Subhourly rainfall in a convection-permitting model

Convection-permitting models (CPMs)—the newest generation of high-resolution climate models—have been shown to greatly improve the representation of subdaily and hourly precipitation, in particular for extreme rainfall. Intense precipitation events, however, often occur on subhourly timescales. The distribution of subhourly precipitation, extreme or otherwise, during a rain event can furthermore have important knock-on effects on hydrological processes. Little is known about how well CPMs represent precipitation at the subhourly timescale, compared to the hourly. Here we perform multi-decadal CPM simulations centred over Catalonia and, comparing with a high temporal-resolution gauge network, find that the CPM simulates subhourly precipitation at least as well as hourly precipitation is simulated. While the CPM inherits a dry bias found in its parent model, across a range of diagnostics and aggregation times (5, 15, 30 and 60 min) we find no consistent evidence that the CPM precipitation bias worsens with shortening temporal aggregation. We furthermore show that the CPM excels in its representation of subhourly extremes, extending previous findings at the hourly timescale. Our findings support the use of CPMs for modelling subhourly rainfall and add confidence to CPM-based climate projections of future changes in subhourly precipitation, particularly for extremes

Ensemble prediction for nowcasting with a convection-permitting model - II: forecast error statistics

A 24-member ensemble of 1-h high-resolution forecasts over the Southern United Kingdom is used to study short-range forecast error statistics. The initial conditions are found from perturbations from an ensemble transform Kalman filter. Forecasts from this system are assumed to lie within the bounds of forecast error of an operational forecast system. Although noisy, this system is capable of producing physically reasonable statistics which are analysed and compared to statistics implied from a variational assimilation system. The variances for temperature errors for instance show structures that reflect convective activity. Some variables, notably potential temperature and specific humidity perturbations, have autocorrelation functions that deviate from 3-D isotropy at the convective-scale (horizontal scales less than 10 km). Other variables, notably the velocity potential for horizontal divergence perturbations, maintain 3-D isotropy at all scales. Geostrophic and hydrostatic balances are studied by examining correlations between terms in the divergence and vertical momentum equations respectively. Both balances are found to decay as the horizontal scale decreases. It is estimated that geostrophic balance becomes less important at scales smaller than 75 km, and hydrostatic balance becomes less important at scales smaller than 35 km, although more work is required to validate these findings. The implications of these results for high-resolution data assimilation are discussed

Improving a convection-permitting model simulation of a cold air outbreak

A convection-permitting local-area model was used to simulate a cold air outbreak crossing from the Norwegian Sea into the Atlantic Ocean near Scotland. A control model run based on an operational configuration of the Met Office UKV high-resolution (1.5 km grid spacing) NWP model was compared to satellite, aircraft and radar data. While the control model captured the large-scale features of the synoptic situation, it was not able to reproduce the shallow (<1.5 km) stratiform layer to the north of the open cellular convection. Liquid water paths were found to be too low in both the stratiform and convective cloud regions. Sensitivity analyses including a modified boundary-layer diagnosis to generate a more well-mixed boundary layer and inhibition of ice formation to lower temperatures improved cloud morphology and comparisons with observational data. Copyright © 2013 Royal Meteorological Society and British Crown Copyright, the Met Offic

Ensemble prediction for nowcasting with a convection-permitting model—I: description of the system and the impact of radar-derived surface precipitation rates

A key strategy to improve the skill of quantitative predictions of precipitation, as well as hazardous weather such as severe thunderstorms and flash floods is to exploit the use of observations of convective activity (e.g. from radar). In this paper, a convection-permitting ensemble prediction system (EPS) aimed at addressing the problems of forecasting localized weather events with relatively short predictability time scale and based on a 1.5 km grid-length version of the Met Office Unified Model is presented. Particular attention is given to the impact of using predicted observations of radar-derived precipitation intensity in the ensemble transform Kalman filter (ETKF) used within the EPS. Our initial results based on the use of a 24-member ensemble of forecasts for two summer case studies show that the convective-scale EPS produces fairly reliable forecasts of temperature, horizontal winds and relative humidity at 1 h lead time, as evident from the inspection of rank histograms. On the other hand, the rank histograms seem also to show that the EPS generates too much spread for forecasts of (i) surface pressure and (ii) surface precipitation intensity. These may indicate that for (i) the value of surface pressure observation error standard deviation used to generate surface pressure rank histograms is too large and for (ii) may be the result of non-Gaussian precipitation observation errors. However, further investigations are needed to better understand these findings. Finally, the inclusion of predicted observations of precipitation from radar in the 24-member EPS considered in this paper does not seem to improve the 1-h lead time forecast skill

Convective Precipitation over Complex Terrain, Current and Future Climate

Given the significance of climate models for assessing climate change impacts, and recent increases in their resolution, there is a need to understand strengths and weaknesses of climate models in reproducing key atmospheric processes, and to assess their performance using accurate ground-based observations. This thesis first investigates the inconsistencies in ground-based observations for cold environments, second, the role of ground-based observations for empirical model validation over complex terrain, and third, uses both observations and model output, to describe a mesoscale process associated with precipitation and their changes in a simulated future climate. Regional climate modelling in a convection-permitting configuration improves simulation of mesoscale systems in which convection initiates and develops, adding value to estimates of convective precipitation compared to models that rely on deep convective parameterization schemes. On the leeside of the Canadian Rocky Mountains in extratropical regions, convective precipitation is influenced by a strong longitudinal gradient of low-level moisture across the foothills. Known as the dryline, this gradient is the result of the convergence of moist air from the interior of the continent and the dry air from the subsidence on the lee side of the Rocky Mountains. The dryline plays a key role in initiating convective precipitation. To find robust answers to questions about a future transient climate, a better understanding is needed of the dryline’s relationship to the location and timing of convective initiation. This research has three objectives: 1) to critically quantify the systematic bias of precipitation measurements on two sides of the northern Canada-U.S. border since the two countries use different standard instrumentation to observe liquid and solid precipitation; 2) to study if a convection-permitting model can reproduce the warm season’s diurnal cycle of precipitation at a continental scale, and 3) to describe a mesoscale mechanism related to the initiation of convective precipitation in the Rocky Mountains vulnerable to climate change at the end of the century. Results show that a correction due to wind-undercatch in monthly solid precipitation is up to 31% during January in the Yukon, whereas across the border in Alaskan stations, it is up to 136%. This correction leads to a smaller and inverted horizontal precipitation gradient in the northern part of the border. In July, the correction for monthly liquid precipitation is around 20% in Alaska and 4% in the Yukon. This inconsistency has to be considered in any regional study using precipitation in cold and windy environments. The research to validate the precipitation diurnal cycle characteristics using a convection-permitting model, uses ground-based observations and a gridded product. Results show that the convection-permitting model can represent the main continental patterns and also represent the precipitation peak transitions from the afternoon to night on the leeside of the Rocky Mountains. However, in the central and eastern region of the study domain, the convection-permitting model performance deteriorates during the diurnal cycle for observed morning peaks (in the central-east U.S.) and overestimates the magnitude of the observed diurnal cycle in the southeast region in the U.S. When a warmer climate scenario is simulated at the end of the century, persistent increase is shown both, in the amplitude of the precipitation diurnal cycle and in the precipitation intensity throughout the domain. The warmer climate simulation also presents an increase in precipitation frequency in the northern region in early summer. These increases may impact the agricultural sector and alter flood risk. Finally, it is found that the convection-permitting model can simulate the dryline, showing an average magnitude of 0.48 g kg-1 km-1 and its maximum intensity being in July. The dryline is present in 37% of the biggest precipitation events (storms with at least one day above the 85% quantile in 13 years period). Although the percentage of the dryline frequency associated with convective initiation in the future scenario is not substantially changed, the dryline is both more intense (0.55 g kg-1 km-1) and narrower. Furthermore, in the simulation of the future climate, an intensification in the north and a dissipation in the southern part of the region was found in a standardized number of occurrences of convective initiation east of the dryline. This finding is associated with a change in the thermodynamical forcing of the most intense precipitation on the selected events in the southern part of the region. By describing the dryline, this research provides a reference point to assess the convective initiation forecasts and offers information on precipitation changes in a warmer scenario at the end of the century

Effects of Increasing Horizontal Resolution in a Convection-Permitting Model on Flood Forecasting: The 2011 Dramatic Events in Liguria, Italy

Abstract Coupling meteorological and hydrological models is a common and standard practice in the field of flood forecasting. In this study, a numerical weather prediction (NWP) chain based on the BOLogna Limited Area Model (BOLAM) and the MOdello LOCale in Hybrid coordinates (MOLOCH) was coupled with the operational hydrological forecasting chain of the Ligurian Hydro-Meteorological Functional Centre to simulate two major floods that occurred during autumn 2011 in northern Italy. Different atmospheric simulations were performed by varying the grid spacing (between 1.0 and 3.0 km) of the high-resolution meteorological model and the set of initial/boundary conditions driving the NWP chain. The aim was to investigate the impact of these parameters not only from a meteorological perspective, but also in terms of discharge predictions for the two flood events. The operational flood forecasting system was thus used as a tool to validate in a more pragmatic sense the quantitative precipitation forecast obtained from different configurations of the NWP system. The results showed an improvement in flood prediction when a high-resolution grid was employed for atmospheric simulations. In turn, a better description of the evolution of the precipitating convective systems was beneficial for the hydrological prediction. Although the simulations underestimated the severity of both floods, the higher-resolution model chain would have provided useful information to the decision-makers in charge of protecting citizens

Representation of precipitation and top-of-atmosphere radiation in a multi-model convection-permitting ensemble for the Lake Victoria Basin (East-Africa)

The CORDEX Flagship Pilot Study ELVIC (climate Extremes in the Lake VICtoria basin) was recently established to investigate how extreme weather events will evolve in this region of the world and to provide improved information for the climate impact community. Here we assess the added value of the convection-permitting scale simulations on the representation of moist convective systems over and around Lake Victoria. With this aim, 10 year present-day model simulations were carried out with five regional climate models at both PARameterized (PAR) scales (12–25 km) and Convection-Permitting (CP) scales (2.5–4.5 km), with COSMO-CLM, RegCM, AROME, WRF and UKMO. Most substantial systematic improvements were found in metrics related to deep convection. For example, the timing of the daily maximum in precipitation is systematically delayed in CP compared to PAR models, thereby improving the agreement with observations. The large overestimation in the total number of rainy events is alleviated in the CP models. Systematic improvements were found in the diurnal cycle in Top-Of-Atmosphere (TOA) radiation and in some metrics for precipitation intensity. No unanimous improvement nor deterioration was found in the representation of the spatial distribution of total rainfall and the seasonal cycle when going to the CP scale. Furthermore, some substantial biases in TOA upward radiative fluxes remain. Generally our analysis indicates that the representation of the convective systems is strongly improved in CP compared to PAR models, giving confidence that the models are valuable tools for studying how extreme precipitation events may evolve in the future in the Lake Victoria basin and its surroundings