Martin S. Singh

Influence of microphysics on the scaling of precipitation extremes with temperature

Simulations of radiative-convective equilibrium with a cloud-system resolving model are used to investigate the scaling of high percentiles of the precipitation distribution (precipitation extremes) over a wide range of surface temperatures. At surface temperatures above roughly 295 K, precipitation extremes increase with warming in proportion to the increase in surface moisture, following what is termed Clausius-Clapeyron (CC) scaling. At lower temperatures, the rate of increase of precipitation extremes depends on the choice of cloud and precipitation microphysics scheme and the accumulation period, and it differs markedly from CC scaling in some cases. Precipitation extremes are found to be sensitive to the fall speeds of hydrometeors, and this partly explains the different scaling results obtained with different microphysics schemes. The results suggest that microphysics play an important role in determining the response of convective precipitation extremes to warming, particularly when ice- and mixed-phase processes are important.

Upward Shift of the Atmospheric General Circulation under Global Warming: Theory and Simulations

Many features of the general circulation of the atmosphere shift upward in response to warming in simulations of climate change with both general circulation models (GCMs) and cloud-system-resolving models. The importance of the upward shift is well known, but its physical basis and the extent to which it occurs coherently across variables are not well understood. A transformation is derived here that shows how an upward shift of a solution to the moist primitive equations gives a new approximate solution with higher tropospherictemperatures.Accordingtothetransformation, allvariables shiftupwardwith warmingbutwith an additional modification to the temperature and a general weakening of the pressure velocity. The applicability of the vertical-shift transformation is explored using a hierarchy of models from adiabatic parcel ascents to comprehensive GCMs. The transformation is found to capture many features of the response to climate change in simulations with an idealized GCM, including the mid- and upper-tropospheric changes in lapse rate, relative humidity, and meridional wind. The transformation is less accurate when applied to simulations with more realistic GCMs, but it nonetheless captures some important features. Deviations from the simulated response are primarily due to the surface boundary conditions, which do not necessarily conform to the transformation, especially in the case of the zonal winds. The results allow for a physical interpretation of the upward shift in terms of the governing equations and suggest that it may be thought of as a coherent response of the general circulation of the mid- and upper troposphere.

Extratropical cyclones in idealized simulations of changed climates

AbstractCyclones are a key element of extratropical weather and frequently lead to extreme events like wind storms and heavy precipitation. Understanding potential changes of cyclone frequency and intensity is thus essential for a proper assessment of climate change impacts. Here the behavior of extratropical cyclones under strongly varying climate conditions is investigated using idealized climate model simulations in an aquaplanet setup. A cyclone tracking algorithm is applied to assess various statistics of cyclone properties such as intensity, size, lifetime, displacement velocity, and deepening rates. In addition, a composite analysis of intense cyclones is performed. In general, the structure of extratropical cyclones in the idealized simulations is very robust, and changes in major cyclone characteristics are relatively small. Median cyclone intensity, measured in terms of minimum sea level pressure and lower-tropospheric relative vorticity, has a maximum in simulations with global mean temperature...

The Relative Humidity in an Isentropic Advection-Condensation Model: Limited Poleward Influence and Properties of Subtropical Minima

AbstractAn idealized model of advection and condensation of water vapor is considered as a representation of processes influencing the humidity distribution along isentropic surfaces in the free troposphere. Results are presented for how the mean relative humidity distribution varies in response to changes in the distribution of saturation specific humidity and in the amplitude of a tropical moisture source. Changes in the tropical moisture source are found to have little effect on the relative humidity poleward of the subtropical minima, suggesting a lack of poleward influence despite much greater water vapor concentrations at lower latitudes. The subtropical minima in relative humidity are found to be located just equatorward of the inflection points of the saturation specific humidity profile along the isentropic surface. The degree of mean subsaturation is found to vary with the magnitude of the meridional gradient of saturation specific humidity when other parameters are held fixed.The atmospheric re...

Increasing potential for intense tropical and subtropical thunderstorms under global warming

Significance A substantial fraction of the world’s most intense thunderstorms occur in the tropics and subtropics, but the response of such storms to climate change remains uncertain. Here, we show that, in simulations of global warming, a measure of the energy available to thunderstorms increases robustly across the tropics and subtropics. Furthermore, we elucidate an important mechanism contributing to such increases in available energy, and we present observational evidence that this mechanism is present in Earth’s atmosphere. By combining theory, observations, and models, our results provide confidence in climate model projections of future intense thunderstorm potential; such model projections are shown to imply large future increases in the frequency of damaging thunderstorm environments in tropical and subtropical regions. Intense thunderstorms produce rapid cloud updrafts and may be associated with a range of destructive weather events. An important ingredient in measures of the potential for intense thunderstorms is the convective available potential energy (CAPE). Climate models project increases in summertime mean CAPE in the tropics and subtropics in response to global warming, but the physical mechanisms responsible for such increases and the implications for future thunderstorm activity remain uncertain. Here, we show that high percentiles of the CAPE distribution (CAPE extremes) also increase robustly with warming across the tropics and subtropics in an ensemble of state-of-the-art climate models, implying strong increases in the frequency of occurrence of environments conducive to intense thunderstorms in future climate projections. The increase in CAPE extremes is consistent with a recently proposed theoretical model in which CAPE depends on the influence of convective entrainment on the tropospheric lapse rate, and we demonstrate the importance of this influence for simulated CAPE extremes using a climate model in which the convective entrainment rate is varied. We further show that the theoretical model is able to account for the climatological relationship between CAPE and a measure of lower-tropospheric humidity in simulations and in observations. Our results provide a physical basis on which to understand projected future increases in intense thunderstorm potential, and they suggest that an important mechanism that contributes to such increases may be present in Earth’s atmosphere.

Scaling of the entropy budget with surface temperature in radiative-convective equilibrium

The entropy budget of the atmosphere is examined in simulations of radiative-convective equilibrium with a cloud-system resolving model over a wide range of surface temperatures from 281 to 311 K. Irreversible phase changes and the diffusion of water vapor account for more than half of the irreversible entropy production within the atmosphere, even in the coldest simulation. As the surface temperature is increased, the atmospheric radiative cooling rate increases, driving a greater entropy sink that must be matched by greater irreversible entropy production. The entropy production resulting from irreversible moist processes increases at a similar fractional rate as the entropy sink and at a lower rate than that implied by Clausius-Clapeyron scaling. This allows the entropy production from frictional drag on hydrometeors and on the atmospheric flow to also increase with warming, in contrast to recent results for simulations with global climate models in which the work output decreases with warming. A set of approximate scaling relations is introduced for the terms in the entropy budget as the surface temperature is varied, and many of the terms are found to scale with the mean surface precipitation rate. The entropy budget provides some insight into changes in frictional dissipation in response to warming or changes in model resolution, but it is argued that frictional dissipation is not closely linked to other measures of convective vigor.

Eddy Influences on the Strength of the Hadley Circulation: Dynamic and Thermodynamic Perspectives

AbstractThe strength of the equinoctial Hadley circulation (HC) is investigated in idealized simulations conducted on an equatorial beta plane in which the zonal width of the domain is varied to either permit or suppress large-scale eddies. The presence of such eddies is found to amplify the HC by a factor of 2–3 in simulations with slab-ocean boundary conditions or with a simple representation of ocean heat transport. Additional simulations in which the eddy forcing is prescribed externally indicate that this amplification is primarily associated with large-scale eddy momentum fluxes rather than large-scale eddy heat fluxes. These results contrast with results from simulations with fixed distributions of sea surface temperature (SST), in which the HC strength has been found to be relatively insensitive to large-scale eddy momentum fluxes.In both the interactive- and fixed-SST cases, the influence of nonlinear momentum advection by the mean flow complicates efforts to use the angular-momentum budget to co...

Exploring the Role of Eddy Momentum Fluxes in Determining the Characteristics of the Equinoctial Hadley Circulation: Fixed-SST Simulations

AbstractThe influence of eddy momentum fluxes on the equinoctial Hadley circulation is explored using idealized simulations on an equatorial beta plane in which the sea surface temperature (SST) distribution is fixed. By comparing simulations run in a wide-domain configuration, in which large-scale eddies are present, to simulations in which the model domain is too narrow to permit baroclinic instability, the role of large-scale eddies in determining the characteristics of the Hadley circulation is elucidated. The simulations also include an explicit representation of deep convection, allowing for an evaluation of the influence of convective momentum transport on the zonal-mean circulation.The simulated eddy momentum fluxes are much weaker in the narrow-domain configuration than in the wide-domain case, and convective momentum transport is found to be of secondary importance. As a result, many characteristics of the narrow-domain Hadley circulation are well described by axisymmetric theory and differ from...

Future Community Efforts in Understanding and Modeling Atmospheric Processes

What: About 160 participants from some 20 countries, representing the weather and climate modeling community, held an international workshop to overview the progress in understanding and modeling atmospheric processes and to discuss promising ideas for future community projects. When: 26 February–2 March 2018 Where: Lorne, Victoria, Australia FUTURE COMMUNITY EFFORTS IN UNDERSTANDING AND MODELING ATMOSPHERIC PROCESSES

Viscous effects in two-layer, unidirectional hydraulic flow

Hydraulic equations are derived for a stratified (two-layer) flow in which the horizontal velocity varies continuously in the vertical. Viscosity is included in the governing equations, and the effect of friction in hydraulically controlled flows is examined. The analysis yields Froude numbers which depend upon the integrated inverse square of velocity but reduce to the original layered Froude numbers when velocity is constant with depth. The Froude numbers reveal a critical condition for hydraulic control, which equates to the arrest of internal gravity waves. Solutions are presented for the case of unidirectional flow through a lateral constriction, both with and without bottom drag. In the free-slip lower boundary case, viscosity transports momentum from the faster to the slower layer, thereby shifting the control point downstream and reducing the flux through the constriction. However, while the velocity shear at the interface between the two layers is reduced, the top-to-bottom velocity difference of the controlled solution is increased for larger values of viscosity. This counter-intuitive result is due to the restrictions placed on the flow at the hydraulic control point. When bottom drag is included in the model, the total flux may increase, in some cases exceeding that of the inviscid solution.