improve noah snow model treatment based on snotel data

Improve Noah snow model treatment based on SNOTEL data

Michael Barlage, Fei Chen, Mukul Tewari, and Kyoko Ikeda

2

Snow modeling in Noah

• Snow physics:– Currently one-layer snow model in Noah V3.0

(based on Koren et al. 1999, JGR)• Snow density change due to compaction, fractional snow

cover, frozen-ground physics

– 3-layer model development by Guo-Yue Niu and Zong-Liang Yang at UT-Austin

– Improving the one-layer snow model (work reported here)

Snow albedo comparison against 94-95 Col de Port (France) data using Livneh’s albedo formulation

Red: Obs, Black: V3, Green: V3 with albedo modification

Albedo Snow Depth

This new albedo scheme was used for Headwater simulations and released in WRF 3.1

4

“Good” Sites: Coupled WRF Results

WRF Precip

WRF SWE

Obs SWE

Sublimation

Melt

5

“Bad” Sites: Coupled WRF Results

6

Radiation Comparison: WRF terrain adjusted vs GOES SRBWRF SW is consistently higher than SRB for “bad” sites

Good SWE sites

WRF solar at four points GOES solar WRF solar at four points GOES solar

Bad SWE sites

Major reasons for early snowmelt in Noah at “bad” sites

• Higher downward solar radiation in WRF after adjusted to sub-grid terrain slope

• High melt amount in early spring– Raise maximum snow albedo to 0.85

• Sublimation in winter– Reduce surface exchange coefficient (Ch) for

nocturnal stable regime– Reduce surface roughness length (Zo) based on

snow depth and forest height

8

Effects of increasing maximum snow albedo

Bad site Good site

Dash lines: Simulation with new max snow albedoSolid line: Default simulation

Legend legendLV: Livneh albedoML: model level forcing85: max albedo set to 0.85

9

Effects of reducing snow-surface Zo and Ch for stable regime

Bad site Good site

Dash lines: Simulation with reduced Zo and ChSolid line: Default simulation

Legend legendLV: Livneh albedoML: model level forcingZE: Zo based on exposed vegSL: Slater stability

10

New adjustment (in red) reduce Ch for stable regime

Bad site Good site

Lower Ch in nighttime stable BL BL is stable through most of days 2 - 5 Legend legendLV: Livneh albedoML: model level forcingCH: WRF MYJ stability

11

Combined effects on long-term SWEGood sites

Dash lines: Simulation with changes in Zo and albedoSolid line: Default simulation(contains new WRF Ch)

By reducing sublimation and early spring melt, these mods delay snow melt

Legend legendLV: Livneh albedoML: model level forcingCH: WRF MYJ stabilityZE: Zo = f(exposed veg)85: Max albedo = 0.85

12

Combined effects on long-term SWEbad sites

Dash lines: Simulation with changes in Zo and albedoSolid line: Default simulation(contains new WRF Ch)

By reducing sublimation and early spring melt, these mods improves timing of snow melt season

Legend legendLV: Livneh albedoML: model level forcingCH: WRF MYJ stabilityZE: Zo = f(exposed veg)85: Max albedo = 0.85

13

Effects of observed SWGood sites

Dash lines: Use of GOES SW as forcingSolid line: Control with WRF stability, albedo, Zo

Legend legendLV: Livneh albedoML: model level forcingCH: WRF MYJ stabilityZE: Zo = f(exposed veg)85: Max albedo = 0.85SW: GOES SW forcing

14

Effects of observed SWBad sites

Dash lines: Use of GOES SW as forcingSolid line: Control with WRF stability, albedo, Zo

Legend legendLV: Livneh albedoML: model level forcingCH: WRF MYJ stabilityZE: Zo = f(exposed veg)85: Max albedo = 0.85SW: GOES SW forcing

15

Combined effects with observed SWGood sites

Dash lines: Simulation with changes in Zo, albedo, GOESSolid line: Default simulation(with new WRF Ch)

By reducing sublimation and early spring melt, these mods delay snow melt

Legend legendLV: Livneh albedoML: model level forcingCH: WRF MYJ stabilityZE: Zo = f(exposed veg)85: Max albedo = 0.85SW: GOES SW forcing

16

Combined effects with observed SWBad sites

Dash lines: Simulation with changes in Zo, albedo, GOESSolid line: Default simulation(with new WRF Ch)

By reducing sublimation and early spring melt, these mods improves timing of snow melt season

Legend legendLV: Livneh albedoML: model level forcingCH: WRF MYJ stabilityZE: Zo = f(exposed veg)85: Max albedo = 0.85SW: GOES SW forcing

17

Full grid effects of stability adjustment and all changes

Left: ControlMiddle: Control + ChRight: Control + Ch + albedo + Zo

Effects are domain-wide but increased time to melt does not propagate out of mountains

Legend legendLV: Livneh albedoML: model level forcingCH: WRF MYJ stabilityZE: Zo = f(exposed veg)85: Max albedo = 0.85SW: GOES SW forcing

Summary and Ongoing Work

• Coupled WRF simulations produce early snowpack disappearance at many SNOTEL sites

• We have a set of simple changes to the existing Noah which delays snowmelt and decreases sublimation

• These changes include a stability adjustment and new Zo formulation that reduce sublimation and an albedo change that delays snowmelt

• We are now running simulations over the full domain to do a more widespread and statistical comparison to SNOTEL sites

19

Site radiation