David M. Romps

Projected increase in lightning strikes in the United States due to global warming

Lightning plays an important role in atmospheric chemistry and in the initiation of wildfires, but the impact of global warming on lightning rates is poorly constrained. Here we propose that the lightning flash rate is proportional to the convective available potential energy (CAPE) times the precipitation rate. Using observations, the product of CAPE and precipitation explains 77% of the variance in the time series of total cloud-to-ground lightning flashes over the contiguous United States (CONUS). Storms convert CAPE times precipitated water mass to discharged lightning energy with an efficiency of 1%. When this proxy is applied to 11 climate models, CONUS lightning strikes are predicted to increase 12 ± 5% per degree Celsius of global warming and about 50% over this century. Climate warming should increase the incidence of lightning strikes in the United States. Striking when hot, and more when hotter Lightning occurs more frequently when it is hotter than when it is colder, but how much more lightning should we expect as global temperatures increase? Currently there are around 25 million lightning strikes per year. Romps et al. constructed a proxy based on the energy available to make air rise in the atmosphere and on precipitation rates to model the frequency of lightning strikes across the continental United States. They predict that the number of lightning strikes will increase by about 12% for every degree of rise in global average air temperature. Science, this issue p. 851

Response of Tropical Precipitation to Global Warming

Abstract Using high-resolution cloud-resolving simulations with different CO2 concentrations, local precipitation fluxes are found to obey Clausius–Clapeyron (CC) scaling. Previous studies of the effect of CO2 concentration on precipitation extremes have used general circulation models, which are poor platforms for studying tropical convection because convection is parameterized. In idealized cloud-resolving simulations, it is possible to identify not only the changes in local precipitation fluxes, but also the factors responsible for those changes. There are many properties of convection that can change as the atmosphere warms, each of which could produce deviations from CC scaling. These properties include the effective water-vapor gradient, cloud pressure depth, and cloud velocity. A simple theory is developed that predicts the changes in these properties consistent with CC scaling. Convection in the cloud-resolving simulations is found to change as predicted by this theory, leading to an ∼20% increase...

Do Undiluted Convective Plumes Exist in the Upper Tropical Troposphere

Using a passive tracer, entrainment is studied in cloud-resolving simulations of deep convection in radiative‐convective equilibrium. It is found that the convective flux of undiluted parcels decays with height exponentially, indicating a constant probability per vertical distance of mixing with environmental air. This probability per distance is sufficiently large that undiluted updrafts are negligible above a height of 4‐5 km and virtually absent above 10 km. These results are shown to be independent of the horizontal grid size within the range of 3.2 km to 100 m. Plumes that do reach the tropopause are found to be highly diluted. An equivalent potential temperature is defined that is exactly conserved for all reversible adiabatic transformations, including those with ice. Using this conserved variable, it is shown that the latent heat of fusion (from both freezing and deposition) causes only a small increase in the level of neutral buoyancy near the tropopause. In fact, when taken to sufficiently low pressures, a parcel with an ice phase ends up colder than it would without an ice phase. Nevertheless, the contribution from fusion to a parcel’s kinetic energy is quite large. Using an ensemble of tracers, information is encoded in parcels at the cloud base and decoded where the parcel is observed in the free troposphere. Using this technique, clouds at the tropopause are diagnosed for their cloud-base temperature, specific humidity, and vertical velocity. Using these as the initial values for a Lagrangian parcel model, it is shown that fusion provides the kinetic energy required for diluted parcels to reach the tropopause.

A Direct Measure of Entrainment

A method is introduced for directly measuring convective entrainment and detrainment in a cloudresolving simulation. This technique is used to quantify the errors in the entrainment and detrainment estimates obtained using the standard bulk-plume method. The bulk-plume method diagnoses these rates from the convective flux of some conserved tracer, such as total water in nonprecipitating convection. By not accounting for the variability of this tracer in clouds and in the environment, it is argued that the bulk-plume equations systematically underestimate entrainment. Using tracers with different vertical profiles, it is also shown that the bulk-plume estimates are tracer dependent and, in some cases, unphysical. The new directmeasurement technique diagnoses entrainment and detrainment at the gridcell level without any recourse to conservedtracers.Usingthismethodinlarge-eddysimulationsofshallowanddeepconvection,itisfoundthat the bulk-plume method underestimates entrainment by roughly a factor of 2. The directly measured entrainment rates are then compared to cloud height and cloud buoyancy. Contrary to existing theories, fractional entrainment is not found to scale like the inverse of height, the cloud buoyancy, or the gradientof cloud buoyancy. On the other hand, fractional detrainment is found to scale linearly with cloud buoyancy. Finally, direct measurement is used to diagnose the spatial distribution of entrainment and detrainment during the evolution of an individual deep cumulonimbus.

The Dry-Entropy Budget of a Moist Atmosphere

The entropy budget has been a popular starting point for theories of the work, or dissipation, performed by moist atmospheres. For a dry atmosphere, the entropy budget provides a theory for the dissipation in terms of the imposed diabatic heat sources. For a moist atmosphere, the difficulties in quantifying irreversible moist processes or the value of the condensation temperature have so far frustrated efforts to construct a theory of dissipation. With this complication in mind, one of the goals here is to investigate the predictive power of the budget of dry entropy (i.e., the heat capacity times the logarithm of potential temperature). Toward this end, the dry-entropy budget is derived for an atmosphere with realistic heat capacities and a solid-water phase, features that were absent from some previous studies of atmospheric entropy. It is shown that the dry-entropy budget may be interpreted as the sum of sources and sinks from six processes, which are, in order of decreasing magnitude, radiative cooling, condensation heating, sensible heating at the surface, wind-generated frictional dissipation, lifting of water, and transport of heat from the melting line to the upper troposphere. This picture leads to an alternative explanation for the low efficiency of the moist atmospheric engine. Numerical simulations are presented from a new cloud-resolving model, Das Atmospharische Modell, which was designed to conserve energy and close the dry-entropy budget. Simulations with and without subgrid diffusion of heat and water are compared to investigate the impact of subgrid parameterizations on the terms in the dry-entropy budget. The numerical results suggest a particularly simple parameterization of wind-generated dissipation that appears to be valid for changes in sea surface temperature and mean wind. The dry-entropy budget also points to various changes in forcings and parameterizations that could be expected to increase or decrease the wind-generated dissipation.

Nature versus Nurture in Shallow Convection

Abstract Tracers are used in a large-eddy simulation of shallow convection to show that stochastic entrainment (and not cloud-base properties) determines the fate of convecting parcels. The tracers are used to diagnose the correlations between a parcel’s state above the cloud base and both the parcel’s state at the cloud base and its entrainment history. The correlation with the cloud-base state goes to zero a few hundred meters above the cloud base. On the other hand, correlations between a parcel’s state and its net entrainment are large. Evidence is found that the entrainment events may be described as a stochastic Poisson process. A parcel model is constructed with stochastic entrainment that is able to replicate the mean and standard deviation of cloud properties. Turning off cloud-base variability has little effect on the results, which suggests that stochastic mass-flux models may be initialized with a single set of properties. The success of the stochastic parcel model suggests that it holds promi...

CAPE in Tropical Cyclones

AbstractConvective available potential energy (CAPE) and the vertical distribution of buoyancy were calculated for more than 2000 dropsonde soundings collected by the NOAA Gulfstream-IV aircraft. Calculations were done with and without the effects of condensate loading, entrainment, and the latent heat of fusion. CAPE showed larger values downshear than upshear within 400 km of the center, consistent with the observed variation of convective intensity. The larger downshear CAPE arose from (i) higher surface specific humidity, (ii) lower midtropospheric temperature, and, for entraining CAPE, (iii) larger free-tropospheric relative humidity.Reversible CAPE had only one-half the magnitude of pseudoadiabatic CAPE. As shown previously, reversible CAPE with fusion closely resembled pseudoadiabatic CAPE without fusion. Entrainment had the most dramatic impact. Entraining CAPE was consistent with the observed radial distribution of convective intensity, displaying the largest values downshear at inner radii. With...

Weak Pressure Gradient Approximation and Its Analytical Solutions

AbstractA weak pressure gradient (WPG) approximation is introduced for parameterizing supradomain-scale (SDS) dynamics, and this method is compared to the relaxed form of the weak temperature gradient (WTG) approximation in the context of 3D, linearized, damped, Boussinesq equations. It is found that neither method is able to capture the two different time scales present in the full 3D equations. Nevertheless, WPG is argued to have several advantages over WTG. First, WPG correctly predicts the magnitude of the steady-state buoyancy anomalies generated by an applied heating, but WTG underestimates these buoyancy anomalies. It is conjectured that this underestimation may short-circuit the natural feedbacks between convective mass fluxes and local temperature anomalies. Second, WPG correctly predicts the adiabatic lifting of air below an initial buoyancy perturbation; WTG is unable to capture this nonlocal effect. It is hypothesized that this may be relevant to moist convection, where adiabatic lifting can r...

Sticky Thermals: Evidence for a Dominant Balance between Buoyancy and Drag in Cloud Updrafts

AbstractThe vertical velocities of convective clouds are of great practical interest because of their influence on many phenomena, including severe weather and stratospheric moistening. However, the magnitudes of forces giving rise to these vertical velocities are poorly understood, and the dominant balance is in dispute. Here, an algorithm is used to extract thousands of cloud thermals from a large-eddy simulation of deep and tropical maritime convection. Using a streamfunction to define natural boundaries for these thermals, the dominant balance in the vertical momentum equation is revealed. Cloud thermals rise with a nearly constant speed determined by their buoyancy and the standard drag law with a drag coefficient of 0.6. Contrary to suggestions that cloud thermals might be slippery, with a dominant balance between buoyancy and acceleration, cloud thermals are found here to be sticky, with a dominant balance between buoyancy and drag.

Numerical Tests of the Weak Pressure Gradient Approximation

AbstractCloud-resolving simulations of convection over a surface temperature hot spot are used to evaluate the weak pressure gradient (WPG) and weak temperature gradient (WTG) approximations. The premise of the relaxed form of WTG—that vertical velocity is equal to buoyancy times a positive time scale—is found to be violated by thick layers of negative buoyancy in steady-state ascent. The premise of WPG—that horizontal divergence and pressure anomalies are collocated—is validated by these simulations. When implemented in a cloud-resolving model, WPG replicates buoyancy transients exceptionally well, including the adiabatic lifting of air below buoyancy anomalies. WTG captures neither this effect nor the associated triggering of moist convection. For steady states, WTG produces vertical velocity profiles that are too top heavy. On the other hand, WPG generates velocity profiles that closely match fully resolved hot-spot simulations. Taken together, the evidence suggests that WPG is a relatively accurate me...