Project Overview
This project investigates the diurnal cycle of clouds and precipitation over Amazon, using km-scale model intercomparison to understand where and how current convection-permitting models or cloud-resolving models diverge from observations/LES in representing congestus and the emergence of organization, aiming to providing concrete targets for turbulence, microphysics, gray-zone physics improvements, convection over land and land-atmosphere coupling. You can also refer to this website GEWEX GoAmazon-MIP for project details
Scientific Motivation
Earth system models are entering the global storm-resolving era (grid spacing ~1-5 km), where storms are explicitly simulated and classic cumulus parameterizations is largely turned off. While km-scale models promise unprecedented realism in representing convection, precipitation, and circulation patterns, gray-zone issue at this spatial resolution persists. Some long-standing issues include the shallow to deep transition and convection organization. The Green Ocean Amazon (GoAmazon 2014/15) experiment revealed several robust diurnal modes of locally-generated deep convection (Tian et al. 2021), among which two dominant ones are: (i) a single-pulse regime with an early-afternoon peak tightly phased with surface forcing, and (ii) a double-pulse regime in which a second round of precipitation occurs with weakened surface forcing. Based on the LES study of Tian and Zhang (2025), a population of precipitating cumulus congestus was shown to moisten the middle troposphere, sustain convection under weak surface forcing, and help facilitate the upscale organization of convection later in the afternoon (Figure. 1).
In contrast, a recent process-level evaluation of a state-of-the-art km-scale SCREAM (Simple Cloud Resolving E3SM Atmosphere Model, Caldwell et al, 2021) shows abrupt shallow-to-deep transitions and an absence of midlevel congestus even at 250-m grid spacing, with delayed peaks and excessive ice water paths at 3 km resolution, which may result from missing or poorly represented physics that hinder realistic congestus development and cloud organization (Donahue et al., 2024; Bogenschutz, et al. 2025). This work highlights broader issues: lack of organized tropical convection and strong sensitivity to resolution/vertical grids that impact cloud transitions (Figure 2).
Figure 3 shows a schematic underlying the single vs double pulse case. Single-pulse case features an abrupt onset-transition-dissipation cycle of a classical diurnal case, whereas double-pulse features a gradual transition in which congestus clouds pre-moisten the environment and leave some atmospheric instability.
Therefore, we propose to have a focused, reproducible km-scale model intercomparison centered on the GoAmazon single- vs. double-pulse cases.
Science Questions
This intercomparison study aims to address several key scientific questions:
1.Transition dynamics: What physical pathways (resolved dynamics vs subgrid turbulence vs microphysics vs vertical resolution) govern the timing and nature (i.e. gradual vs abrupt) of the shallow-to-deep transition?
2. Congestus and convective maintenance: To what extent do models reproduce the congestus stage e.g., the observed trimodal vertical cloud-fraction structure (shallow–congestus–deep) seen in GoAmazon observations and LES? Why would some models bypass congestus development?
3. Organization metrics: How do models differ in their representation of the lifecycle of convective cells, their growth, merger and dissipation and the associated size evolution and distribution? What physical processes and their representations are key for models to reproduce the convective cell morphology and organization, e.g., turbulent mixing and transport, mid-troposphere moistening, cold-pool effects?
4. Environmental controls and predictability: How sensitive are the precipitation onset timing, the number of precipitation pulses, and the peak intensities to early-morning humidity and large-scale advective tendencies in different models?
Models and Configurations
Reference Large Eddy Simulation (LES): The baseline simulation will use SAM (System Atmospheric Modeling, Khairoutdinov and Randall, 2003) at 250 m resolution over a 128 x 128 km domain, consistent with the benchmark study.
Cloud Resolving Models (CRMs) (~1–5 km), and Single Column Models (SCMs): Participating models will include a range of convection-permitting or cloud-resolving models (~1–5 km), as well as SCM configurations. Examples include SCREAM in doubly periodic mode and other km-scale models contributed by partner institutions. Where available, groups are encouraged to include higher-resolution nested or limited-area simulations (100–500 m) to assess convergence behavior and contrast it with parameterized gray-zone responses. Though our focus will be on km-scale model comparison, we would also welcome participation from modeling centers that are interested in contributing SCM configuration.
Configuration: All models will use doubly periodic domains with harmonized vertical grids and I/O specifications. Shared microphysics options will be used where feasible, and aerosol assumptions will be standardized to enhance comparability of microphysical responses.
- Perturbation Suite: A small, diagnosis-oriented set of sensitivity experiments will accompany the baseline runs
- Vertical resolution refinement tests (coarse → fine), motivated by documented sensitivities in km-scale models.
- Targeted SGS turbulence adjustments (e.g., Bogenschutz et al. 2025) to test shallow-cloud depth and transition mechanisms.
- Controlled humidity-profile swaps in the boundary layer and mid-troposphere, reflecting LES-identified controls on pulse timing and maintenance.
- Optional additional cases, such as Colombia diurnal-precipitation events, may be included where relevant.
Data Products
The website provides access to forcing files, surface fluxes, and related metadata generated or used by the project.