Thermally forced winds due to topography: Slope winds case of study over Costa Rica

Marcela Ulate

CR-AVE

Motivation

Ticosonde 2006

Objective of the mission: Study the tropical upper troposphere and lower stratosphere and to provide information for comparison to satellite observations.During CR-AVE the NASA WB-57F aircraft with 29 instruments was used during the January 14-February 12 period.

Ticosonde was the local project from CR-AVE. It consisted on radiosonde launches 4 times at day. The period overlapped with CR-AVE, but radiosondes were also launched during mid- June, July and mid – August 2006.Similar field campaigns took place during June-July August 2004,2005 and 2007.

Slope winds

• Thermally forced , driven by between air in direct contact with the slope and air at the same altitude not affected directly by the heating or cooling.

• Day time heating of a mountain slope produces upslope winds.

• Night time cooling of a mountain slope produces down slope winds.

r h B

Basic Momentum equations

Basic State Flow (away from the gravity current):Motionless.Homogeneous.Stationary

Flow associated with the slope heating or cooling

Turbulent fluctuations

From Mahrt, 1982

From Haiden 2003

U: Down-slope velocity scaleL: Down-slope length scaleV: Cross-slope velocity scaleL: Cross-slope length scaleH: Depth Scale

V/L <<U/L

r

r v 0

u

x w

z0 H

U

L~ w

0

Quasi-hydrostatic equilibrium

Radiosonde Data

WRF SimulationWRF version 3.2Real Case.3Domains :

90Km (75e-w x 55 n-s)30Km (148e-w x 94 n-s)10Km (70e-w x 72n-s)

28 vertical levels.2-way nesting.Boundary conditions from ERA-Interim data updated every 6h.

Parameterizations for : PBL, LWR, SWR, Convective processes. Period simulated: July 16 2006 – July 21 2006Output interval: 1h.

00Z 06Z

12Z18Z

00Z 06Z

12Z18Z

00Z 06Z

12Z18Z

ConclusionsData limitations: The information that can be extracted from one isolated sounding is limited, and it is dominated for local effects.

The radiosonde data and the model data present a similar pattern in the surface wind during the period of study. The vertical profiles of the simulations present different patterns than the sounding data, this could be due to the resolution and also the fact that the radiosonde change location during its upward motion.

The model is able to capture the basics characteristics of the slope winds, with limitations related with resolution.

The results would probably change if other parameterization are used with special emphasis on the PBL scheme.

The “ideal case” to study the slope winds would be the use of idealized simulations, with higher resolution.

Model run shows, as hypothesized before, that the low-levels winds are orographically forced

Thanks

References• Haiden, T. 2003: On the pressure field in the slop wind layer. J. Atmos.

Sci., 60, 1632-1636.

• Kossmann, M. And Fiedler, 2000: Diurnal momentum budget analysis of thermally induced slope winds. Meteor. Atmos. Phys., 75, 195-215.

• Mahrt, L. 1982: Momentum balance of gravity flows. J. Atmos. Sci., 39, 2701-2711.

• Schuman, U., 1990: Large-eddy simulation of the up-slope boundary layer. Quart. J. Roy. Meteor. Soc., 116, 637-670.