Presentation on theme: "Ten Fifteen Years of Development on UMISM: Application to Advance and Retreat of the Siple Coast Region James L Fastook Jesse V Johnson Sean Birkel We."— Presentation transcript:
Ten Fifteen Years of Development on UMISM: Application to Advance and Retreat of the Siple Coast Region James L Fastook Jesse V Johnson Sean Birkel We thank the NSF, which has supported the development of this model over many years through several different grants.
INTRODUCTION: UMISM ● The University of Maine Ice Sheet Model has expanded from the original mass and momentum ICE DYNAMICS solver which forms its core, to include components for calculating: – THERMODYNAMICS for internal ice temperatures. – ISOSTASY for the response of the bed to changing ice load. – BASAL WATER for the distribution and movement of water produced by basal melting, used to predict regions where sliding occurs.
INTRODUCTION: (continued) – CLIMATE for the response of the net accumulation rates to changing climate and ice sheet configuration. – ICE SHELF/CALVING for the grounding line response to the changing ice sheet. – EMBEDDED model, for better resolution with reasonable run times. ● All of these are important improvements that increase the accuracy of the physics, as well as the realism and utility of the model.
THERMODYNAMICS ● Energy Conservation: – 1-D temperature profiles. – Vertical diffusion and advection. – NO horizontal diffusion (OK). – NO horizontal advection (maybe NOT OK). ● Surface Boundary conditions: Temperature (from CLIMATE). ● Basal Boundary conditions: Geothermal heat flux OR Presence of water (from BASAL WATER). ● Other heat sources: Internal shear and/or Basal sliding (both from ICE DYNAMICS).
THERMODYNAMICS (continued) ● INPUT: – Surface temperature and mass balance (from CLIMATE). – Geothermal heat flux. – Internal heat sources (from ICE DYNAMICS). – Presence of water (from BASAL WATER). ● OUTPUT: – Internal temperatures (for ICE DYNAMICS). – Basal temperature and melt/freeze rates (for BASAL WATER).
ISOSTASY ● Several models: – Pseudo-elastic, hydrostatically-supported plate (SLOW). – Viscous point loading (FAST). – Visco-elastic plate (COMING SOON). ● INPUT: – Ice thickness (from ICE DYNAMICS). ● OUTPUT: – Bed elevation (for ICE DYNAMICS and CLIMATE).
BASAL WATER ● Conservation of water: – Basal melt/freeze rates as source. – Movement down hydrostatic gradient. – Diffusive, Advective, Loss to aquifer terms. ● INPUT: – Ice thickness (from ICE DYNAMICS). – Bed (from ISOSTASY). – Melt/freeze rates (from THERMODYNAMICS). ● OUTPUT: – Basal water amount (for ICE DYNAMICS to define sliding area and magnitude).
CLIMATE (several) ● Simple lapse rate: – Surface temperature from latitude and elevation lapse rates. – Accumulation proportional to temperature. ● (warm: LOTS, cold: LITTLE) – Positive Degree Days from latitude-dependent imposed seasonal amplitude. – Ablation proportional to PDD. – Net Mass Balance = Accumulation – Ablation.
CLIMATE (continued) ● INPUT: – Ice Elevation (from ICE DYNAMICS). – Bed (from ISOSTASY). – Latitude. ● OUTPUT: – Surface temperature (for THERMODYNAMICS). – Net Mass Balance (for ICE DYNAMICS and THERMODYNAMICS).
CLIMATE (better) ● NCEP2 data-based: From re-analysis gridded monthly-mean temperature and monthly total precipitation. – Partition precipitation into SNOW or RAIN depending on monthly mean temperature adjusted for “climate knob”and lapse rate-based elevation change. ● Accumulation = SNOW. – Count PDD from monthly mean temperatures adjusted for “climate knob” and lapse rate-based elevation change. ● Ablation proportional to PDD. – Net Mass Balance = Accumulation – Ablation. ● INPUT/OUTPUT: the same.
CLIMATE (further...) ● GCM result-based: – LGM configuration provided as topography input to GCM (Bromwich). – Results: gridded monthly-mean temperature and monthly total precipitation. – Same treatment as NCEP2 data-based. – NCEP2 data-based: “Modern” climate. – GCM results-based: “Ice Age” climate. – Switch between the two by some proxy. ● (sea level ??? internally calculated). ● INPUT/OUTPUT: the same.
ICE SHELF/CALVING ● A parameterization of Weertman slab thinning is used to control advance and retreat of the grounding line through application of an ablation rate (i.e., thinning) in elements containing a grounding line. ● Improvements include a more complete treatment of the ice shelf that forms in front of the grounding line. This will require a more complete treatment of the longitudinal stresses (the Morland equations), currently absent from the model.
EMBEDDED MODELS ● High-resolution, limited domain – runs inside ● Low-resolution, larger domain model. ● Modeling the whole ice sheet allows margins to be internally generated. – No need to specify flux or ice thickness along a boundary transecting an ice sheet. ● Specification of appropriate Boundary Conditions for limited-domain model, based on spatial and temporal interpolations of larger-domain model.
APPLICATION TO SIPLE COAST: A GLACIAL CYCLE with 3 climate proxies. ● Temperature proxies for the “climate knob” are derived from deep cores, Vostok, Byrd, and Taylor. ● A glacial cycle beginning at 88 KBP is run to the present. ● “Climate knob,” areal extent, and flotation volume are shown.