensemble of high-resolution future projections over Belgium at convectionpermitting resolutions. 3. Couple these model simulations to seven local-impact models for impact studies. 4. Present an overview of the ongoing climate modeling activities in Belgium. 5. Provide coherent climate information for Belgium targeted to end-users, backed by: (i) a unified framework for the H-Res climate runs and (ii) uncertainty estimations on the climate change signal; 6. Provide and present a climate-impact report for stakeholders and the general public that highlight the most important results of the project.BRAIN-beBRAIN-be - Belgian Research Action through Interdisciplinary Network