Transcript Title Slide

An Overview of the NoahDistributed Land Surface Model
David J. Gochis, Wei Yu, Fei Chen, Kevin
Manning
WRF Land Surface Modeling Workshop
Sep. 13, 2005
Brief Rationale for NoahDistributed
• Standard land surface parameterizations characterize
exchanges of radiation, heat, mass and momentum between
the land and atmosphere
• Historically, treatment of terrestrial hydrology has been
simplified 1-d formulations
• With high resolution implementations/applications there is
now a need to explicitly account for enhanced hydrological
processes:
Violating early assumptions…
1. Surface runoff can not assumed to be “captured” by a stream channel
2. Lateral transfers from one cell may form significant input to adjacent
cell
Brief Rationale for NoahDistributed
• Standard land surface parameterizations characterize
exchanges of radiation, heat, mass and momentum
between the land and atmosphere
• Historically, treatment of terrestrial hydrology has
been simplified 1-d formulations
• With high resolution implementations/applications
there is now a need to explicitly account for
enhanced hydrological processes:
– Higher resolution capabilities  land surface
heterogeneity
– Earth systems-biogeochemcial cycling
– Mitigation of “high-impact” weather events (e.g. floods)
Outline
• Brief overview of the Noah LSM
• Noah-distributed core features
• Implementation of Noah-distributed into the
NCAR/HRLDAS framework
• Ongoing and planned upgrades to Noahdistributed
Community Noah Land Surface
Model – Recent Enhancements
• Recent Enhancement of the Community Noah LSM (released in WRF
V2.0, May 2004) ‘Noah-Unified’
– “Nearly”-identical implementations of Noah LSM development
effort: NCAR, NCEP, U.S. Air Force Weather Agency, NASA,
university community
– Fully modularized, F90 code conventions
– Seasonal surface emissivity
• surface emissivity is introduced as function of landuse
• Added surface emissivity in surface energy balance equation
for both snow and non-snow surfaces
– Urban model improvements (the simple approach) such as
• Large roughness length
• Low surface albedo
• Large thermal capacity and thermal conductivity
Overland Flow Processes in Noah-Router
(NCAR Tech Note: Gochis and Chen, 2003)
IF (Surface Head > Retention Depth) 
Route Water as Overland Flow
• New Parameters: retention depth,
surface roughness
• Ponded water in excess of retention
depth subject to overland flow
• Overland flow: fully-unsteady,
explicit, finite-difference, 2dimensional diffusive wave (generally
applicable to length scales < 1km)
2-Dimensional
Diffusive Wave
Overland Flow Routing
Ogden, 1997
dhdx  (hi 1, j  hi , j ) / gsize
sfx  SOX i, j  dhdx  1E  30
( sfx / ABS ( sfx)) h5 3dt
qx 
dx
Dynamic modeling of land-surface hydrology
with ‘Noah-Router’: Ponded Water Processes
(NCAR Tech Note: Gochis and Chen)
• New Parameters: None
Direct Evaporation
Surface Runoff 
Surface Head
• Currently no formulation for
partial area coverage
Issue: May need to revise infiltration formulation
• Ponded water consists of:
residual
‘infiltration excess’
when using routed runoff
to ofcalibrate:
from previous time step and
Surface runoff can not assumed to be “captured”
by water
a stream channel
routed surface
Re-infiltration
• Direct evaporation of ponded
water reduces potential
evaporation (no adj. for
temp/albedo)
Ponded Water Evaporation
and Re-infiltration
• Ponded water not evaporated is
subject to infiltration
Subsurface Flow Routing Noah-Router
(NCAR Tech Note: Gochis and Chen)
Surface Exfiltration from
Saturated Soil Columns
Lateral Flow from
Saturated Soil Layers
Saturated Subsurface Routing
Wigmosta et. al, 1994
• New Parameters: Lateral Ksat, n –
exponential decay coefficient
• Critical initialization value: water
table depth
• 8-layer soil model (2m – depth,
sealed bottom boundary)
• Quasi steady-state saturated flow
model, 2-d (x-,y-configuration)
• Exfiltration from fully-saturated soil
columns
  SOX i, j  dzdx  1E  30

( gsize  LKSAT  SOLDEP )
tan 
n
z


hh  1 

 SOLDEP 
qsubx    hh
n
Noah-Distributed Core Features
• Present issues in treatment of subsurface
routing:
– Frozen soil adjustment to soil water
• Remove soil ice from total soil moisture and route
only liquid component
– Update of conductivity as a function of soil
temp/fraction of frozen soil
– Inclusion of variable depth soils
Subgrid Routing
•
Noah LSM is run at a variety of
grid spacings
•
Subsurface and overland flow
routing need to be performed on a
terrain grid (< 1 km)
•
Required fields are
aggregated/disaggregated using a
simple averaging scheme
Noah land surface
model grid
Routing Subgrids
• Soil water, infiltration excess,
routing parameters
•
•
Can offer significant computational
savings compared to full resolution
implementations of Noah LSM
Sacrifice detail in current
formulation
AGGFACTR = 4
Noah-Distributed Core Features
• Subgrid disaggregation:
proposed new method
carrying over weighting
factors between LSM
model executions
• Eliminates the “loss” of
distributed information
between routing timesteps
Noah land surface
model grid
Routing Subgrids
Noah-Distributed Software Features
• F90, up to date with recent version in HRLDAS
• Routing routines (1-d and 2-d) are contained within a
single module (all agg./disagg. Routines will be included
into routing module)
• Routing and sub-grid options are switch-activated though
a namelist file
• Options to output sub-grid state and flux fields to WRF
consistent netcdf files
• Basic Flow:
LSM > Disagg. > Subsfc > Overland > Agg. > LSM > …
Outline
• Brief overview of the Noah LSM
• Noah-distributed core features
• Implementation of Noah-distributed into the
NCAR/HRLDAS framework
• Ongoing and planned upgrades to Noahdistributed
NCAR-HRLDAS
(High Res. Land Data Assim. System)
• Rationale and basics of HRLDAS:
– Create globally-deployable variable resolution
equilibrated land surface conditions for NWP
initializations
• Current static & forcing data:
Time
Variable
Interval
Dataset
Grid Resolution
Reference
Precipitation
Hourly
NEXRAD Stage IV
4 km
Fulton et al, 1998
Surface
Meteorology
Hourly
EDAS
40 km
Rogers et al., 1995
Land Use
Static
USGS-24 Category
1 km
Loveland et al., 1995
Greenness
Fraction
Monthly
n/a
0.15 degree
Gutman and Ignatov, 1998
Soil
Classification
Static
STASGO-16 Category
1 km
Miller and White, 1998
Overland Flow
Roughness
Coefficient
Static
n/a
Mapped to Land Use
Adapted from Vieux, 2001
HRLDAS
• Recent tests over International H2O Project
(IHOP) domain
• 18 month execution 1 Jan, 2001 – 30 Jun, 2002
Top Layer Soil Moisture (fraction):
Total Column Soil Moisture (mm):
Total Surface Evapotranspiration (mm):
Ponded Water Evaporation (mm):
Outline
• Brief overview of the Noah LSM
• Noah-distributed core features
• Implementation of Noah-distributed into the
NCAR/HRLDAS framework
• Ongoing and planned upgrades to Noahdistributed
Future Upgrades to Noah
Distributed
• Improve runtime performance:
– 2-d vs 1-d formulations
• DEM-based steepest descent method is much faster
• Strictly DEM based routing (kinematic) problematic in flat areas
where change in sfc water influences flow direction (e.g. backwater)
– Working on a compromise algorithm
• default to DEM based routing
• check for backwater
• Perform search
– 1-d: 129 model days/wall clock d vs. 2-d: 97 model days/wall
clock day (~1/3 faster)
Future Upgrades to Noah
Distributed
• Complete parallelization
• Couple to stream channel model (via
DESWAT project)
• Develop better method to nudge/assimilate
groundwater and river/stream stage into
modeling system
• Develop enhanced method to characterize
stream-aquifer exchange