Simulating expected elevation dependent warming (EDW) mechanisms in a dynamically-downscaled perturbed physics climate model ensemble over the Himalayan region

Abstract:

Current theoretical climatology suggests three key climate processes – snow cover contribution to surface albedo, cloud cover prevalence and near surface water vapour – influencing the surface energy balance are expected to exhibit elevation-gradients in global warming-driven changes. These gradients are in turn expected to act as mechanisms contributing to EDW. This study examines the simulation of these mechanisms and their influence on projections of EDW in a dynamically downscaled transient perturbed physics ensemble (PPE). The downscaling experiment in question is the Hadley Centre Regional Model version 3 PRECIS configuration (HadRM3P) 25km simulation over the South Asian domain driven by the MetOffice 17-member QUMP (Quantifying Uncertainty in Model Projections) ensemble of the Hadley Centre Climate Model version 3 (HadCM3). Use of the multi-member PPE enables quantification of uncertainty in projected changes in climate variables – albedo, cloud cover, water vapour and near surface temperature – while the spatial resolution of a RCM improves insight into the role of elevation in projected rates of change.

This work specifically addresses the Regional Climate Model (RCM) representation of expected EDW mechanisms by calculating vertical profiles (relative to modelled surface elevation of downscaled grid cells) for changes in:

[1] albedo, i.e. the ratio of future to control period albedo where albedo is calculated as one minus the ratio of absorbed surface solar radiation to incoming surface solar radiation;

[2] shortwave cloud radiative effect (CRE), i.e. the ratio of future to present CRE where CRE is calculated as incoming “top of atmosphere” shortwave radiation minus incoming surface shortwave radiation;

[3] near surface water vapour -- in terms of specific humidity (Qair) – and related down-welling longwave radiation, but because previous EDW research has shown non-linearity in Q_air radiative influence, changes in Qair is evaluated in both delta (additive) and relative (multiplicative) terms.

The RCM representation of expected EDW mechanisms will be placed in the context of model validation and bias assessment using a combination of global meteorological reanalyses and available local observations.