# Towards a Digital Twin Prototype of Alpine Glaciers: Proposal for a Possible Theoretical Framework
## 2 Glaciers Monitoring
### Overviewing Glacier Modelling Approaches  
In the last decades, many authors have been using data from glacier inventories and have combined them with models and parameters to predict future ice mass evolution. Extensive endeavours have been dedicated to developing models to describe ice flow, encompassing its intricate physical phenomena and their relationships with climate. Changes in glaciers play a crucial role in water availability concerns and their reduction can even heighten the occurrence of geo-hazards [61]. Due to data availability, nowadays, physical modelling of ice sheets and glaciers at high spatial resolution is possible. Different models have been developed to understand and study the evolution of glaciers, mainly relying on time series of in situ measurements. Models can be, however, classified depending on: (i) the spatial scale of analysis (single glacier, glacier system, etc. ); (ii) on the type of modelled parameters; and (iii) the degree of complexity versus accuracy. All these criteria are somehow inter-related. In fact, simple models are, in general, fed with few data and require limited computational efforts. These features make them suitable for performing a first estimate about the behaviour of a high number of glaciers over wide regions. These are in general referred to as 4D Multi Glaciers (see Section 3.2). When aiming, on the other hand, at generating future scenarios based on expected or simulated climatic conditions, possibly looking for an estimated glacier \"expiration date\", more advanced models have to be considered (hereinafter called Glaciers Evolution Models--GEM, see Section 3.2). Both approaches have to be included in a DT as different output environments that different users can ask for, depending on the application they are facing. A third type of output one can be interested in is a mere 3D representation, useful for didactic, cultural and dissemination purposes, easy to be accessed and managed by citizens. Presently, the reference modelling experience of the GEM type is the Open Global Glacier Model (OGGM). Its starting point is the Randolph Glacier Inventory, and the main goal is the simulation of past and future evolution of all of the 216,502 inventoried glaciers worldwide (as of RGI V6). OGGM works similarly to the most of global and regional models. Its simulations rely on auxiliary data such as glacier boundaries, topographic descriptors and climatic observations. Expected outputs can be summarized as follows: (i) glacier shape, defined through the adoption of topographic descriptors (e.g., hyposometry); (ii) estimates of the glacier total ice volume; (iii) maps describing bedrock topography; (iv) climatic-related mass balance and frontal ablation; and (v) glacier dynamic evolution under various climate conditions completed with estimates of associated uncertainties [62; 63]. Some operational outputs can, in general, be automatically or semi-automatically obtained: (i) flow lines; (ii) glaciers catchment areas; (iii) mass balance; and (iv) ice thickness and its variation over time. An eventual increase in the complexity of the model determines a significant amount of the additional computational cost of simulations, that, to be somehow limited, involves approximations mainly related to a reduced geometrical and temporal resolution of inputs. The great differences that glaciers around the world present make the adoption of a single global model too challenging and, possibly, inadequate. The approach that is presently proposed is one relying on regional models. A region is defined as an area presenting common climatic features and all glaciers that belong to the same climatic area can be integrated in a single model. What \"common climatic conditions\" means depends on the scale of the modelling. That means that a \"global\" similarity can, eventually, be split in many \"local\" similarities if one focuses over a specific (but unitary) territorial context.