Marginal snowpacks are a critical part of the hydrology and ecology of temperate mountain regions. However, their sensitivity to temperature increase is uncertain due to their phenology which sits between seasonal and ephemeral snowpacks. In this paper, we investigate the sensitivity and the potential response of marginal snowpacks to a warmer climate by leveraging simulated snow series under various combinations of mean winter temperature (T) and precipitation (P), and varied meteorological conditions over 21 years, at 24 mountain sites globally. Marginal snowpacks are found to be more sensitive to temperature increases compared to both seasonal, and ephemeral snowpacks. According to all considered T and P combinations, 64% of marginal snowpacks would transition to ephemeral or disappear, compared to only 21% and 24% for seasonal and ephemeral snowpacks, respectively. The study reveals new insight on the impacts of a warming climate on marginal snowpacks, including: the advance of melt-out dates (-21 days) and a reduction in snowmelt contribution to runoff (-11%). Further, we estimate the global distribution of marginal snowpack based on T and P from ERA5-Land reanalysis. The total area covered by marginal snowpack would decrease by 2.7% under +1º C, with a more significant decreases under +2 ºC (5.7%) and +3 ºC (10.5%) warming. This study places marginal snowpacks at the forefront of where climate change will pose the greatest threats to mountain and montane water resources and environmental systems globally, with profound shifts even under optimistic future climate scenarios (+1ºC).

A document by Amadou COULIBALY. Click on the document to view its contents.

A document by Shourya Mukherjee. Click on the document to view its contents.

A new cirrus cloud parameterization for the ICOLMDZ atmospheric general circulation model (AGCM) is presented. It is built using prognostic equations for cloud fraction and cloud water vapor, and the processes that affect these quantities are parameterized. In particular, the tendency in ice crystal mass concentration due to changes in water phases is calculated, as is the macroscale mixing of cirrus clouds with their environment. The parameterization simulates ice supersaturation in clear sky, an ubiquitous metastable phenomenon in the upper troposphere that is necessary for the formation of in situ cirrus clouds. Our parameterization also allows for cirrus clouds to be sub- and supersaturated with respect to ice. It is evaluated on a case study of a warm conveyor belt cirrus cloud forming and dissipating above the Paris region. The representation of high cloud cover is improved with the new parameterization relative to the standard version of the cloud scheme in comparison to observations, as is the representation of water vapor. In particular, the upper tropospheric dry bias from ICOLMDZ is corrected, although it is replaced by a wet bias in some cases. As for the standard parameterization, ice fallspeed velocity is a major parameter for tuning the ice water content, but the new parameterization allows ice water content and cloud cover to be tuned separately.

A document by William Keely. Click on the document to view its contents.