the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Apr 2024
Convection-permitting climate model representation of severe convective wind gusts and future changes in southeastern Australia
Abstract. Previous research has suggested that the frequency and intensity of surface hazards associated with thunderstorms and convection, such as severe convective winds (SCWs), could potentially change in a future climate due to global warming. However, because of the small spatial scales associated with SCWs, they are unresolved in global climate models, and future climate projections are uncertain. Here, we evaluate the representation of SCW events in a convection-permitting climate model (Bureau of Meteorology Regional Projections for Australia, BARPAC-M), run over southeastern Australia for December–February months. We also assess changes in SCW event frequency in a projected future climate for the year 2050, and compare this with an approach based on identifying large-scale environments favourable for SCWs from a regional parent model (BARPA-R). This is done for three different types of SCW events that have been identified in this region, based on clustering of the large-scale environment. Results show that BARPAC-M representation of the extreme daily maximum wind gust distribution is improved relative to the gust distribution simulated by the regional parent model. This is due to the high spatial resolution of BARPAC-M output, as well as partly resolving strong and short-lived gusts associated with convection. However, BARPAC-M significant overestimates the frequency of simulated SCW events, particularly in environments having steep low-level temperature lapse rates. A future decrease in SCW frequency under steep lapse rate conditions is projected by BARPAC-M, along with less frequently favourable large-scale environments. In contrast, an increase in SCW frequency is projected under high surface moisture conditions, with more frequently favourable large-scale environments. Therefore, overall changes in SCWs for this region remain uncertain, due to different responses between event types, combined with historical model biases.
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RC1: 'Comment on egusphere-2024-322', Andreas F. Prein, 27 Apr 2024
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The manuscript “Convection-permitting climate model representation of severe
convective wind gusts and future changes in southeastern Australia” by Brown et al. investigates how future convective wind gusts might change over Southeastern Australia. Previous research suggests that surface hazards from thunderstorms, like severe convective winds (SCWs), may alter with climate change, yet global climate models struggle to resolve these due to their small scale, leading to uncertain projections. The authors find that SCW events using a convection-permitting climate model (BARPAC-M) over southeastern Australia for December–February, and comparing with a regional parent model (BARPAR) improved representation of extreme wind gusts in BARPAC-M but overestimation of SCW frequency, particularly in certain environments. Projected changes in SCW frequency for 2050 show uncertainties, with potential decreases under certain conditions and increases under others, highlighting the complexity of future SCW trends in the region. The study is very well structured, and written, and the images and text are of high quality. The differentiation of wind gusts into different categories adds a lot of novel insights about model biases and future climate change impacts on these extremes. This is one of the best-written and interesting papers that I have read in a while. I have only a couple of minor suggestions for changes and recommend publishing this manuscript after those are addressed.
General comment
Adding more discussion on the importance of climate internal variability on your results would be beneficial. You mention the high degree of spatial and temporal variability of convective gusts already but making the role of internal climate variability more explicit would be important (see e.g., Deser et al. 2012). Internal climate variability could easily be the dominant source of uncertainty in your future climate projections.
Deser, C., Phillips, A., Bourdette, V. and Teng, H., 2012. Uncertainty in climate change projections: the role of internal variability. Climate dynamics, 38, pp.527-546.Specific Coments:
- Is BARPAC-M online or offline nested into BARPAR? The online nesting would have the benefit of providing higher-temporal resolution at the lateral boundaries which generally reduces that spatial spinup in the high-resolution domain.
- How did you account for boundary effects in BARPAC-M? Do you use a sponge zone and did you exclude boundary grid cells from the analysis?
- L100: It is optimistic to assume that the resolved gust in BARPAR is 10-min if you have a 5-min time step. I would assume that your resolved temporal scales are at least 4Dt based on numerical considerations and model diffusivity.
- In the analysis of Fig. 2 you directly compare the grid cell wind gust from the models with observed gusts at point locations. Should the model be able to capture point-scale wind gusts? I would assume that the model wind speed should be lower than that observed at point locations since the model is representing a spatial (e.g., grid cell) average wind gust, which in case of ERA5 and BARPAR is a quite large area.
- 3: Maybe using a log y-axis would make this figure easier to read.
- L223: Why are you using the 6 km speed here? Are you assuming that this is the source height of downdrafts?
- 6: Please add a legend that describes the circle sizes.
Citation: https://doi.org/10.5194/egusphere-2024-322-RC1 -
RC2: 'Comment on egusphere-2024-322', Andreas F. Prein, 27 Apr 2024
reply
The manuscript “Convection-permitting climate model representation of severe convective wind gusts and future changes in southeastern Australia” by Brown et al. investigates how future convective wind gusts might change over Southeastern Australia. Previous research suggests that surface hazards from thunderstorms, like severe convective winds (SCWs), may alter with climate change, yet global climate models struggle to resolve these due to their small scale, leading to uncertain projections. The authors find that SCW events using a convection-permitting climate model (BARPAC-M) over southeastern Australia for December–February, and comparing with a regional parent model (BARPAR) improved representation of extreme wind gusts in BARPAC-M but overestimation of SCW frequency, particularly in certain environments. Projected changes in SCW frequency for 2050 show uncertainties, with potential decreases under certain conditions and increases under others, highlighting the complexity of future SCW trends in the region. The study is very well structured, written, and the images and text are of high quality. The differentiation of wind gusts into different categories adds a lot of novel insights about model biases and future climate change impacts on these extremes. This is one of the best written and interesting papers that I have read in a while. I have only a couple of minor suggestions for changes and recommend publishing this manuscript after those are addressed.
General comment:
- Adding more discussion on the importance of climate internal variability on your results would be beneficial. You mention the high degree of spatial and temporal variability of convective gusts already but making the role of internal climate variability more explicit would be important (see e.g., Deser et al. 2012). Internal climate variability could easily be the dominant source of uncertainty in your future climate projections.
Deser, C., Phillips, A., Bourdette, V. and Teng, H., 2012. Uncertainty in climate change projections: the role of internal variability. Climate dynamics, 38, pp.527-546.
Specific comments:
- Is BARPAC-M online or offline nested into BARPAR? The online nesting would have the benefit of providing higher-temporal resolution at the lateral boundaries which generally reduces that spatial spinup in the high-resolution domain.
- How did you account for boundary effects in BARPAC-M? Do you use a sponge zone and did you exclude boundary grid cells from the analysis?
- L100: It is optimistic to assume that the resolved gust in BARPAR is 10-min if you have a 5-min time step. I would assume that your resolved temporal scales are at least 4Dt based on numerical considerations and model diffusivity.
- In the analysis of Fig. 2 you directly compare the grid cell wind gust from the models with observed gusts at point locations. Should the model be able to capture point scale wind gusts? I would assume that the model wind speed should be lower than that observed at point locations since the model is representing a spatial (e.g., grid cell) average wind gust, which in case of ERA5 and BARPAR is a quite large area.
- 3: Maybe using a log y-axis would make this figure easier to read.
- L223: Why are you using the 6 km speed here? Are you assuming that this is the source height of downdrafts?
- 6: Please add a legend that describes the circle sizes.
Citation: https://doi.org/10.5194/egusphere-2024-322-RC2 - Adding more discussion on the importance of climate internal variability on your results would be beneficial. You mention the high degree of spatial and temporal variability of convective gusts already but making the role of internal climate variability more explicit would be important (see e.g., Deser et al. 2012). Internal climate variability could easily be the dominant source of uncertainty in your future climate projections.
Data sets
Simulated severe convective wind events and environments from the Bureau of Meteorology Atmospheric Regional Projections for Australia (BARPA) Andrew Brown, Andrew Dowdy, Todd P. Lane, Chun-Hsu Su, Christian Stassen, and Harvey Ye https://zenodo.org/records/10521068
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