ARM Observations Validate Climate Model for Tropical Cirrus Clouds

Comstock, J. M., Pacific Northwest National Laboratory

General Circulation and Single Column Models/Parameterizations

Cloud Modeling

Comstock, J.M., C. Jakob, Evaluation of tropical cirrus cloud properties derived from ECMWF model output and ground based measurements over Nauru Island, 2004, Geophys. Res. Ltr, Vol.31, L10106, doi:10.1029/2004GL019539.

Composite statistics compare ARM measurements and ECMWF model-derived cloud properties of (a) base height, (b) cloud top height, and (c) high cloud visible optical depth for tropical cirrus clouds.

In a study funded by the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Program, researchers used ground-based observations to assess simulations of tropical cirrus derived from the European Climate Model for Weather Forecasting (ECMWF). As described in Geophysical Research Letters (May 2004) the researchers used lidar and radar measurements of cirrus occurrence obtained between April and November 1999 from ARM's instrumented field site on Nauru Island to assess the accuracy of the ECMWF model in predicting tropical cirrus. Using a subdivided model grid box of 100 independent samples to represent cloud variability, comparisons of cloud top height and optical depth over the eight month period indicated good agreement between the model and measurements. Additionally, both the model and observations showed that tropical cirrus fall into two distinct categories: cirrus near convection (anvils) and cirrus detached from convection.

Thin cirrus clouds near the tropopause layer—the layer of the Earth's atmosphere where the troposphere turns into the stratosphere—can have significant impacts on radiative heating in the upper troposphere. This is due, in part, to the ice crystals that make up tropical cirrus clouds; these crystals tend to absorb and re-emit infrared radiation, which can increase warming in the atmosphere. Because of the important role of tropical cirrus in modulating the radiation budget, the ability of large-scale models to correctly simulate their occurrence is important in predicting the role of these clouds on climate. This is especially true in the tropics, where deep convection, heavy precipitation, and consistently warm temperatures act as a "breeding ground" for extensive and persistent cirrus cloud sheets.

To examine the conditions under which the specific cirrus forms, the researchers also conducted a case study of measured and modeled typical thin cirrus in the tropopause over Nauru. Of the 32 cases examined, 56% were associated with convection, and 66% were influenced by tropospheric waves, as determined in the model's vertical velocity field. These waves occur just below cloud base and appeared to modulate cirrus cloud cover and ice water content.

Ground-based measurements provided by ARM's instrumented field sites allow comparisons such as these to assess strengths and weaknesses in climate models. Although there is some difficulty in comparing a model grid box with single point measurements (such as those provided by ARM instruments), this research showed that the model predicts both convective anvils and isolated tropopause cirrus reasonably well. In this case, the model exhibited sufficient skill in simulating tropical cirrus characteristics such that it may be useful as a tool for identifying and understanding cirrus formation mechanisms and large-scale features of tropical cirrus over time.