The RegCM5 model has been tested over the entire set of CORDEX-CORE domains (see Figure S1 in the Supplementary Material), which were previously simulated with the RegCM4.7 version (Coppola et al., 2021a; Giorgi et al., 2022). Additionally, the model was tested for the first time at a convection-permitting (CP) resolution over a pan-European domain. For each domain, multiple observations and reanalysis data have been utilized for model assessment, as reported in Table 2.
Table 2
Observed Datasets
|
Domain
|
Variables
|
Data type
|
Spatial Resolution
|
Temporal Resolution
|
Period
|
Reference
|
CPC_Global
|
Global Land
|
PRECIP
TMAX
TMIN
|
Gridded, Station based
|
0.50 degrees
|
DAILY
|
1979–2021
|
Chen et al. (2008)
|
TRMM
|
Tropics
|
PRECIP
|
Satellite observation based
|
0.25 degrees
|
3-HOURLY
|
1998–2017
|
Kummerow et al. (2000)
|
MSWEP
|
Global
|
PRECIP
|
Derived by optimally merging a range of gauge, satellite, and reanalysis estimates
|
0.10 degrees
|
DAILY
|
1979–2020
|
Beck et al. (2019)
|
GPCC
|
Global
|
PRECIP
|
Gridded, Station based
|
0.25 degrees
|
MONTHLY
|
1891–2020
|
Schneider et al. (2022
|
GPCC
|
Global
|
PRECIP
|
Gridded, Station based
|
1.0
|
DAILY
|
1982–2020
|
Schamm et al. (2014)
|
CRU
|
Global Land
|
PRECIP
TMEAN
|
Station based
|
0.50 degrees
|
MONTHLY
|
1901–2015
|
Harris et al. (2020)
|
APHRO
|
India and East Asia
|
PRECIP
|
Grid
|
0.25 degrees
|
DAILY
|
1951–2007
|
Yatagai et al. (2009)
|
E_OBS
|
Europe Land
|
PRECIP
TMAX TMIN
|
Grid
|
0.25 degrees
|
DAILY
|
1950–2015
|
Cornes et al. (2018)
|
CN05.1
|
China
|
PRECIP
TMEAN
|
Station based
|
0.25 degrees
|
DAILY
|
1961–2012
|
Wu & Gao (2013)
|
ERA5
|
Global
|
WIND,
PRECIP,
CLOUD FRACTION, CLOUD WATER, CLOUD ICE,
MEAN SEA LEVEL PRESSURE, TMEAN
|
Reanalysis
|
0.25 degrees
|
HOURLY
|
1940- Present
|
Hersbach et al. (2020)
|
IBTrACS
|
Global
|
TROPICAL CYCLONES TRACK
|
Merging datasets from different agencies
|
-
|
DAILY
|
1842- Present
|
Knapp et al. (2010, 2018)
|
REGNIE
|
Germany
|
PRECIP
|
Station based
|
1 km
|
DAILY
|
1961–2014
|
Rauthe et al.(2013)
|
RADKLIM
|
Germany
|
PRECIP
|
Radar based (rain gauges calibration)
|
1 km
|
HOURLY
|
2001–2009
|
Kreklow et al. (2020)
|
SPAIN02
|
Spain
|
PRECIP
|
Station based
|
0.11 degrees
|
DAILY
|
1971–2010
|
Herrera et al. (2010)
|
CARPATCLIM
|
Carpatians
|
PRECIP
|
Station based
|
0.1 degrees
|
DAILY
|
1961–2010
|
Szalai et al. (2013)
|
ENG_REGR
|
Great Britain
|
PRECIP
|
Station based
|
5 km
|
DAILY
|
1990–2010
|
http://www.precisrcm.com/Erasmo/ncic.uk.11.tgz
|
COMEPHORE
|
France
|
PRECIP
|
Reanalysis based on radar and rain gauges
|
1 km
|
HOURLY
|
1997–2017
|
Tabary et al. (2012)
|
GRIPHO
|
Italy
|
PRECIP
|
Station based gridded dataset
|
3 km
|
HOURLY
|
2001–2016
|
Fantini (2019)
|
EURO4M
|
Alps
|
PRECIP
|
Station based gridded dataset
|
5 km
|
DAILY
|
1971–2008
|
Isotta et al. (2014a)
|
PTHBV
|
Sweden
|
PRECIP
|
Station based
gridded dataset
|
4 km
|
DAILY
|
1961–2011
|
https://opendata-download-metanalys.smhi.se
Johansson (2000)
|
METNO
|
Norway
|
PRECIP
|
Station based gridded dataset
|
1 km
|
DAILY
|
1980–2008
|
Mohr et al. (2009)
|
RdisaggH
|
Switzerland
|
PRECIP
|
Combination of rain-gauge data and radar measurements
|
1 km
|
HOURLY
|
2003–2010
|
Wüest et al. (2010)
|
CEH-GEAR
|
Great Britain
|
PRECIP
|
Rain-gauge based gridded dataset
|
1 km
|
HOURLY
|
1990–2016
|
Lewis et al. (2022)
|
All simulations use ERA5 reanalysis fields (Hersbach et al., 2020) as initial and lateral boundary conditions. Specific model configurations for each domain, including spatial resolution and the simulation period, are provided in Table 3.
Table 3
Model configuration for each domain.
DOMAIN
|
Period
|
Horizontal Resolution
|
Vertical Resolution
|
Boundary Layer Scheme
(ib ltyp)
|
Cumulus convection scheme (icup_lnd/ocn)
|
Moisture
scheme (ipptis)
|
Cloud fraction algorithm (icldfrac)
|
Dynamical Land
Use
|
Australasia
|
2000–2009
|
25 km
|
30 levels
|
Holtslag PBL
|
Tiedtke/
Tiedtke
|
Explicit
moisture
Nogherotto/
Tompkins
|
SUBEX
|
NO
|
East Asia
|
2000–2009
|
25 km
|
30 levels
|
Holtslag PBL
|
Tiedtke/
Tiedtke
|
Explicit moisture Nogherotto/Tompkins
|
Xu-Randall empirical
|
NO
|
South East Asia
|
2000–2009
|
25 km
|
30 levels
|
Holtslag PBL
|
Tiedtke/
Tiedtke
|
Explicit moisture Nogherotto/Tompkins
|
SUBEX
|
NO
|
South America
|
2000–2009
|
25 km
|
30 levels
|
Holtslag PBL
|
Tiedtke/
Tiedtke
|
Explicit moisture Nogherotto/Tompkins
|
SUBEX
|
NO
|
Central America
|
2000–2009
|
25 km
|
30 levels
|
Holtslag PBL
|
Tiedtke/
Tiedtke
|
Explicit moisture Nogherotto/Tompkins
|
SUBEX
|
NO
|
Europe
|
2000–2004
|
3 km
|
30 levels
|
Holtslag PBL
|
None/
None
|
Explicit moisture Nogherotto/Tompkins
|
Xu-Randall empirical
|
NO
|
1980–2010
|
12 km
|
30 levels
|
Holtslag PBL
|
Tiedtke/
Tiedtke
|
Explicit moisture Nogherotto/Tompkins
|
Xu-Randall empirical
|
YES
|
South Asia
|
2000–2009
|
25 km
|
30 levels
|
Holtslag PBL
|
Tiedtke/
Tiedtke
|
Explicit moisture Nogherotto/Tompkins
|
Xu-Randall empirical
|
NO
|
North America
|
2000–2009
|
25 km
|
30 levels
|
Holtslag PBL
|
Tiedtke/
Tiedtke
|
Explicit moisture Nogherotto/Tompkins
|
Xu-Randall empirical
|
NO
|
Africa
|
2000–2009
|
25 km
|
30 levels
|
Holtslag PBL
|
Tiedtke/
Tiedtke
|
Explicit moisture Nogherotto/Tompkins
|
SUBEX
|
NO
|
The model validation was conducted over the set of sub-regions identified in the AR6 WGI IPCC report covered by the RegCM5 domains The regions are described by Iturbide et al. (2020). Various metrics were computed to validate the model, encompassing both mean climate and extreme climate distribution, as shown in Table 4.
Table 4
Metrics used for model validation.
Metric
|
Definition
|
Unit
|
Tmean
|
Daily mean 2-m temperature
|
°C
|
Tmax
|
Daily maximum 2-m temperature
|
°C
|
Tmin
|
Daily minimum 2-m temperature
|
°C
|
pr
|
Daily/hourly total precipitation
|
mm/day, mm/hr
|
pr-frq
|
Total number of wet days/hours (i.e., days/hours with total precipitation greater than 1/0.5 mm )
|
day/year
|
pr-int
|
Average amount of wet-day and wet-hour precipitation
|
mm/day, mm/hr
|
p99
|
The 99th percentile of the precipitation distribution over the time period considered
|
mm/day, mm/hr
|
p99.9
|
The 99.9th percentile of the precipitation distribution over the time period considered
|
mm/day, mm/hr
|
cl
|
Cloud Fraction
|
%
|
clw
|
Cloud Liquid Water
|
mg/kg
|
cli
|
Cloud Ice
|
mg/kg
|
Mean seasonal bias
The mean seasonal bias for 2 meter, mean, maximum and minimum temperature (Tmean, Tmax, and Tmin respectively), mean precipitation (pr), precipitation intensity and frequency (pr-int and pr-frq), as well as the annual 99th percentile (p99), were used for the validation of the model mean climatology (definition of the metrics can be found in Table 4). For temperature, the model results are compared with observations from the Climate Research Unit (CRU) dataset. For mean precipitation, the reference dataset is the Global Precipitation Climatology Centre (GPCC), and for precipitation intensity/frequency and p99 the Climate Prediction Center (CPC) is used as reference. The seasonal means are first calculated over the baseline period (1980 to 2010 for Europe and 2000 to 2009 for all other domains) at the original resolutions and are subsequently interpolated (distance-weighted average for temperature, and nearest neighbour for precipitation and related metrics) to the resolution of the observations. The area-weighted averages of all variables are then computed over the AR6 WGI IPCC regions contained within each domain, and the biases are then derived by taking the difference between the simulated and observed values. The mean bias over all the regions, is obtained in the same way, except that the area-weighted average is calculated over all grids of all domains.
Precipitation distribution
Boxplots were computed for daily precipitation in all regions considered, from RegCM4, RegCM5 and observations. We use the station-based data from CPC except for Europe, for which the observation dataset is E-OBS. Due to the steepness of the distribution, the box plots include the 5th and 95th and 99th percentiles.
Note that over some regions, and particularly the Mediterranean, RegCM4 exhibited a notable overestimation of extreme events due to the occurrence of numerical grid point storms, a problem that is considerably improved in RegCM5. Therefore, in the box plots, events with excessively large amounts in RegCM4 were excluded by adjusting the figure axes limits to align with the distribution from observations and RegCM5.
Hourly precipitation distributions for the period 2000–2004 were calculated for the RegCM5 CP and 12 km simulation over Europe and compared with high-resolution hourly observations over Italy, Switzerland, Germany, France and Great Britain (see Table 2). Furthermore, results were compared with the ERA5 reanalysis estimates. Distributions are calculated by taking all available time steps and grid points within each dataset considered. Some of the observational datasets did not have observations at the start of the RegCM5 simulations (e.g Switzerland observational dataset starts in 2003). Therefore, in order to consider a consistent time period for the observations and model simulations, we used the first five available years for each of the observational datasets (e.g. Switzerland 2003–2007).
Daily precipitation distributions are calculated for 2000–2004 for the Europe RegCM5 model simulations, ERA5 and all available observations in the simulated region. In addition to the observational datasets mentioned above, daily precipitation estimates from Sweden, Norway, Spain and the Carpatians are also available (see Table 2). All the biases were computed interpolating each observational dataset on the model grid.
Precipitation sub daily analysis
Seasonal diurnal cycles of precipitation were computed for Europe, analysing both the 12 km and the 3 km simulations. The comparison was carried out against ERA5 data as well as different sub-regional hourly observation datasets: GRIPHO (Italy), RdisaggH (Switzerland), RADKLIM (Germany), COMEPHORE (France) and CEH-GEAR (Great Britain). Each high-resolution dataset was interpolated on the coarser model grid and the daily cycle was computed spatially averaging only in the region covered by observations.
Precipitation intensity and frequency for the hourly observation and RegCM5 datasets were calculated using hourly minimum precipitation thresholds of 0.1 mm/hr and 0.5 mm/hr in order to investigate the uncertainties in the data at very low intensities, which can strongly influence the biases. Note that the choice of threshold does not influence the p99.9 estimates as the whole distribution (including dry hours) is used to calculate this variable.
Taylor diagram
Taylor diagrams were used to validate the mean seasonal precipitation and temperature against several reference datasets. For precipitation, the model results are compared with ERA5, CRU, MSWEP, CPC, and GPCC. For temperature, ERA5 and CRU are used, except for additional observation datasets for Europe and East Asia. Specifically, for Europe, precipitation and daily mean temperature are compared against E-OBS, while for several subregions of East Asia, they are compared against APHRO and CN05.1. For each subregion, the gridded seasonal averages of the observed and simulated data are used to calculate the area-weighted centered pattern correlation and the ratio between the simulated and observed standard deviations, which are then used to generate the diagrams.
Cloud distributions
Vertical profiles were computed over each region for the mean seasonal cloud fraction, cloud liquid water and cloud ice in June-July-August (JJA) and December-January-February (DJF) using twelve pressure levels: 1000, 925, 850, 700, 600, 500, 400, 300, 250, 200, 150 and 100 hPa.The calculations were done for both RegCM5 and the ERA5 reanalysis data and covered the period 2000–2009 for all domains, except for Europe, for which 1980–2010 was used.
Upper level circulations
Composites of zonal and meridional wind were computed for 3 different pressure levels, i.e., 850, 500, and 250 hPa. RegCM5 includes a function to perform this task, called sigma2p. This function is first executed to interpolate the wind components from the sigma coordinates to pressure levels. Wind at the selected levels is then extracted, and its seasonal means are calculated over the baseline period. Results of different domains are subsequently interpolated onto the grids of the reference dataset, i.e., ERA5, using distance-weighted average mapping. The composite of global wind is then obtained by directly combining the wind of all domains. In cases where there is an overlap between multiple domains, the average is calculated. For ERA5, wind at the three pressure levels averaged over 2000–2009 is used for all domains except for Europe, where the 1980–2010 average is employed. Wind of the reference dataset is then masked with respect to the RegCM composite to facilitate an intuitive comparison between the two.
Tropical and extratropical cyclones
Tropical and extratropical cyclones were tracked in each domain, but a graphical representation was created by combining all domains into a single map. Three different algorithms for identifying and tracking tropical cyclones (Reboita et al., 2010; Fuentes-Franco et al., 2014, 2017; Hodges, 1994, 1995, 1999) were employed, while one algorithm was used for extratropical systems (Reboita et al., 2010).
a) Reboita et al. (2010)’s algorithm
This algorithm identifies and tracks tropical and extratropical cyclones using cyclonic relative vorticity every 6 hours (0000, 0600, 1200, and 1800 UTC). Before applying the algorithm, the horizontal wind components at 925 hPa (zonal and meridional) are interpolated to a grid with a resolution of 1.5º x 1.5º in latitude and longitude. Once the data are provided to the algorithm, relative vorticity is computed and smoothed to reduce noisy features using the Cresmann (1959) method. The algorithm consists of three main steps: (1) initially, in a specific time step of the dataset, it searches for the minimum relative vorticity by comparing each grid point value with those of 24 surrounding points (nearest-neighbour method). A grid point is a cyclone center candidate when a minimum of relative vorticity is found and is smaller or equal to a threshold (defined as -1 x 10− 5 s− 1); (2) the coordinates of the grid point identified in (1) are located in the following time step and a new search by the minimum of relative vorticity is performed in an area defined by the 24 grid points around this grid point. Next, the nearest-neighbour method is applied in each of the 24 grid points ; and (3) once two positions (minima of relative vorticity) are known, the algorithm calculates the displacement velocity of the cyclone center and uses it as an initial estimate (first guess) to locate the cyclone center in the following time step. This procedure continues until the dissipation (cyclolysis) of the cyclone. When cyclolysis occurs, the algorithm returns to the specific time step of the initial identification and searches for other grid points that could be a cyclone center, and all three steps are repeated.However, after the identification of the cyclone position at a given time step, the relative vorticity is interpolated on a high resolution grid just covering an area around the cyclone center (limited to a radius of 250 km far from the cyclone center) and a new search for the minimum of relative vorticity is performed to obtain a more precise location of the cyclone center. Only cyclones with lifetime equal to or higher than 24 hours and equal to or lower than 10 days are included in the statistics. It is important to highlight that we will present all synoptic cyclonic systems detected by the algorithm and not only extratropical and tropical cyclones. For selection of a specific cyclone type, the tracking output would need to be used as input to the Cyclone Phase Space (CPS; Hart, 2003), which analyses the vertical structure of the systems.
b) Fuentes-Franco et al. (2014, 2017)’ algorithm
This algorithm, named Kyklop (Fuentes-Franco et al., 2017), is configured to work with three variables (near-surface wind speed at 10 m, mean sea level pressure (MSLP), and sea surface temperature (SST) and with the time frequency and horizontal resolution (see https://github.com/kyklop-climate/kyklop/blob/master/kyklop/kyklop.py) of the NetCDF file provided as input. In this study, 3-hourly data (0000, 0300, 0600, 0900, 1200, 1500, 1800, and 2100 UTC) are used. The Kyklop algorithm has two main procedures: (1) it searches for grid points that are candidates to be a tropical cyclone in all time steps and (2), subsequently performs the matching of grid points to determine the cyclone’s trajectory. This logical sequence differs from Reboita et al. (2010) in that, in their approach, once a grid point is identified as a cyclone center candidate, it is tracked until cyclolysis. In (1), for each time step, Kyklop searches grid points that satisfy the following criteria: wind speed > 20 m s− 1, MSLP < 1005 hPa, and SST > 25°C. As these conditions may be satisfied by some neighbouring grid points, the centroid of the area encompassed by these grid points is considered as the center of the tropical cyclone. In (2), for each detected cyclone grid point in a specific time step, its tracking (following positions) is carried out by checking on next time steps if there are grid points that meet the conditions presented in (1) within a radius of 6º × 6º longitude–latitude. These conditions need to persist for at least 24 hours.
c) Hodges (1994, 1995, 1999)’ algorithm
Hodges (1994, 1995, 1999) named his algorithm TRACK, which searches for various types of cyclones based on relative vorticity. However, this algorithm can also be configured for identifying only tropical disturbances. In this case, the TRACK uses the zonal and meridional wind components at different vertical levels (10 m, 850, 700, 600, 500 400, 300 and 200 hPa), and at 6-hour intervals (0000, 0006, 1200 and 1800 UTC). The identification of tropical disturbances involves three main steps: (1) pre-processing filtering, (2) tracking performed following Hodges’s references, and the (3) post-tracking filtering - an additional procedure integrated within TRACK (Hodge et al., 2017). The data used in this study were first interpolated to a regular grid of 0.25º x 0.25º before being processed by TRACK. In step (1), the algorithm calculates the vertically averaged relative vorticity between 850 − 600 hPa. Subsequently, a spectral filter (triangular truncation) is applied, retaining wavenumbers between 6 and 63, in order to remove the noise associated with the smallest spatial scales and the large-scale background. In step (2), the nearest-neighbor method is applied to the processed data from step (1) to identify all tropical disturbances (tropical cyclones will be separated from all systems in step 3). Unlike Reboita et al. (2010), TRACK standardizes the relative vorticity field to positive values in both hemispheres, so it identifies the cyclonic disturbances by maxima of relative vorticity, and, in addition, it applies a threshold: candidates for tropical disturbance need to have relative vorticity > 5 x 10− 6 s− 1 (in the Southern Hemisphere the field is scaled by -1). The tropical disturbance location is then refined using a B-spline interpolation. Additionally, the algorithm refines the tracks by minimizing a cost function for track smoothness. The final step (3) is post-tracking filtering, selecting only the tropical cyclones from all tracked tropical disturbances. Tropical cyclones are identified based on three parameters describing their structure: presence of coherent vertical symmetry (presence of a maximum of relative vorticity at each vertical level), warm core, and high near-surface wind speeds. These three parameters must be satisfied for at least 2 days, with a minimum of 24 hours over the ocean. To identify the symmetry, the scheme searches the maximum relative vorticity at the vertical levels (850, 700, 600, 500 400, 300 and 200 hPa). The algorithm uses the location of tropical disturbance computed at the 850 − 600 hPa level as the starting point. and then a circle with a radius of 5º (geodesic) is delimited. The maximum relative vorticity is then searched inside this area, and the location of this maximum is used as reference for the level above and this procedure is repeated until the uppermost level. The warm core is calculated as the difference between the relative vorticity fields at 850 and 200 hPa (at T63 resolution) and must be greater than 6 x 10− 5 s− 1 (indicating stronger winds near the surface than at upper levels). Additionally, the 10-m wind speed must be greater than 17.5 m s− 1 and is searched within a 6° radius from the cyclone center identified using the vorticity average between 850 − 600 hPa.
All algorithms provide as output the latitude and longitude at each time step of the cyclone’s lifecycle and other features such as MSLP, relative vorticity etc., depending on the algorithm .With the tracking information, it is possible to compute the track density, which is the number of cyclones passing by an area of 1º x 1º divided by the area of this box. We compared the RegCM5 performance in reproducing the cyclonic systems against the ERA5 reanalysis when working with the Reboita et al. (2010) algorithm and against the International Best Track Archive for Climate Stewardship (IBTrACS, version v04; Knapp et al. 2010, 2018) for the other algorithms. IBTrACS collects observed tropical cyclone data from 11 agencies around the world covering all major ocean basins and provides 6-hour data of tropical cyclones locations.