What Controls the Strength of the Hydrological Cycle and Subtropical Relative Humidity in an Idealized Model?

Abstract:

The strength of the hydrological cycle is a critical component to understand in predicting climate change. A closely connected variable is the moisture content of the subtropical free troposphere. Here, the idealized Held-Suarez (HS) dynamic core is utilized to study moisture effects through controlled experiments (Held and Suarez, 1994, BAMS). The HS core involves a spherical rotating planet with a dry atmosphere forced by a Newtonian relaxation of temperature toward a prescribed zonally symmetric equilibrium temperature as well as planetary boundary layer drag represented by Rayleigh damping.

With fixed temperature and circulation patterns, clouds can be isolated. To this end, the HS model is run as a base case with a passive water tracer (no clouds) as well as with a cloud scheme used in comprehensive climate models. Without clouds, water is precipitated whenever saturation is reached, but with a cloud scheme, cloud formation and precipitation is influenced by microphysical factors as well as processes such as re-evaporation.

In the base case, the HS model yields a global average precipitation value of 2.39 mm/day. With clouds, the value increases to 3.10 mm/day. While both these values are relatively proximate to observations, the difference in the values suggests that cloud processes have an important bearing on the strength of the hydrological cycle (in the absence of radiative balance).

The idealized HS model yields a qualitatively realistic relative humidity distribution, but the most noticeable climatological discrepancy is the subtropics which are too dry. However, with the introduction of a cloud scheme, these dry values improve noticeably, implying that missing cloud microphysics are a contributing factor to the base case’s dry bias.

Through various sensitivity studies and an analysis of the moisture budget, implications are drawn for a broader understanding of what controls subtropical relative humidity and thus the strength of the hydrological cycle.