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A brief introduction to

UMCES Chesapeake Bay Model

Yun Li and Ming Li

University of Maryland Center for Environmental Science

VIMS, SURA Meeting

Oct-1-2010

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Grid

Forcingwind

river

SST (sea surface temperature)

Boundary

Model Sensitivitywind

background diffusivity

resolution

Outline

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– Model domain includes mainstem, 8 major tributaries a piece of coastal ocean

– Curvilinear orthogonal grid

– 20 stretched layers

– Resolution Low (80x120) <1km in cross-channel 2~3km in along-channel deepest point is 26m.

High (160x240) <500m in cross-channel 0.5~2km in along-channel deepest point is 40.5m.

Deep channel is well resolved in the high-resolution model

Gri

d

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Fo

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– Wind Data Sourcehttp://www.wunderground.com

6 wind stations, hourly data

– Interpolationwind speed is linearly interpolated along latitude and longitude

– Wind speed to stress

where

– Amplification Factor (Xu et al. 2002; Wang and Johnson 2000)

UUCdair

310063.061.0 UCd

Station N-S comp E-W comp

NIA(KORF) 1.37 1.25

PRS(KNHK) 2.05 1.43

BWI(KBWI) 1.50 1.00

Wind

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Fo

rcin

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– River Data SourceUSGS monitoring stations, discharge (m3/s) and temperature (<monthly)

salinity is set to zero

– Major TributariesSusquehanna (1) 01578310Patapsco (3) 01583500 01586210 01586610Patuxent (1) 01594440Potomac (1) 01646500Rappahannock (1) 01668000York (2) 01673000 01674500James (3) 02037500 02041650 02042500Choptank (1) 01491000

– Sea Level at Riverine Boundary

New CPP: PSOURCE_FSCHAPMAN allow incoming wave but avoid reflection

River

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Fo

rcin

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– SST Data SourceChesapeake Bay Program along-channel observation (monthly or biweekly)

– InterpolationSurface temperature (<1m) is linearly interpolated along latitude

– ConfigurationNew CPP: SST_RELAXATION (must undef QCORRECTION)Nudging is performed every 6 hours.

SST

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Bo

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ryModel only has open boundary at eastern edge.

– Data SourcesTidesFive major components M2, S2, N2, K1, O1 from Oregon State U. global inverse tidal model TPXO

Subtidal Sea Leveldetided component from NOAA historical data at Duck, NC

T and Slinear interpolation from WOA2005, monthly Levitus climatology

ubar and vbarzeros at boundary

– Configuration sea level: FSCHAPMAN 2D momentum: EAST_M2FLATHER 3D momentum: EAST_M3RADIATION T and S: EAST_TRADIATION EAST_TNUDGING (1 day)

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Se

ns

itiv

ity

original wind

amplified wind

observation

Salinity, Nov 11, 1996

Wind

Along channel distribution in a low-runoff period.

Using the amplification factors, the model produces a well-mixed surface layer in better agreement with the observation

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10-4m2/s

5x10-5m2/s

10-5m2/s

10-6m2/s

Vertical Diffusivity

A comparison of along-channel salinity distribution between four model runs with different vertical diffusivity (Ks) shows that

– the stratification increases as Ks decreases.

– the along-channel salinity gradient increases as Ks decreases.

– model prediction becomes less sensitive when Ks is reduced below 10-5m2/s.

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k-kl,80x120

k-kl,160x240

observation

Salinity, April 23, 1997

Resolution

Increasing model resolution is important to resolve the narrow deep channel, which is the main conduit for the landward salt transport.

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– Special features in UMCES Chesapeake Bay Model

1. Both low- and high-resolution configurations are available

2. Apply Chapman’s condition for sea level elevation at Riverine boundary to avoid wave reflection.

3. SST nudging to observation with a time scale of 6hr.

4. Using wind amplification factors, the model produces a well-mixed surface layer

– Areas of Improvement

1. Model resolution in the deep channel

2. Turbulent mixing near the pycnocline (Li et al. 2005)

3. Adjustment of observational wind (Xu et al. 2002; Li et al. 2005)

Summary