Mean Climate and Tropical Rainfall Variability in Aquaplanet Simulations Using the Model for Prediction Across Scales‐Atmosphere

Abstract

Aquaplanet experiments are important tools for understanding and improving physical processes simulated by global models; yet, previous aquaplanet experiments largely differ in their representation of subseasonal tropical rainfall variability. This study presents results from aquaplanet experiments produced with the Model for Prediction Across Scales-Atmosphere (MPAS-A)-a community model specifically designed to study weather and climate in a common framework. The mean climate and tropical rainfall variability simulated by MPAS-A with varying horizontal resolution were compared against results from a recent suite of aquaplanet experiments. This comparison shows that, regardless of horizontal resolution, MPAS-A produces the expected mean climate of an aquaplanet framework with zonally symmetric but meridionally varying sea-surface temperature. MPAS-A, however, has a stronger signal of tropical rainfall variability driven by convectively coupled equatorial waves. Sensitivity experiments with different cumulus parameterizations, physics packages, and vertical grids consistently show the presence of those waves, especially equatorial Kelvin waves, in phase with lower-tropospheric convergence. Other models do not capture such rainfall-kinematics phasing. These results suggest that simulated tropical rainfall variability depends not only on the cumulus parameterization (as suggested by previous studies) but also on the coupling between physics and dynamics of climate and weather prediction models. Plain Language Summary Continuous improvements of weather and climate models are necessary to advance both our understanding and ability to predict high-impact weather under present and future climates. One particular area requiring model improvements is the representation of tropical rainfall variability happening at subseasonal timescales. With this issue in mind, we produced experiments using a simplified version of a global model called an aquaplanet-an Earth-like water-covered surface devoid of land, topography, sea-ice, or seasons. We first demonstrate that the model used in this study, whichwas specifically designed to study weather and climate, simulates a mean climate that is consistent with other aquaplanet experiments. We also demonstrate that the model is able to capture tropical rainfall variability driven by phenomena called convectively coupled Kelvin waves. Because other models do not capture this variability, we show experiments changing various components of the model to explore the influence of those components on the representation of Kelvin waves. Those experiments demonstrate that, contrary to previous perceptions, the component that approximates deep cumulus clouds is not the only responsible component. Instead, the ability to produce a certain phasing between winds and rainfall appears to explain whether a model can capture Kelvin waves.

Resolved Convection Improves the Representation of Equatorial Waves and Tropical Rainfall Variability in a Global Nonhydrostatic Model

“…Nevertheless, the fact that even the 120‐km MPAS simulated Kelvin waves supports Rios‐Berrios et al. (2020), who demonstrated that MPAS‐A is a “Kelvin‐wave friendly” model in that the model develops robust Kelvin waves regardless of the choice of cumulus or other physical parameterization schemes.…”

“…In the simulations with parameterized convection, the wave amplitude is smaller than in the observations, and the wave packets are less distinct (Figures 4c-4e). Nevertheless, the fact that even the 120-km MPAS simulated Kelvin waves supports Rios-Berrios et al (2020), who demonstrated that MPAS-A is a "Kelvin-wave friendly" model in that the model develops robust Kelvin waves regardless of the choice of cumulus or other physical parameterization schemes.…”

“…(2013) and Rios‐Berrios et al. (2020) demonstrate that the characteristics of equatorial waves are strongly affected by model formulation (of which convection parameterization is a crucial aspect), to the point that different models with different convection schemes produce completely different wave modes and precipitation patterns. Bengtsson et al.…”

“…In the present study, we demonstrate that models with explicitly resolved convection rather than parameterized convection produce more realistic equatorial waves and rainfall patterns.Model studies indicate that convection parameterization is one-if not the-dominant reason for the deficiencies in current weather and climate models. Nakajima et al (2013) and Rios-Berrios et al (2020) demonstrate that the characteristics of equatorial waves are strongly affected by model formulation (of which…”

Differences in Tropical Rainfall in Aquaplanet Simulations With Resolved or Parameterized Deep Convection

“…(2020a); Rios‐Berrios et al. (2020b), used uniform cell spacing of 120, 30, and 15 km everywhere. The fourth experiment is the first of its kind, employing a variable‐resolution mesh with 3 km cell spacing between 20°S and 20°N gradually transitioning to 15 km cell spacing poleward of 30°S and 30°N.…”