Interpreting Seasonal and Interannual Hadley Cell Descending Edge Migrations via the Cell-Mean Rossby Number

Journal of Climate · 2025-06-27

Abstract The poleward extent of Earth’s zonal-mean Hadley cells varies across seasons and years, which would be nice to capture in a simple theory. A plausible, albeit diagnostic, candidate from Hill et al. combines the conventional two-layer, quasigeostrophic, baroclinic instability-based framework with a less conventional assumption that each cell’s upper-branch zonal winds are suitably captured by a single, cell-wide Rossby number, with meridional variations in the local Rossby number neglected. We test this theory against ERA5 reanalysis data, finding that it captures both seasonal and interannual variations in the Hadley cell zonal winds and poleward extent fairly well. For the seasonal cycle of the Northern Hemisphere (NH) cell poleward edge only, this requires empirically lagging the prediction by 1 month, for reasons unclear to us. In all cases, the bulk Rossby number value that yields the most accurate zonal wind fields is approximately equal to the actual, diagnosed cell-mean value. Variations in these cell-mean Rossby numbers, in turn, predominantly drive variations in each cell’s poleward extent. All other terms matter much less—including the subtropical static stability, which, by increasing under global warming, is generally considered the predominant driver of future Hadley cell expansion. These results argue for developing a predictive theory for the cell-mean Rossby number and for diagnosing its role in climate model projections of future Hadley cell expansion.

Titan's weather, climate, and paleoclimate

Elsevier eBooks · 2025-01-01 · 1 citations

Contributors

Elsevier eBooks · 2025-01-01

Clear‐Sky Convergence, Water Vapor Spectroscopy, and the Origin of Tropical Congestus Clouds

AGU Advances · 2025-02-01 · 2 citations

Abstract Congestus clouds, characterized by their vertical extent into the middle troposphere, are widespread in tropical regions and play an important role in Earth's climate system. However, fundamental questions regarding their formation and prevalence remain unanswered. Here, we endeavor to answer how congestus cloud tops form by detraining preferentially at altitudes between 5 and 6 km and why this detraining outflow is invigorated by drier mid‐tropospheric conditions. We construct a clear‐sky radiative‐convective framework of congestus cloud‐top formation that is grounded in the discovery of an important spectroscopic property of water vapor. In this mass‐ and energy‐conserving framework, convective detrainment maximizes at a height of 5 and 6 km due to a swift decline in radiative cooling in clear‐sky regions. This decline is, in turn, a consequence of water vapor spectroscopy: more specifically, a drop in the number of strong absorption lines in the water vapor rotation band. In a simple spectral model, we link this spectroscopic property to the shape of the rotation band, which can be approximated as the product of a power law and a sine wave representing the band's deviation from statistical log‐linearity. The characteristic “C”‐shaped relative humidity profile in the tropics further strengthens the outflow in drier mid‐level conditions by amplifying vertical decreases in the clear‐sky cooling rate. Essential to this process are strong RH gradients, which are most pronounced under the driest conditions and induce a vertical decrease in the optical depth lapse rate across the mid‐troposphere.

Interpreting seasonal and interannual Hadley cell descending edge migrations via the cell-mean Rossby number

arXiv (Cornell University) · 2024-11-21

The poleward extent of Earth's zonal-mean Hadley cells varies across seasons and years, which would be nice to capture in a simple theory. A plausible, albeit diagnostic, candidate from Hill et al (2022) combines the conventional two-layer, quasi-geostrophic, baroclinic instability-based framework with a less conventional assumption: that each cell's upper-branch zonal winds are suitably captured by a single, cell-wide Rossby number, with meridional variations in the local Rossby number neglected. We test this theory against ERA5 reanalysis data, finding that it captures both seasonal and interannual variations in the Hadley cell zonal winds and poleward extent fairly well. For the seasonal cycle of the NH cell poleward edge only, this requires empirically lagging the prediction by one month, for reasons unclear to us. In all cases, the bulk Rossby number value that yields the most accurate zonal wind fields is approximately equal to the actual, diagnosed cell-mean value. Variations in these cell-mean Rossby numbers, in turn, predominantly drive variations in each cell's poleward extent. All other terms matter much less -- including the subtropical static stability, which, by increasing under global warming, is generally considered the predominant driver of future Hadley cell expansion. These results argue for developing a predictive theory for the cell-mean Rossby number and for diagnosing its role in climate model projections of future Hadley cell expansion.

Clear-sky convergence and the origin of tropical congestus clouds

2024-06-10

Congestus clouds, characterized by their vertical extent into the middle troposphere, are widespread in tropical regions and play an important role in Earth's climate system by their contribution to cloud radiative forcing, atmospheric humidification, and surface rainfall. However, their spatial distribution-in particular the abundance of stratiform clouds sourced by the outflow from congestus cloud tops is inaccurately captured by state-of-the-art climate models, suggesting that fundamental questions regarding their formation, dynamics, and climate impact remain unanswered. Here, we demonstrate the existence of a clear-sky water vapor absorption feature that lends insight into how congestus cloud tops form by detraining preferentially at altitudes between 5-6 km and why they are more prevalent in dry mid-tropospheric conditions. Convective detrainment maximizes at a height of 5-6 km due to a swift decline in radiative cooling in clear-sky regions. This decline is, in turn, a consequence of the absorption feature: more specifically, a non-uniform density of strong absorption lines in the water vapor rotation band. The increased prevalence of congestus clouds in drier mid-tropospheric conditions may be due to stronger vertical gradients in the clear-sky cooling rate, which lead to stronger outflow at 5-6 km. We speculate that, in partnership with stability and entrainment, radiation could significantly and systematically influence mid-tropospheric buoyancy and therefore congestus cloud top formation.

A Simple Model for the Emergence of Relaxation‐Oscillator Convection

Journal of Advances in Modeling Earth Systems · 2024-11-01 · 6 citations

Abstract Earth's tropics are characterized by quasi‐steady precipitation with small oscillations about a mean value, which has led to the hypothesis that moist convection is in a state of quasi‐equilibrium (QE). In contrast, very warm simulations of Earth's tropical convection are characterized by relaxation‐oscillator‐like (RO) precipitation, with short‐lived convective storms and torrential rainfall forming and dissipating at regular intervals with little to no precipitation in between. We develop a model of moist convection by combining a zero‐buoyancy model of bulk‐plume convection with a QE heat engine model, and we use it to show that QE is violated at high surface temperatures. We hypothesize that the RO state emerges when the equilibrium condition of the convective heat engine is violated, that is, when the heating rate times a thermodynamic efficiency exceeds the rate at which work can be performed. We test our hypothesis against one‐ and three‐dimensional numerical simulations and find that it accurately predicts the onset of RO convection. The proposed mechanism for RO emergence from QE breakdown is agnostic of the condensable, and can be applied to any planetary atmosphere undergoing moist convection. To date, RO states have only been demonstrated in three‐dimensional convection‐resolving simulations, which has made it seem that the physics of the RO state requires simulations that can explicitly resolve the three‐dimensional interaction of cloudy plumes and their environment. We demonstrate that RO states also exist in single‐column simulations of radiative‐convective equilibrium with parameterized convection, albeit in a different surface temperature range and with much longer storm‐free intervals.

Seasons, Shorelines, and Their Effects on the Tropical Circulation and Hydrological Cycle

Journal of the Atmospheric Sciences · 2024-06-28 · 1 citations

Abstract This work is a direct continuation of McKinney et al., who attempted to create a planet with Earth-like temperatures and physical properties but with precipitation and circulation patterns that were Titan-like. McKinney et al. attempted to do so by changing only three basic planetary parameters: the ratio of dry land to ocean on the surface, the rotation period, and the volatility of the condensable. Each of these parameters is varied from an Earth-like value to a Titan-like one to analyze the climate transition between these two planetary archetypes. In this work, we expand on McKinney et al. by including a seasonal cycle and increasing the number of diagnostic criteria for determining Titan-like dynamics. The simulations use Earth-like obliquity and an Earth-like solar constant. We find that the presence of a dry land strip extending to at least 55°N/S is most effective at creating Titan-like climatic conditions on an otherwise Earth-like planet, such as high-latitude summer precipitation maxima and a low-humidity equator. In contrast, slow rotation and high atmospheric vapor abundance have minimal climatic impacts despite being characteristic features of Titan. Our experiments show that it is not difficult to produce distinctly Titan-like features in an Earth-like GCM with minimal changes to its fundamental parameters. This suggests that Earth-like planets could have a large range of global climate states throughout their history just through changes in topography. Similarly, Titan may have experienced more Earth-like climate states in periods where its tropics were wetter.

Data and code for Spaulding-Astudillo and Mitchell (2023c), "Clear-sky convergence and the origin of tropical congestus clouds"

Zenodo (CERN European Organization for Nuclear Research) · 2023-11-15 · 1 citations

Data and code for Spaulding-Astudillo and Mitchell (2023c), "Clear-sky convergence and the origin of tropical congestus clouds". The Python code used to create Figures 1, 3-6 and supplementary Figure 1 are included in separate directories. Within each directory, we include experimental data generated by the Reference Forward Model (RFM; Dudhia 2017) that is needed to reproduce the figures. To create the figures, run the provided code in a Jupyter notebook or from the command line. Helper functions for data processing within the figure scripts have the nomenclature ..._data.py. Note that the provided code only allows you to visualize RFM data, not produce it yourself. To do so, you must download and run RFM yourself (https://eodg.atm.ox.ac.uk/RFM/). In addition, Figure 1 requires spatio-temporally averaged data from MERRA-2, ERA5, and CloudSat/CALIPSO (Betrand et al. 2023), which we provide in pickle files. See the main text for data citations, which will allow you to access the original, non-averaged climate data. RFM data directories have the nomenclature /ctrl-..... For example, ctrl-RHmid-75-zmid-7-uniform-1 where:RHmid=75% is the relative humidity at a mid-tropospheric height zmid=7.5 km. Uniform=True indicates that the tropospheric relative humidity is constant with height. When uniform=False, the relative humidity follows a C-shaped distribution between the surface and the tropopause. The helper functions in ..._data.py allow you to read the RFM output from .asc files. The nomenclature of the RFM output files is standardized: https://eodg.atm.ox.ac.uk/RFM/.

Data for Spaulding-Astudillo and Mitchell (2023), "Effects of varying saturation vapor pressure on climate, clouds, and convection"

Zenodo (CERN European Organization for Nuclear Research) · 2023-04-10

Data for Spaulding-Astudillo and Mitchell (2023), "Effects of varying saturation vapor pressure on climate, clouds, and convection" The main directories have a common nomenclature: e.g., TEST_1360_FSC_288K_1p0, where 1360 is the insolation, FSC indicates that it is a full-sky radiation run, 288 K is the initial surface temperature, and the multiplicative factor on the saturation vapor pressure of water is 1p0, meaning 1.0. In every directory, each .nc file contains 5 years of model output. There are 5 types of .nc files, which correspond to different output streams of ECHAM6. The main output stream is ..._echam.nc.