HR: 0830h
AN: GC31B-0186    [PDF]
TI: A study of relative humidity dynamics using water vapor trajectories in an idealized GCM
AU: * Galewsky, J
EM: jg2282@columbia.edu
AF: Department of Applied Physics and Applied Mathematics, Columbia University 500 W. 120th St., Room 200, New York, NY 10027 United States
AU: Sobel, A
EM: ahs129@columbia.edu
AF: Department of Applied Physics and Applied Mathematics, Columbia University 500 W. 120th St., Room 200, New York, NY 10027 United States
AB: We present a novel technique for diagnosing the mechanisms controlling the water vapor distribution in an idealized general circulation model (GCM) through the use of multiple passive tracers to track the trajectories of unsaturated air. The model atmosphere is divided into a number of axisymmetric regions, each of limited meridional and vertical extent. When a parcel of air is saturated in a given region, the value of the tracer for that region is set to unity there, and the tracer values for the other regions are set to zero there. The time-mean, zonal-mean tracer fields show the typical pathways that air parcels take between one occurrence of saturation and the next. Because saturation water vapor mixing ratio is a function only of temperature, and mixing ratio is conserved for unsaturated parcels, these tracer fields can be used together with the temperature field to reconstruct the water vapor field. Under a given climate change, the water vapor field reconstructed by this method may perhaps be thought of as a first correction to the assumption of constant relative humidity (the simplest possible model). This method also allows changes in the water vapor field which occur in a climate change to be partitioned into changes due to temperature and changes due to circulation. Results will be presented from a model consisting of the dry GCM of Held and Suarez plus advection of a passive water vapor field which is emitted at the surface, and assumed to rain out immediately upon saturation, but whose latent heat of vaporization is zero so that it does not affect the circulation. In this simple model, the largest errors in the reconstructed water vapor fields are those due to sampling (use of a finite number of tracers). An interesting preliminary result is that, even for a very simple, small climate change in which the radiative-convective equilibrium temperature (relaxation towards which is the only thermal forcing of the model) is increased by one degree everywhere in the atmosphere, the relative humidity change is determined substantially by circulation changes as well as by temperature changes.
DE: 0322 Constituent sources and sinks
DE: 1620 Climate dynamics (3309)
DE: 1655 Water cycles (1836)
SC: Global Climate Change [GC]
MN: 2003 Fall Meeting