# Presentation: 571642

## Evolution of the cross-equatorial planetary boundary layer in the eastern Pacific

- Simon de Szoeke
- October 27, 2004

## Outline

- Background
- Questions
- Large eddy simulations
- EPIC 2001 95°W observations
- Conclusions
- Future work

**Notes:**
Background (3 slides)
Questions (1)

## Why care about the tropical planetary boundary layer (PBL)?

- Tropical meteorology is a coupled problem.
- Sea surface temperature (SST) induces atmospheric pressure gradients in the PBL.
- Wind stress affects the ocean currents.

## PBL processes affect the climate system

- Clouds respond to surface, turbulent, and radiative fluxes.
- Clouds affect the heat budget of the atmosphere and upper ocean.
- Small-scale processes feed back on the general circulation.
- PBL evolution upstream of the ITCZ preconditions convection.

## Eastern Pacific boreal fall SST

- 95°W

**Notes:**
Equatorial east Pacific: PACS region
SST: equatorial and coastal upwelling, east Pacific warm pool
introduce region of next slide

## TMI SST, MODIS cloud, and QuikSCAT wind image:boreal fall

- stratocumulus

- clearer

- ITCZ

- slowing

- resumes speed

- Rob Wood

- EQ

- 95°W

**Notes:**
SST pattern: hemispheric asymmetry, NH east Pac warm pool, equatorial CT, warm front and TIWs
Clouds: Sc (organized ~20km) in SH, Sc just north of front (<5km), shallow cumulus 20deg to wind direction (20km), ITCZ off map, but some convection seen at 91W.
Winds: SE in SH, note esp. reduced wind speed over the cold tongue, wind speed resumes at WF, big gradient in QuikSCAT windspeed across SST gradient. Winds northward at 5N and bending eastward north of here.

## The eastern Pacific circulation inmeridional cross-section

- S. Esbensen & W. Kessler

## How do SST gradients affect the PBL structure and surface wind?
How do surface and radiative fluxes affect the cross equatorial flow?
What is the effect of clouds on the PBL?

- How do surface and radiative fluxes affect the cross equatorial flow?
- What is the effect of clouds on the PBL?

- Questions

## Ways forward

- Modeling
- regional
- large eddy simulations (LES)
- EPIC 2001 experiment
- 95°W meridional-verticalcross-sections

**Notes:**
End of introduction: next up, EPIC 95W

## PBL models

| 1D vertical | 
Boundary conditions:
fluxes
subsidence
Parameterize:
mixing
entrainment
Find the vertical structure of the PBL.

 |
| --- | --- |
|  |  |

- 1D Lagrangian(Wakefield and Schubert, 1987)

- Lagrangian

- stationary

- Compute the Lagrangian trajectory of a PBL column.
- Compute conditions along the trajectory.
- Apply the boundary conditions to the 1D model.
- Find the Lagrangian evolution of the PBL.

- 1D mixed-layer model(Lilly, 1968)

## PBL models

| 1D vertical | eddy-resolving models |
| --- | --- |
|  |  |

- 1D mixed-layer model(Lilly, 1968)

- 1D Lagrangian(Wakefield and Schubert, 1987)

- Lagrangian

- stationary

- Lagrangian LES(Bretherton et al., 1999)

- 3D LES

## The cross-equatorial Lagrangian

- Compute the trajectory from the**September-October 2001 NCEP 1000-925 hPa** average wind.
- Approximate the trajectory as meridional.
- zonal advection, SST, free tropospheric profile, Coriolis parameter, and solar radiation change as a function of latitude.

- Rob Wood

## The LES

|  | longitude | latitude | altitude |
| --- | --- | --- | --- |
| domain size | 3.2 km | 3.2 km | 3 km |
| resolution | 50 meters | 50 meters | 25 meters |
| number of points | 64 | 64 | 120 |

- The large eddy simulation resolves turbulent eddies one decade into the inertial subrange of a mixed PBL.
- It requires a typical time step of 3 seconds.

## LES physical schemes

- DHARMA: flux-limited forward-in-time advection scheme (Stevens and Bretherton, 1996).
- Periodic lateral boundaries.
- COARE bulk flux algorithm surface fluxes (Fairall et al., 1996).
- BUGSrad: 18-band two-stream radiative transfer (Stephens et al*.*, 2001).
- Simple bulk microphysics (Wyant et al., 1997)

## Forcings on the LES

**Subsidence** is 4 mm/s above the PBL.
- Above the PBL inversion, the profile is relaxed to the NCEP free-tropospheric profile.
- The **pressure gradient force** is prescribed by NCEP reanalysis.
**Zonal advective tendencies** are prescribed by NCEP.

## Peculiarities of the Lagrangian model framework

- It implicitly includes meridional advection.
- Pressure gradients are prescribed, and do not evolve with the thermal structure of the PBL.
- It assumes the column is well mixed, so that all levels are advected by the mean PBL wind.

## Initial conditions

- Initialized at 8°S, 95°W.
- Initial profiles and soundings from the NOAA ship *Ron Brown:*

- NCEP

- shipsoundings

- model

## Surface boundary condition: SST

## SST and surface air temperature

- SST

- Tair

## Potential temperature (K)

- 10 K

- altitude (km)

- time (days)

- degrees latitude

## specific humidity (g kg-1)

- altitude (km)

- degrees latitude

- cloud

- total water

## GOES visible images for EPIC 95°W flights

- 10°N

- 10°S

- EQ

**Notes:**
20 degree C SST isotherm demarcates cloudiness transitions
variability in the PBL.
-Mesoscale variability (Sc cells, Sc -> Cu rolls) is evident, but can’t be resolved by an LES.

## Heat budget

- radiative divergence

- drizzle

- entrainment

- surface flux

**Notes:**
next: heat budget

## Heat budget (W m-2)

- Heat budget (W m-2)

- entrainment

- rain

- surface

- radiative

- degrees latitude

- storage

## winds (m s-1)

- altitude (km)

- degrees latitude

- zonal

- meridional

## EPIC 95°W mean dropsondecross-sections

- altitude (km)

- meridional wind

**Notes:**
not shown: humidity profiles. moist air returns S above boundary layer, so the free troposphere is often as moist as the boundary layer itself.

## Mixed PBLat 1°N

- Mixed PBLat 1°N

- Stable over the equatorial cold tongue

- Potential temperature (K)

- meridional wind (m/s)

- altitude (km)

- altitude (km)

- model

- dropsondes

## What governs the surface wind?

- Lindzen and Nigam shallow water model (1987)
- Wallace et al. surface stability (1989)

## Lindzen and Nigam model

- no pressure gradient at inversion

- cold

- cold

- warm

- warm

*p*

*p*

- divergence

- convergence

- H

- L

- inversion

- ocean

- SST gradients induce hydrostatic pressure gradients, which accelerate the winds.

- atmospheric PBL

## Wallace et al.: surface stability distributes momentum

- cold

- warm

- divergence

- Stable stratification supports wind shear.

- Neutral or unstable stratification mixes down momentum.

*v*

*θ*

*v*

*θ*

- mixed

- stable

## EPIC 95°W C-130 turbulence profiles

- mean vertical velocity variance

- altitude
- (km)

- degrees latitude

**Notes:**
axes

## Turbulence and cloud profiles

- altitude
- (km)

- degrees latitude

**Notes:**
inversion from in-situ theta profiles
entrainment inferred to be 12+-1 mm/s, though one outlier with a weaker inversion had a much higher entrainment rate

## Turbulent kinetic energy (m2 s-2)

- altitude (km)

- degrees latitude

## Turbulent meridional wind sources at the warm SST front

- degrees latitude

- altitude (km)

- m s-2

## Momentum budgets

- m s-1 day-1

- degrees latitude

- meridional

- zonal

- Coriolis

- entrainment

- storage

- surface

- large
- scale

*geostrophic*

*accelerating*

*frictional*

## The role of pressure gradients

- The changing temperature has an effect on the pressure gradient.
- A regional model simulates this.
- The maximum wind is found downstream of the maximum pressure gradient.

- Justin Small

## How do SST gradients affect the PBL structure and surface wind?

- Over cold SST (warm advection) the lower PBL is stably stratified, turbulence is weak> shear > weak surface wind.
- Over warm SST (cold advection) the lower PBL is turbulent and mixed> strong surface wind.
- PBL thermal structure also influences the wind through the pressure gradient.

- Summary

## What is the effect of surface fluxes?

- Surface latent and sensible heat fluxes depend on the air-sea temperature difference: zero over cold tongue; 20 W m-2 sensible, 190 W m-2 latent over warm SST.
- Turbulence and wind speed near the surface affect the fluxes.
- Thickest cloud is seen over the cold-advection (warm SST) PBL.

## What is the effect of radiative flux?

- Radiative flux is an important contributor to the heat budget, and to turbulence through buoyancy.
- especially true in the warm-advection PBL, where surface fluxes are small

## Cloud radiative cooling maintains cloud over the cold water

- degrees latitude

- altitude (km)

## Cloud-hysteresis:cloud depends on initial conditions

- degrees latitude

- altitude (km)

## Diurnal cycle increases the patchiness of clouds

- degrees latitude

- altitude (km)

- solar radiation at 3 km (W m-2)

## What about clouds?

- Clouds reflect solar radiation.
- Infrared radiative flux divergence at the cloud top is an important positive feedback for maintaining thin stratocumulus in the absence of surface fluxes.
- “cloud hysteresis”
- Patchiness: Mesoscale and diurnal variability would act to reduce the modeled cloud amount over the cold tongue.

## Future work

- Comparison among LES, regional models, and observations.
- IPRC RCM
- MODIS
- EPIC 95°W
- Testing coupled processes: coupled regional modeling.
- IPRC ROAM
- Synoptic variability of the cross-equatorial flow and the ITCZ.

## Future work

- In the vicinity of SST fronts, what is the effect on the ocean of the fine-scale wind stress due to stratification of the lower-boundary layer and the hydrostatic pressure gradient?
- Do the atmospheric models in coupled GCMs sufficiently resolve divergences and curls in the wind stress?