Observations and Parameterization of Boundary Layer Observations and Parameterization of Boundary Layer Structures and Clouds at the ARM TWP Nauru SiteStructures and Clouds at the ARM TWP Nauru Site

Bruce Albrecht, Virendra Ghate, and Dan VossBruce Albrecht, Virendra Ghate, and Dan Voss11 Pavlos KolliasPavlos Kollias22

11University of MiamiUniversity of Miami22Brookhaven National LaboratoryBrookhaven National LaboratoryIntroduction:

The observations from the ARM TWP-Nauru site provide a unique opportunity to fair-weather cumulus clouds and the associated boundary layer structure. In this study an atmospheric mixed layer representation of the subcloud layer for application in fair-weather cumulus regimes is used to evaluate a diagnostic model of the near surface temperature and moisture associated with a given sea surface surface temperature and wind speed. The validity of treating the Nauru (night-time) site as a marine site is demonstrated.

(1 ) /RM T vo vA BQ k C U T T z

Model Description:

An atmospheric mixed layer representation of the subcloud layer for application in marine fair-weather cumulus regimes (Albrecht et al., 1977) is used to develop a diagnostic model of air-sea temperature and moisture differences. This model assumes local steady state conditions and cloud base height fixed at the mixed layer LCL. The subcloud layer radiative heating rate QRM from this model (where advection is negligible) can be written as

zB=cloud base height; k=0.2; CT is a bulk aerodynamic transfer coefficient.

Conclusions:

Observations from Nauru demonstrate the utility of using a simple boundary layer model to represent air-sea differences. Night-time longwave radiative cooling in the subcloud layer is found to be remarkably constant for the period analyzed. Despite variations in wind speeds, sea-air temperature differences, and boundary layer depth, longwave cooling calculated as a residual from these quantities was on average ~2.5 oC day-1 with only small month-to-month variability. These values compared well with monthly averaged LW cooling rates from calculations by Mather and McFarlane. Temperature, moisture, and LCL/cloud-base variations in the boundary layer showed a dependence consistent with the model.

References:

Bruce A. Albrecht, Alan K. Betts, Wayne H. Schubert Bruce A. Albrecht, Alan K. Betts, Wayne H. Schubert and Stephen K. Cox. 1979: and Stephen K. Cox. 1979: Model of the Thermodynamic Structure of the Trade-Wind Boundary Layer: Part I. Theoretical Formulation and Sensitivity Tests. . J. Atmos. Sci., J. Atmos. Sci., 36,73–89.36,73–89.

Mather, J., S. McFarlane, M. Miller, and K. Miller, Mather, J., S. McFarlane, M. Miller, and K. Miller, 2007: Cloud Properties and Associated Radiative 2007: Cloud Properties and Associated Radiative Heating Rates in the Tropical Western Pacific. Heating Rates in the Tropical Western Pacific. JGR JGR (in Press) (in Press)

Monthly averaged values of terms used to evaluate the residual longwave cooling. Cloud base height is from ceilometer (lowest 25%) estimates.

T0-TA (oC)

0.5 1.0 1.5 2.0 2.5

Q0-

QA (

g/kg

)

5

6

7

8

9

10

Monthly averaged sea-air mixing ratio difference as a function of sea-air temperature differences showing consistency with model.

To-TA (oC)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Clo

ud

Bas

e H

eig

ht (

m)

500

550

600

650

700

750

800

850

900

Residual longwave cooling rates compared with those calculated (Mather and MacFarlane; Mather et al., 2007). LWup is σTo

4 and LWdwn from observations.

LW cooling rates (residuals and calculated) as a function of difference between LWup and LWdw. Line is fit to calculated values.

QRM residual cooling and cooling calculated from LW fluxes using regression from above.

Residual longwave cooling rates calculated using cloud base height and those using LCL.

Observations showing correlation between cloud base height (mixed layer depth) and sea-air temperature difference that is consistent with model

Monthly averaged Nauru and QuikScat winds showing good correlation with 3 ms-1 offset.

Sea surface temperature (NCEP/Reynolds) and Nauru air temperature.

BL Forcing

SST

Air Temp

Winds

SSTs

Months after January 1999

0 4 8 12 16 20 24

IR C

oolin

g (o C

/day

)

0

1

2

3

4

QRM (residual)QRM (from LW fluxes)