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Dennis L. Hartmann
,
Brittany D. Dygert
,
Peter N. Blossey
,
Qiang Fu
, and
Adam B. Sokol

Abstract

The vertical profile of clear-sky radiative cooling places important constraints on the vertical structure of convection and associated clouds. Simple theory using the cooling-to-space approximation is presented to indicate that the cooling rate in the upper troposphere should increase with surface temperature. The theory predicts how the cooling rate depends on lapse rate in an atmosphere where relative humidity remains approximately a fixed function of temperature. Radiative cooling rate is insensitive to relative humidity because of cancellation between the emission and transmission of radiation by water vapor. This theory is tested with one-dimensional radiative transfer calculations and radiative–convective equilibrium simulations. For climate simulations that produce an approximately moist adiabatic lapse rate, the radiative cooling profile becomes increasingly top-heavy with increasing surface temperature. If the temperature profile warms more slowly than a moist adiabatic profile in midtroposphere, then the cooling rate in the upper troposphere is reduced and that in the lower troposphere is increased. This has important implications for convection, clouds, and associated deep and shallow circulations.

Open access
Marina Dütsch
,
Eric J. Steig
,
Peter N. Blossey
, and
Andrew G. Pauling

Abstract

The West Antarctic Ice Sheet (WAIS) may have collapsed during the last interglacial period, between 132 000 and 116 000 years ago. The changes in topography resulting from WAIS collapse would be accompanied by significant changes in Antarctic surface climate, atmospheric circulation, and ocean conditions. Evidence of these changes may be recorded in water-isotope ratios in precipitation archived in the ice. We conduct high-resolution simulations with an isotope-enabled version of the Weather Research and Forecasting Model over Antarctica, with boundary conditions provided by climate model simulations with both present-day and lowered WAIS topography. The results show that while there is significant spatial variability, WAIS collapse would cause detectable isotopic changes at several locations where ice-core records have been obtained or could be obtained in the future. The most robust signals include elevated δ 18O at SkyTrain Ice Rise in West Antarctica and elevated deuterium excess and δ 18O at Hercules Dome in East Antarctica. A combination of records from multiple sites would provide constraints on the timing, rate, and magnitude of past WAIS collapse.

Open access
Mario A. Lopez
,
Dennis L. Hartmann
,
Peter N. Blossey
,
Robert Wood
,
Christopher S. Bretherton
, and
Terence L. Kubar

Abstract

A methodology is described for testing the simulation of tropical convective clouds by models through comparison with observations of clouds and precipitation from earth-orbiting satellites. Clouds are divided into categories that represent convective cores: moderately thick anvil clouds and thin high clouds. Fractional abundances of these clouds are computed as a function of rain rate. A three-dimensional model is forced with steady forcing characteristics of tropical Pacific convective regions, and the model clouds are compared with satellite observations for the same regions. The model produces a good simulation of the relationship between the precipitation rate and optically thick cold clouds that represent convective cores. The observations show large abundances of anvil cloud with a strong dependence on rain rate, but the model produces too little anvil cloud by a factor of about 4 and with a very weak dependence on the rain rate. The observations also show probability density functions for outgoing longwave radiation (OLR) and albedo with maxima that correspond to extended upper-level cold clouds, whereas the model does not. The sensitivity of the anvil cloud simulation to model parameters is explored using a two-dimensional model. Both cloud physical parameters and mean wind shear effects are investigated. The simulation of anvil cloud can be improved while maintaining a good simulation of optically thick cloud by adjusting the cloud physics parameters in the model to produce more ice cloud and less liquid water cloud.

Full access