Effects Of Moist Convection Research Articles

Tropical cyclone flow asymmetries induced by a uniform flow revisited

AbstractWe investigate the hypothesized effects of a uniform flow on the structural evolution of a tropical cyclone using a simple idealized, three‐dimensional, convection‐permitting, numerical model. The study addresses three outstanding basic questions concerning the effects of moist convection on the azimuthal flow asymmetries and provides a bridge between the problem of tropical cyclone intensification in a quiescent environment and that in vertical shear over a deep tropospheric layer. At any instant of time, explicit deep convection in the model generates flow asymmetries that tend to mask the induced flow asymmetries predicted by the dry, slab boundary layer model of Shapiro, whose results are frequently invoked as a benchmark for characterizing the boundary layer‐induced vertical motion for a translating storm. In sets of ensemble experiments in which the initial low‐level moisture field is randomly perturbed, time‐averaged ensemble mean fields in the mature stage show a coherent asymmetry in the vertical motion rising into the eyewall and in the total (horizontal) wind speed just above the boundary layer. The maximum ascent occurs about 45° to the left of the vortex motion vector, broadly in support of Shapiro's results, in which it occurs ahead of the storm, and consistent with one earlier more complex numerical calculation by Frank and Ritchie. The total wind asymmetry just above the boundary layer has a maximum in the forward right sector, which is in contrast to the structure effectively prescribed by Shapiro based on an inviscid dry symmetric vortex translating in a uniform flow where, in an Earth‐relative frame, the maximum is on the right.

Impacts of Convective Lifetime on Moist Geostrophic Adjustment

Abstract This paper presents a theoretical study of the effects of moist convection on geostrophic adjustment in an infinite channel. The governing equations correspond to a linearized shallow water system of equations for the atmosphere first vertical baroclinic mode, which is coupled to a vertically averaged moisture equation. The coupling is through a parameterization that represents precipitation. The transient behavior and final state of the flow initially at rest with active precipitation limited to half of the channel is investigated, both numerically and analytically. It is shown that an initial imbalance resulting from precipitation induces a circulation that dries out the nonprecipitating region and further enhances precipitation. This interaction between precipitation and dynamics leads to a sharper temperature gradient and stronger jet in the final state, when compared to the dry adjustment. Unlike in the dry case, the moist geostrophic adjustment cannot be entirely determined from the initial unbalanced flow, since it depends on the time scale for convection. Analytic approximations are derived in limits of both fast and slow convective adjustment time.

Effects of Moist Convection on Hurricane Predictability

Abstract This study exemplifies inherent uncertainties in deterministic prediction of hurricane formation and intensity. Such uncertainties could ultimately limit the predictability of hurricanes at all time scales. In particular, this study highlights the predictability limit due to the effects on moist convection of initial-condition errors with amplitudes far smaller than those of any observation or analysis system. Not only can small and arguably unobservable differences in the initial conditions result in different routes to tropical cyclogenesis, but they can also determine whether or not a tropical disturbance will significantly develop. The details of how the initial vortex is built can depend on chaotic interactions of mesoscale features, such as cold pools from moist convection, whose timing and placement may significantly vary with minute initial differences. Inherent uncertainties in hurricane forecasts illustrate the need for developing advanced ensemble prediction systems to provide event-dependent probabilistic forecasts and risk assessment.

Numerical Simulations of Forced Shallow-Water Turbulence: Effects of Moist Convection on the Large-Scale Circulation of Jupiter and Saturn

AbstractTo test the hypothesis that the zonal jets on Jupiter and Saturn result from energy injected by thunderstorms into the cloud layer, forced-dissipative numerical simulations of the shallow-water equations in spherical geometry are presented. The forcing consists of sporadic, isolated circular mass pulses intended to represent thunderstorms; the damping, representing radiation, removes mass evenly from the layer. These results show that the deformation radius provides strong control over the behavior. At deformation radii <2000 km (0.03 Jupiter radii), the simulations produce broad jets near the equator, but regions poleward of 15°–30° latitude instead become dominated by vortices. However, simulations at deformation radii >4000 km (0.06 Jupiter radii) become dominated by barotropically stable zonal jets with only weak vortices. The lack of midlatitude jets at a small deformation radii results from the suppression of the beta effect by column stretching; this effect has been previously documented in the quasigeostrophic system but never before in the full shallow-water system. In agreement with decaying shallow-water turbulence simulations, but in disagreement with Jupiter and Saturn, the equatorial flows in these forced simulations are always westward. In analogy with purely two-dimensional turbulence, the size of the coherent structures (jets and vortices) depends on the relative strengths of forcing and damping; stronger damping removes energy faster as it cascades upscale, leading to smaller vortices and more closely spaced jets in the equilibrated state. Forcing and damping parameters relevant to Jupiter produce flows with speeds up to 50–200 m s−1 and a predominance of anticyclones over cyclones, both in agreement with observations. However, the dominance of vortices over jets at deformation radii thought to be relevant to Jupiter (1000–3000 km) suggests that either the actual deformation radius is larger than previously believed or that three-dimensional effects, not included in the shallow-water equations, alter the dynamics in a fundamental manner.

Effects of Moist Convection on Mesoscale Predictability

In a previous study by the authors, it was shown that the problematic numerical prediction of the 24‐25 January 2000 snowstorm along the east coast of the United States was in some measure due to rapid error growth at scales below 500 km. In particular they found that moist processes were responsible for this strong initial-condition sensitivity of the 1‐2-day prediction of mesoscale forecast aspects. In the present study they take a more systematic look at the processes by which small initial differences (‘‘errors’’) grow in those numerical forecasts. For initial errors restricted to scales below 100 km, results show that errors first grow as small-scale differences associated with moist convection, then spread upscale as their growth begins to slow. In the context of mesoscale numerical predictions with 30-km resolution, the initial growth is associated with nonlinearities in the convective parameterization (or in the explicit microphysical parameterizations, if no convective parameterization is used) and proceeds at a rate that increases as the initial error amplitude decreases. In higherresolution (3.3 km) simulations, errors first grow as differences in the timing and position of individual convective cells. Amplification at that stage occurs on a timescale on the order of 1 h, comparable to that of moist convection. The errors in the convective-scale motions subsequently influence the development of meso- and larger-scale forecast aspects such as the position of the surface low and the distribution of precipitation, thus providing evidence that growth of initial errors from convective scales places an intrinsic limit on the predictability of larger scales.

Moist convection and the vertical structure and water abundance of Jupiter's atmosphere

We explore the consequences of moist convection on Jupiter with a one-dimensional version of the cumulus parameterization used in the GISS general circulation model. The model predicts the collective effects of an ensemble of moist convective plumes on a conditionally unstable atmosphere. Heating/cooling and drying/moistening of the large-scale environment occur through compensating subsidence, detrainment of updraft air at cloud top, and evaporation and melting of falling condensate. Dry convective adjustment and stratiform cloud formation are included to remove superadiabatic lapse rates and supersaturated humidities, respectively. The model also transports parahydrogen fraction as a passive tracer. We make two different assumptions about how convection operates on Jupiter. In the first scenario we assume that convection dominates all other processes, modifying an initial temperature and moisture profile until it reaches neutral stability. Three regimes are possible: Pure moist convective, mixed moist-dry convective, and primarily dry convective. The outcome depends on the assumed deep water abundance, efficiency of condensate evaporation, and initial temperature profile. Relative humidity just below cloud top is low in the moist convective regime but increases steadily with depth. Isolated dry convection layers in the mixed regime produce sharp vertical variations of relative humidity and multiple water cloud layers. In all cases, severe water depletion is possible only over a narrow range of altitudes. In particular, the water vapor profile inferred by Bjoraker et al. (1986 , Astrophys. J. 311, 1058–1072) cannot be reconciled with the effects of moist convection, regardless of whether the deep abundance is subsolar, solar, or supersolar. Significant cloud-base stable layers a scale height or less in depth form when condesate evaporation and dry convection are important, but the degree of stabilization is always considerably less than the theoretical limit. Convective transports produce subequilibrium parahydrogen fractions and small or negative gradients near the visible cloud level. We also examine the alternative assumption that a quasiequilibrium prevails between moist convection and other processon Jupiter. In this scenario we calculate the cumulus mass flux consistent with the Jovian internal heat flux and diagnose the large-scale vertical advection required to balance the convective heating. The equilibrium configuration for near-neutral static stability is a series of thin convection layers capped by thin cloud layers and a “staircase” vertical profile of temperature, humidity, and parahydrogen fraction.

BIFURCATIONS FROM STATIONARY TO PERIODIC STATES AND TWO KINDS OF LOW-FREQUENCY OSCILLATIONS OF SUBTROPICAL FLOW PATTERNS

A parameterization for convective heating is introduced into the diabatic term of a two-layer, low-or- der spectral model for estimating the effects of moist convection on bifurcations fro mstationary to period- ic states of flow patterns. In certain parameter combinations, thermal forcing with the parameterization can generate bifurcations from stationary to periodic states. However, the forcing without it does not ex- cite such bifurcations. The time scales corresponding to their periodic states are about two-three weeks. In other parameter combinations, the thermal forcing either with or without the convective heating can generate the bifurcations from stationary to periodic states; but the effects of convective heating shorten the time scales and accelerate the amplifications of the excited periodic oscillations. The instabilities of the stationary states, which represent a uniform distribution of the subtropical flow pattern at the upper layer of the model atmosphere, may form two kinds of low-frequency oscilla- tions. One of them is the oscillation of subtropical high centers in the zonal direction. The other is the oscillation of strength of subtropical highs while the locations of their centers are quasistationary. Con- vective heating is able to influence the two kinds of low-frequency oscillations.

The Effects of Moist Convection and Water Vapor Radiative Processes on Climate Sensitivity

Abstract The primary interest of the present study is to examine the sensitivity of climate to radiative perturbations such as increases in CO2 and solar insolation for surface temperatures warmer than present day global averaged values (Ts> 290 K). The climate sensitivity, defined here as the change in Ts, is examined with the aid of a one-dimensional radiative-convective model. The solar insolation in the model is varied from 880 to 1840 W m−2 to obtain a wide range of Ts, from 255 to 325 K. We examine in detail the dependence of the computed ΔTs, on the following processes which are known to be important in the warmer regions (e.g., tropics) of the present day atmosphere: convective parameterizations (fixed lapse-rate, moist-adiabatic adjustment and cumulus adjustment); H2O vertical distribution; and H2O longwave radiative treatment. The climate sensitivity is shown to vary nonlinearly with Ts and to depend strongly on: (i) convective processes; (ii) H2O continuum absorption; and (iii) upper tropospher...