Response of Cloud Formation and Cloud Radiative Effects to Volatility of Water Vapor in Terrestrial Planetary Atmospheres

Abstract:

We examine a one-dimensional radiative convective equilibrium (1D-RCE) model, with water vapor and cloud feedbacks. The moisture parameters of the 1D-RCE are modified to enable variation of the saturation vapor pressure by a specified factor. Our method explores how the volatility of water vapor affects the potential for cloud formation and cloud radiative effects in moist planetary atmospheres. We begin with an Earth-like aquaplanet with an atmosphere made of the present-day mix of dry components and share the results of factors between 0.01 (dry limit) and 2 (moist limit).

We use the Sundqvist cloud scheme, which is a function of the atmospheric relative humidity (RH). We show that the number of tropospheric layers conducive to cloud-formation depends on the competition between the vertical gradients of temperature and moisture in equilibrium. Above the lifting condensation level (LCL), the lapse rate is well-approximated as dry and moist adiabatic in the dry and moist limits, respectively. The vertical gradient of moisture is established through physical transport mechanisms (convection, vertical diffusion) and microphysical phase-change processes in the cloudy and clear parts of the column. In both limits below the LCL (i.e. the planetary boundary layer), the RH tends to increase with decreasing pressure owing to water vapor being well-mixed and the lapse rate dry adiabatic. In the dry limit, the RH is at saturation throughout the troposphere, resulting in a single layer of troposphere-filling clouds (TFC). Surprisingly, TFC exhibit a significant net radiative warming effect, resulting in (~20 K) warmer surface temperatures in the dry limit than the moist. In the moist limit, the RH decreases from the LCL to the middle troposphere (where the vertical gradient of moisture is largest) and increases from the middle troposphere to tropopause. The distribution of RH in the moist limit therefore facilitates the formation of two-cloud layers (low & high), one above the LCL & one in the upper troposphere. Our results at higher factors (>2) also qualitatively resemble the moist greenhouse climate states explored in other recent studies.