Extreme heat index rises several times faster than the air temperature

Summer 2023 broke many records, but how much of a role did global warming play? Using Texas as a case study, it is shown here that global warming has caused the most extreme values of the heat index to increase in Texas by, on average, about 8 to 11 °F.

Romps, ERL, 2024

Is Tw = 35 °C the right threshold for human survivability?

In studies, it is common to use a wet-bulb temperature of 35 °C as a threshold for human survivability. But the wet-bulb temperature is not necessarily a good proxy for heat stress. Here, we find that Tw = 35 °C underestimates hyperthermic conditions and overestimates fatal conditions.

Lu and Romps, ERL, 2023

Disputing observational evidence of warm-phase invigoration

The high-profile result of the GoAmazon campaign was purported evidence that air pollution was making storms more energetic via "warm-phase invigoration." Here, we reexamine those data and find no evidence of warm-phase invigoration. To the contrary, the data rule out any substantial effect.

Öktem, Romps, and Varble, JAS, 2023

First validation of the heat-index model with laboratory data

The heat index is widely used in the United States to warn the public about dangerous heat and humidity, but it has not been validated in the laboratory. Using laboratory data from Penn State, we find that the heat-index model accurately predicts the onset of hyperthermia, providing some needed validation.

Lu and Romps, JAP, 2023

Air pollution unable to intensify storms as hypothesized

It is believed that air pollution makes storms more intense, and a leading hypothesis for how this works would require that rising clouds have relative humidities around 110%. We show that this does not happen.

Romps, Latimer, Zhu, Jurkat-Witschas, Mahnke, Prabhakaran, Weigel, and Wendisch, GRL, 2023

Future windthrow mortality of the Amazon's trees

A leading cause of death for trees in the Amazon is windthrow, i.e., being knocked over by high winds in a storm. Global warming makes storms more intense and, as shown here, this is likely to make death by windthrow commonplace over about 50% more of the Amazon by the end of the century.

Feng, Negron-Juarez, Romps, and Chambers, Nature Comm, 2022

The heat index has been underestimated, by a lot

The National Weather Service (NWS) uses an approximation to extrapolate the heat index beyond temperatures and humidities for which it was originally calculated. Now that we can calculate those values, we find that the NWS has underestimated the heat index by as much as 20 degrees Fahrenheit.

Romps and Lu, ERL, 2022

Nuclear winter is made possible by water vapor

Scientists at a nuclear-weapons laboratory have argued that nuclear winter is impossible because the soot from urban firestorms cannot reach the stratosphere. Using theory and simulation, we show that this conclusion is incorrect. The researchers were misled by their neglect of water vapor.

Tarshish and Romps, JGR Atmospheres, 2022

Extending the heat index to work in future heat waves

The heat index is an "apparent" or "feels like" temperature used by the National Weather Service to warn of dangerously hot and humid conditions. It is based on a physiological model of human thermoregulation that becomes unphysical at conditions soon to be experienced with global warming. We fix that.

Lu and Romps, JAMC, 2022

Why the forcing from carbon dioxide is logarithmic

The forcing from carbon dioxide is logarithmic in its concentration, but competing explanations have been given for this fact. Line-by-line calculations and a simple model show that the logarithmic scaling is a consequence of the particular absorption spectrum of CO2, not the atmosphere's lapse rate.

Romps, Seeley, and Edman, J Climate, 2022

A closure for turbulent plumes

Nuclear war has the potential to severely disrupt Earth's climate (causing a so-called nuclear winter) depending on how high into the atmosphere the soot from citywide firestorms gets lofted. Here, progress is made on this problem: direct numerical simulations (DNS) are used to find the virtual origin of bouyancy-sourced plumes.

Tarshish and Romps, JAS, 2022

Reducing GCM convergence time by a factor of ten

The "bump" in equilibrium climate sensitivity (ECS) in a global aquaplanet simulation probed with 51 simulations equilibrated to different CO2 concentrations from 400 ppmv to over 200,000 ppmmv. These simulations were made computationally feasible using a rapid equilibration technique that has many potential applications.

Zhang, Bloch-Johnson, Romps, and Abbot, JAMES, 2021

Prediction for cloud spacing confirmed

Thuburn and Efstathiou (2020) predicted theoretically that the wavelength of the dominant eddies in a convective boundary layer should equal 2√2 times the boundary-layer depth. Shallow cumuli sit at the tops of those eddies, providing an opportunity to evaluate this prediction using stereo photogrammetry.

Öktem and Romps, JAS, 2021

Are shallow clouds bubbles or plumes?

Without shallow cumulus clouds, which cool Earth by reflecting sunlight to space, conditions on land would be nearly 10 °F hotter. To understand how this cooling power might change, we need to understand aspects of their lifecycle that have not yet been measured. Here, we quantify the degree to which these clouds are bubbles vs. plumes and active vs. forced.

Romps et al., JAS, 2021

Future of fire and permafrost in the Arctic

The carbon locked up in Arctic permafrost is sensitive to changes in fire and vegetation. It is found here that summer lightning in the Arctic is likely to more than double by the end of century, with implications for lightning-strike tundra wildfires and associated carbon release from permafrost.

Chen et al., Nature CC, 2021

The CACTI field campaign in Argentina

The mobile ARM Stereo Cameras, developed and overseen by our group, provide a unique dataset for CACTI: they measure the evolving envelope of the clouds as they develop during the day, providing information on turret sizes, ascent speeds, and the development of anvil clouds.

Varble, ..., Öktem, ..., Romps, ..., BAMS, 2021

Analytic solutions for convective aggregation

Prior work (Romps, 2014) derived analytic expressions for the relative humidity and lapse rate in an atmosphere with zero mean ascent (RCE). Here, the solutions are extended to positive mean ascent. These solutions explain why convective aggregation makes the convecting patch moister, the domain drier, and the lapse rate smaller.

Romps, JAS, 2020

Lecture notes on the energetics of the tropical atmosphere

These lectures, delivered at the Les Houches Summer School on the Fundamental Aspects of Turbulent Flows in Climate Dynamics, introduce simple models for the energetics of the tropical atmosphere. Topics include bulk-plume convection, convective available potential energy, gravity waves, and free-troposphere humidity.

Romps, Les Houches, 2020

ECS increases with warming in a cloud-resolving model

Cloud-resolving models are wonderful tools for exploring the effect of elevated carbon dioxide on a convecting atmosphere, but they are expensive to run. Here a technique is used to accelerate (by 30x) the equilibration of these simulations, allowing us to study how the equilibrium climate sensitivity (ECS) changes with warming.

Romps, J Climate, 2020

Will global warming increase lightning in the tropics?

Two proxies for lightning — CAPExP and ice flux — make two very different predictions for the future of tropical lightning: CAPExP predicts a large increase, while the ice flux predicts a moderate decrease. Here, we use cloud-resolving simulations to query lightning proxies over the United States and the tropics.

Romps, GRL, 2019

What do newspapers tell readers about climate change?

Although climate change is arguably the most urgent issue of our time, studies show that the general public knows little about climate science. In a quest to understand why, we investigate how often five basic climate facts are conveyed in The New York Times news articles covering climate change from 1980 to 2018.

Romps and Retzinger, ERC, 2019

Which hypothesis is correct: FAT or FiTT?

The Fixed Anvil Temperature (FAT) hypothesis is widely accepted, but we show that FAT is supported neither theoretically nor empirically. Instead, a Fixed Tropopause Temperature (FiTT) is found: for a 50-K change in sea-surface temperature, the temperature of the tropopause changes by less than 2 K.

Seeley, Jeevanjee, and Romps, GRL, 2019

Why are there tropical cloud anvils?

In the tropics, the highest frequency of cloudiness is in the upper troposphere. But, why? The conventional wisdom says that the peak in cloudiness occurs where convective detrainment is highest. Here, we argue that this is wrong. Instead, cloudiness is greatest in the cold upper troposphere because the sublimation of ice is so slow there.

Seeley, Jeevanjee, Langhans, and Romps, GRL, 2019

How good a lightning proxy is CAPE x P?

CAPE x P has been used to predict changes in lightning with global warming, but how good is this proxy? As it turns out, quite good! As shown here, the CAPE x P proxy correctly predicts the seasonal maps of lightning and its spatially varying diurnal cycle. Shown here is the hour of maximum (top) CAPE x P and (bottom) lightning for JJA.

Romps, Charn, Holzworth, Lawrence, Molinari, and Vollaro, GRL, 2018

Why does precipitation increase with warming?

Why does global precipitation increase at 2% / K? Here, we explain why. The net upwelling radiative flux F is a fixed function of temperature, F = Ft - k (T - Tt)n, where k and n are positive constants and subscript t denotes the tropopause. Since n = 2 and the depth of the troposphere is 100 K, precipitation increases at 2% / K.

Jeevanjee and Romps, PNAS, 2018

A new stereo-photogrammetric cloud observatory

Shallow cumulus clouds provide a large and uncertain feedback on global warming, but there has not been the means to quantify their individual life cycles in detail. Here, we describe a new ring of stereo cameras that is providing the needed 4D data on a 50-meter grid with a 20-second time step.

Romps and Öktem, BAMS, 2018

Radiative feedbacks accelerate tropical cyclogensis

Convective aggregation is a bizarre phenomenon where, in numerical simulations, rain clouds clump together into a single blob. How relevant is this numerical oddity to the real world? In this study, the feedbacks at work are shown to dramatically accelerate the formation of hurricanes, pointing to a real-world impact.

Muller and Romps, PNAS, 2018

An exact analytic expression for the LCL

For nearly 200 years, atmospheric scientists have relied on approximations to the height of the lifting condensation level (LCL). Here, the exact, explicit, and analytic expression for the LCL is derived, as is an analagous expression for the lifting deposition level (LDL). These expressions depend only on basic physical parameters.

Romps, JAS, 2017

How quickly do gravity-waves dissipate?

What happens to convective heating after it is deposited in the troposphere? This question is fundamental to atmospheric dynamics, but the only known solutions have assumed an artifical rigid lid at the tropopause. This paper presents the first Green's function for buoyancy in a troposphere overlain by a realistic stratosphere.

Edman and Romps, JAS, 2017

How to infer cloud sizes from chord measurements

A cloud's size plays an important role in its dynamics, its ability to form rain, and its radiative forcing. But, the distribution of cloud sizes can be difficult to measure. This paper explores ways to calculate the size distribution from linear sampling, e.g., from vertically pointing lidar or from aircraft.

Romps and Vogelmann, JAS, 2017

The first analytic solution for RCE and CAPE

Severe weather is predicted to become more frequent with global warming, but those predictions are based on warming-induced increases in CAPE, for which we have had no theory. This paper presents the first analytical expression for CAPE. It explains the Clausius-Clapeyron (CC) scaling of CAPE, and its divergence from CC scaling at very warm temperatures.

Romps, JAS, 2016

Ice processes have no practical influence on CAPE

On average, the largest cloud buoyancies occur in the upper troposphere. Common wisdom states that this results from the extra buoyancy provided by the latent heat of fusion. This paper shows that this is a fallacy: the profiles of convective buoyancy and vertical velocity are the same in a world with and without ice.

Seeley and Romps, GRL, 2016

Introducing the Stochastic Parcel Model

In global climate models, the poor treatment of "entrainment" (the mixing of clear air into clouds) is the largest source of uncertainty in how much the Earth is expected to warm. This paper describes the Stochastic Parcel Model, a convective parameterization that treats entrainment the way it operates in nature and that is also deterministic and fast enough for use in global climate models.

Romps, JAMES, 2016

An analytic model for cold pools

Many aspects of Earth's climate (e.g., cloud cover, precipitation, and radiation fluxes) depend on the sizes and lifetimes of cold pools, which can trigger storm clouds when they collide. Here, we present analytical expressions for the evolution of cold pools, including their maximum sizes and lifetimes. These results are successfully validated with large-eddy simulations.

Romps and Jeevanjee, QJRMS, 2016

Why does CAPE increase with warming?

The energy available to storms, which is measured by CAPE, is predicted to increase dramatically with warming, leading to more severe storms and more frequent lightning. But, why does this increase in CAPE occur? We show that the increases in CAPE are driven by a ballooning of upper-tropospheric buoyancy, which is correctly predicted by the zero-buoyancy plume model.

Seeley and Romps, GRL, 2016

Why is it hard for bouyant air to rise off the surface?

Stationary buoyant parcels of fluid do not accelerate at a rate equal to their buoyancy. Instead, they accelerate at a smaller rate, which is found by solving a Poisson equation. This paper shows that the proximity of buoyant parcels to the surface greatly impacts their acceleration. This finding has implications for the triggering of new thermals, which originate at the surface.

Jeevanjee and Romps, QJRMS, 2016

Why are the edges of cold pools so humid?

Rings of high-humidity air at the edges of cold pools serve as important precursors for deep convection. But, how do those rings form? The conventional wisdom is that evaporation of precipitation is responsible. Here, we test this idea and find, instead, that enhanced surface fluxes are the true source of the high-humidity rings.

Langhans and Romps, GRL, 2015

Stereo photogrammetry confirms thermals are sticky

Stereo photogrammetry allows us to make quantitative three-dimensional measurements of cloud sizes and speeds. Here, data on dozens of cloud thermals are used to show that drag -- recently argued to be negligible -- is, to the contrary, a dominant term in their momentum balance, and wave drag is the likely culprit.

Romps and Öktem, GRL, 2015

New clouds are birthed with a shove

What physical process gives birth to convective clouds? There are two possibilities: warm and humid air launches off the surface due to its buoyancy, or that air gets pushed up by colder air that collides with it. Here, we cleanly decompose the forces into those due to buoyancy and inertia to find the answer.

Jeevanjee and Romps, JAS, 2015

Thermals are sticky, not slippery

Vertical velocities in clouds are important for many phenomena, including hail, lightning, and stratospheric moistening, but there is no consensus on what sets those speeds. In this paper, we identify whether clouds are slippery (acceleration equals buoyancy) or sticky (buoyancy equals drag).

Romps and Charn, JAS, 2015

Forecasting a warming-induced increase in severe weather

GCMs disagree on the answer to the following question: will global warming cause an increase or decrease in summertime severe weather over the United States? In pursuit of a resolution, we test a hypothesis that the models that do a good job of simulating today's severe weather will be in better agreement about the future of summertime severe weather.

Seeley and Romps, J Climate, 2015

Lightning will increase over U.S. by 50% by 2100

For hundreds of millions of years, lightning has shaped the evolution of terrestrial species through its triggering of wildfires. In this study, we validate a new proxy for lightning and use it in global climate models to forecast that global warming is likely to generate a 50% increase in U.S. lightning strikes over the 21st century.

Romps, Seeley, Vollaro, and Molinari, Science, 2014

Tracking individual water molecules in an LES

The relative humidity of the atmosphere depends on the precipitation efficiency (PE) of convective clouds. In this study, we measure precipitation efficiency at a new level of detail using water-molecule-following Lagrangian particles. The results show how the PE of water vapor varies depending on where it enters the cloud.

Langhans, Yeo, and Romps, JAS, 2015

An analytical expression for free-tropospheric humidity

Water vapor is the dominant greenhouse gas in the atmosphere, but what processes set its concentration? This paper provides a simple set of equations for relative humidity and its changes with global warming. In particular, it predicts that relative humidity will remain a fixed function of temperature as the atmosphere warms.

Romps, J Climate, 2014

Description and validation of stereo photogrammetry

Using methods from the field of computer vision, this paper details how to reconstruct oceanic clouds in 3D using two digital cameras. These techniques are then applied, and validated, using a pair of stereo cameras in Miami, Florida.

Öktem, Prabhat, Lee, Thomas, Zuidema, and Romps, JAOT, 2014

An improved method for single-column modeling

This paper eliminates a spurious gravity-wave resonance in a method used to test convective parameterizations for GCMs. The resonance appears as the dashed-blue spike in the figure, while the solid-red curve shows the new scheme, which matches the solid-black benchmark.

Edman and Romps, JAS, 2014

An analytical model for cumulus damping of wind shear

In this work, analytical solutions are found for the descent and damping of vertical features in a moist-convecting atmosphere. It is shown that wind profiles with large wavelengths damp slower and descend slower than those with short wavelengths.

Romps, JAS, 2014

Cold pools frustrate convective aggregation

This study finds that cold pools are responsible for the abrupt transition to convective aggregation as domain size increases. In their absence, aggregation occurs at all scales.

Jeevanjee and Romps, GRL, 2013

Evidence for rapid recycling of air in and out of clouds

How can we reconcile the low entrainment rates obtained from mass-flux budgets with the high values obtained from direct measurement? Using Lagrangian particles in a large-eddy simulation of cumulus congestus, we find that clouds rapidly entrain both environmental air and their own detritus, reconciling the two values.

Yeo and Romps, JAS, 2013

Conceptual errors in convective-momentum-transport schemes

Global climate models use parameterizations to model the effect of convection on the atmosphere's momentum budget. This paper finds that a very simple scheme, which neglects the horizontal pressure force, performs better than the schemes currently used in GCMs.

Romps, JAS, 2012

WPG captures the phenomenon of steady cold-air ascent

Cloud-resolving simulations are used to validate the underlying theory and predictions of the weak pressure gradient (WPG) scheme in this paper. WPG is shown to be able to model the stacked layers of negative buoyancy in regions of steady-state ascent.

Romps, JAS, 2012b

Analytical arguments for choosing WPG over WTG

The weak pressure gradient (WPG) scheme is an alternative to the weak temperature gradient (WTG) scheme for modeling a column of atmosphere coupled to its environment via gravity waves. This paper uses pencil-and-paper derivations to argue that WPG is more accurate than WTG.

Romps, JAS, 2012a

Why is the distribution of lightning so asymmetric in hurricanes?

This study provides an explanation for the asymmetrical distribution of lightning in hurricanes. Using data from thousands of dropsondes, condensate loading and entrainment are found to be key mechanisms controlling convective ascent.

Molinari, Romps, Vollaro, and Nguyen, JAS, 2012

Deep convection originates as air near the surface

This study finds that boundary-layer parcels detrained by clouds in the free troposphere come primarily from within 100 meters of the surface. This has important implications for how convective instability is calculated in the atmosphere.

Romps and Kuang, JAS, 2011

The first "global" warming experiment in an LES

This is the first study of precipitation extremes in a high-resolution cloud-resolving model. Contrary to results from GCMs, convective updrafts become more vigorous when CO2 is doubled, contributing to higher rain rates.

Romps, JAS, 2011

What controls the HDO composition of the stratosphere?

Here, we report on the first investigation of stratospheric HDO using a steady-state cloud-resolving simulation. Simulating a Walker cell over an 8000-km-wide domain, the convective injection of ice and the generation of cirrus by gravity waves are found to be dominant controls on HDO in the stratosphere.

Blossey, Kuang, and Romps, JGR, 2010

The first direct measurement of entrainment in an LES

The process of convective entrainment has enormous influence on clouds and the Earth's climate sensitivity, but it has not been possible to measure entrainment directly -- at the grid-cell level -- in large-eddy simulations (LES). This paper reports on the first technique for making this measurement at the native resolution of the simulation.

Romps, JAS, 2010

How frequently do hurricanes punch into the stratosphere?

Tropical cyclones, called hurricanes in the North Atlantic, have some of the most intense convection in the world, capable of reaching up into the stratosphere. In this study, we quantify the fraction of convective excursions into the stratosphere that are caused by tropical cyclones.

Romps and Kuang, GRL, 2009

Nurture rules over nature with regards to entraining clouds

Within an ensemble of clouds and even within a single cloud, parcels of cloudy air detrain at a variety of different heights. What causes this spectrum of behavior: differences in the properties of those parcels at the cloud base, or differences in how they entrain above the cloud base? This paper answers that question.

Romps and Kuang, JAS, 2010