Fig 1.
Geographical locations of model domains and weather stations.
a) The HDRPS (2.5 km) domain over North America used to drive our model simulations, b) The high-resolution domains at 1km (blue rectangle) and 250m (green rectangle) and c) details of land use on the 250 m grid. The grayscale shows the building fraction, with main roads added in white. The green-red scale shows the main type of vegetation at the grid point. Weather stations are shown (in black: Hourly observations; in grey: Daily observations) with their corresponding national identification (refer to table in the supporting materials for details of stations). Panel b base map is taken from OpenStreetMap, available under the Open Database License (https://www.openstreetmap.org/copyright).
Fig 2.
Observed temperature and precipitation for the 2018 event.
Observed hourly a) surface air temperature and b) precipitation accumulation at different stations. Precipitation data is missing for Mirabel-Intl and Ste-Anne de Bellevue stations during that period.
Fig 3.
Observed temperature and precipitation for the 2019 event.
Same as Fig 2 for July 2019 case study.
Fig 4.
Timeseries of observed and simulated surface variables.
Observed (black) and simulated (blue) surface air temperature (TT, a and b) and dew point temperature (TD, c and d) at station McTavish (WTA, a and c) and St-Hubert (YHU, b and d) for the 2018 event. The blue shading shows the ensemble spread.
Fig 5.
Observed and simulated rainfall for the 2018 event.
a-d) Timeseries of 1h-precipitation accumulation in four different stations along the precipitation system: a) Ste Anne de Bellevue (up-wind of the city), b) McTavish (downtown), c) St-Hubert (suburb next to the downtown) and d) Assomption (down-wind). X-axis is the hour on July 17th 2018 (in local time). e-f) 24-h precipitation accumulation from 2018-07-16 2000 LST to 2018-08-17 2000 LST from e) CaPA analysis and f) CTL run (ensemble average), with colored circles representing observed accumulation values at available surface stations. The black arrow shows the global trajectory of the system. Made with Natural Earth (naturalearthdata.com), under the public domain license (https://creativecommons.org/publicdomain/).
Fig 6.
Observed and simulated rainfall for the 2019 event.
a-d) Timeseries of 1h-precipitation accumulation in four different stations along the precipitation system: a) Assomption (up-wind of the city), b) McTavish (downtown), c) Pierre-Elliott-Trudeau (next to the downtown) and d) St-Hubert (down-wind). X-axis is the hour on July 11th 2019 (in local time). e-f) 24-h precipitation accumulation from 2019-07-11 0800 LST to 2019-07-12 0800 LST from e) CaPA analysis and f) CTL run (ensemble average), with colored circles representing observed accumulation values at available surface stations. The black arrow shows the global trajectory of the system. Made with Natural Earth (naturalearthdata.com), under the public domain license (https://creativecommons.org/publicdomain/).
Fig 7.
Changes in averaged 2-m air temperature, 2-m dew point and heat index for the 2018 event.
Spatial timeseries of the difference in 2-m air temperature (a, d, g), 2-m dew point (b, e, h) and heat index (c, f, i) between NOURB-CTL (a, b, c), ALB-CTL (d, e, f) and VEG-CTL (g, h, i) model runs for the 2018 event. The black line is the spatial average on the island of Montreal of the difference between the sensitivity and the CTL experiments. The gray area is the 5th to 95th percentile and represents the spatial variability on the island of Montreal. Vertical lines show 00:00 local time. Made with Natural Earth (naturalearthdata.com), under the public domain license (https://creativecommons.org/publicdomain/).
Fig 8.
Changes in surface air temperature, dew point, relative humidity and heat index for the 2018 event.
Anomalies in air temperature (a, b, c), dew-point temperature (d, e, f), relative humidity (g, h, i) and heat index (j, k, l) between NOURB and CTL experiments. The three columns correspond to different times: 2018-07-16 12:00 LST (left), 2018-07-16 18:00 LST (center) and 2018-07-17 00:00 LST (right). Surface winds for the CTL experiment are shown (in knots). Made with Natural Earth (naturalearthdata.com), under the public domain license (https://creativecommons.org/publicdomain/).
Fig 9.
Difference in accumulated precipitation between NOURB and CTL experiments for the 2018 event (a) and the 2019 event (b). Brown signifies more precipitation when the city is present. Made with Natural Earth (naturalearthdata.com), under the public domain license (https://creativecommons.org/publicdomain/).
Fig 10.
Skew-T diagram on 2018-07-18 00:00 LST (a, b), 2019-07-11 18:00 LST (c, d) and 2019-07-11 19:00 LST (e, f) for the CTL experiment (left, a, c, e) and NOURB experiment (right, b, d, f) at the closest grid point to McTavish station. The red line is the temperature, the green line is the dew-point temperature and the black line is the air parcel lifted. The blue and red shaded areas represents the layers where convective inhibition (CIN) and convective available potential energy (CAPE) is present, respectively.
Fig 11.
Model reflectivity for 2019 event.
Maximum reflectivity at different times for the 2019 event for CTL (left) and NOURB (right) experiment. Times are, from top to bottom row, 1800, 1900, 2000, 2100 and 2200 LST on 2019-07-12. Made with Natural Earth (naturalearthdata.com), under the public domain license (https://creativecommons.org/publicdomain/).