Research Staff: Andrew Ackerman

Condensation clouds of silicate and iron in brown dwarfs, and water in extrasolar giant planets, likely play a leading-order role in the emitted (and reflected) spectra from these objects. Until now, a theoretical model has not been available to calculate the vertical profile of condensate opacity in the atmospheres of these objects, which is a critical component of any model of their emergent (and reflected) spectra. We have developed such a model which predicts the vertical profile of condensate mass and its distribution over particle size.

The model assumes a steady-state balance between turbulent mixing (upwards) of vapor and condensate and sedimentation of condensate (downwards), in which the vapor concentration is limited to not exceed the saturation vapor pressure. The sedimentation rate is scaled to the convective velocity scale by a prescribed dimensionless parameter, f_rain, which is also tied to the modal radius of condensate particles (assumed to follow a log-normal size distribution). Measurements and model simulations of terrestrial clouds show that f_rain is less than unity for stratocumulus, but increases to much greater values for deeper convection such as found in the trade cumulus regime. High values of f_rain are expected for skewed circulations (narrow updrafts, broad downdrafts) and/or small concentrations of condensation nuclei.

We have compared calculations from our model with measurements of the ammonia cloud on Jupiter, and find agreement for moderate values of f_rain (~ 3). The measurements are rather uncertain, but the model is able to fit a number of independent constraints, including cloud physical thickness, optical depth, and average particles size.

We have also computed theoretical cloudy atmospheres in brown dwarfs and an extrasolar giant planet. The model is able to reproduce the measured color trend (a blueward shift) for the transition between L and (cooler) T dwarfs (see Figure). Sedimentation in the model limits the physical thickness of the silicate (and iron) clouds, allowing these clouds to sink below the visible atmosphere (chromosphere) for progressively cooler brown dwarfs. The model fits the data reasonably well for values of f_rain between 3 and 5, though the blueward shift in the observations is more extreme than the model is able to reproduce. We hypothesize that the appearance of holes as the clouds descend into the troposphere (where convection is expected to be strongest), provides for a more rapid blueward shift than our horizontally homogeneous model is able to explain. We are presently working on a simple treatment of fractional cloudiness to address this issue.

Collaborators: Mark Marley, SST branch

Point of Contact: Andrew Ackerman, 650/604-3522, ack@sky.arc.nasa.gov

J-K color of brown dwarfs as a function of effective temperature. Measurements are shown as symbols; the prototypical T-dwarf Gliese 228 B is shown as a filled symbol. Four cases are shown for the present model: evolution with no clouds, and with clouds with different values of the model parameter f_rain (as given). Also shown are color trends (for objects of 30 and 60 Jupiter masses) from the Chabrier model, denoted "COO", in which there is no sedimentation. The lack of condensate sedimentation in that model results in an unrealistic redward trend (characteristic of a blackbody) in the transition regime between L and T dwarfs.