By: Charlotte Lang
There’s a debate on the planet of polar local weather and ice sheet floor modelling about world (GCM) versus regional (RCM) fashions and either side is making an attempt to persuade the opposite that they do higher: world modellers insist that regional fashions don’t add a lot to the subject whereas regional modellers attempt to persuade them that their fashions can complement and even enhance the outputs of world fashions.
Though initially I used to be a regional modeller as a consumer of MAR (Modèle Atmosphérique Régional, Fettweis et al., 2017) in Liège, Belgium, I’ve since joined the darkish aspect as a world modeller in December 2020 after I began a postdoc at NCAS.
Do these 2 sorts of fashions actually need to be opposed? Can’t they be utilized in a complementary approach?
Let’s overview just a few of some great benefits of every of them.
Decision: benefit RCMs
For a very long time, the largest benefit of regional local weather fashions was their skill to run at the next spatial decision (~10 km) than extremely time consuming GCMs (~100 km), permitting them to higher characterize smaller scale processes and to be calibrated for particular climates (Fig. 1). For Greenland for instance, it means a greater illustration of the slender ablation zone, the marginal space of the ice sheet the place the quantity of soften occurring on the floor in summer time exceeds the mass gained in winter via snowfall. In Svalbard, it means a greater illustration of the hilly topography that gave its identify to the principle island Spitsbergen (“pointed mountains” in Dutch) and a greater illustration of orographic precipitation.
Determine 1: Illustration of GCMs and RCMs. Supply: Ambrizzi et al. (2018); Determine 1.
Then again, working on a restricted space means RCMs need to be informed what occurs at their boundaries, which need to be “pressured” at common time intervals by the outputs of a world mannequin. That’s the largest criticism one can have towards RCMs: they’ll solely be pretty much as good because the mannequin used to power their boundaries. Feed them with a “good” GCM they usually would possibly even enhance their outcomes (and due to this fact a greater illustration of orographic precipitation); feed them with a biased GCM and their outputs will show bigger biases as nicely.
Ice sheet floor processes: benefit RCM
When the progress in computing allowed GCMs to extend their spatial decision, us polar local weather and ice sheet floor modellers might nonetheless argue that we might higher simulate the interactions between the local weather and the ice sheets floor with RCMs. Certainly, some RCMs resembling Liège’s MAR or IMAU’s RACMO (Noël et al., 2018) embody snow modules permitting specific simulation of the vitality and mass switch between the environment and the floor of the ice sheets. World fashions for his or her half didn’t embody such advanced fashions and customers needed to resort to forcing less complicated and sometimes empirical snow fashions with their local weather, lacking a number of the suggestions between the local weather and the snow floor.
The rise of Earth System Fashions: benefit GCMs
Extra lately, regional local weather fashions suffered a blow with the event of a brand new class of world fashions, Earth System Fashions (ESM). ESMs, just like the British UKESM (Sellar et al., 2019), are advanced fashions coupling many elements of the Earth System: environment, ocean, vegetation, biogeochemistry… UKESM additionally features a advanced mannequin for the floor of ice sheets coupled to a thermo-mechanical ice sheet mannequin (Smith et al., 2021) simulating the dynamics of the ice sheets, permitting it to have in mind the impact of adjusting ice sheets on the local weather and vice-versa. This characteristic is of specific curiosity in long run future projections (>2100) because the floor of the Greenland ice sheet is anticipated to decrease because it melts, additional rising the near-surface temperature and due to this fact floor soften via the melt-elevation suggestions that RCMs and their mounted geometry can’t characterize (Fig. 2). A altering ice sheet geometry might additionally modify the atmospheric circulation on and across the ice sheet, together with a weakening (resp. strengthening) of the katabatic winds, which might additional improve (resp. dampen) the optimistic suggestions loop.
Determine 2: Soften-elevation suggestions in a hard and fast vs dynamical ice sheet geometry.
Efforts have been made to couple RCMs to ice sheet fashions, just like the PARAMOUR-EOS challenge (https://www.elic.ucl.ac.be/users/klein/PARAMOUR/index.html) aiming at coupling a number of RCMs to ice sheet and ocean fashions over the Greenland and Antarctic ice sheets however the usage of coupled fashions continues to be fairly marginal in polar climatology.
Moreover, the system of elevation courses in UKESM permits it to downscale the local weather and floor variables wanted to power the ice sheet mannequin from the decrease decision atmospheric grid onto a a lot increased decision ice sheet grid. And right here one of many final benefits of working with an RCM disappears.
But regional local weather fashions are nonetheless helpful in lots of features. Pressured by reanalyses, they’ll quickly produce excessive decision simulations of regional climate occasions like the acute soften of the floor of the Greenland Ice sheet in 2012 or the lethal floods that affected Belgium and Germany this summer time. Pressured by an ESM local weather, they’ll quickly run with a deal with particular floor ice sheet processes whose numerical illustration want enhancing or are but to be included within the fashions. RCMs will not be lifeless (but)!
References:
[1] Ambrizzi, T., M. Reboita, R. Rocha, and M. Llopart, 2018: The state-of-the-art and basic features of regional local weather modeling in South America. Annals of the New York Academy of Sciences, 1436, https://doi.org/10.1111/nyas.13932.
[2] Fettweis, X., and Coauthors, 2017: Reconstructions of the 1900–2015 Greenland ice sheet floor mass stability utilizing the regional local weather MAR mannequin. The Cryosphere, 11, 1015–1033, https://doi.org/10.5194/tc-11-1015-2017.
[3] Noël, B., and Coauthors, 2018: Modelling the local weather and floor mass stability of polar ice sheets utilizing RACMO2 – Half 1: Greenland (1958 – 2016). The Cryosphere, 12, 811–831, https://doi.org/10.5194/tc-12-811-2018.
[4] Sellar, A. A., and Coauthors, 2018: UKESM1: Description and analysis of the U.Ok. Earth System Mannequin. Journal of Advances in Modeling Earth Programs, 11, 4513–4558, https://doi.org/10.1029/2019MS001739.
[5] Smith, R. S., and Coauthors, 2021: Coupling the U.Ok. Earth System Mannequin to dynamic fashions of the Greenland and Antarctic ice sheets. Journal of Advances in Modeling Earth Programs, 13, e2021MS002520, https://doi.org/10.1029/2021MS002520.
