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Brian E. Mapes

Abstract

The problem of closure in cumulus parameterization requires an understanding of the sensitivities of convective cloud systems to their large-scale setting. As a step toward such an understanding, this study probes some sensitivities of a simulated ensemble of convective clouds in a two-dimensional cloud-resolving model (CRM). The ensemble is initially in statistical equilibrium with a steady imposed background forcing (cooling and moistening). Large-scale stimuli are imposed as horizontally uniform perturbations nudged into the model fields over 10 min, and the rainfall response of the model clouds is monitored.

In order to reduce a major source of artificial insensitivity in the CRM, a simple parameterization scheme is devised to account for heating-induced large-scale (i.e., domain averaged) vertical motions that would develop in nature but are forbidden by the periodic boundary conditions. The effects of this large-scale vertical motion are parameterized as advective tendency terms that are applied as a uniform forcing throughout the domain, just like the background forcing. This parameterized advection is assumed to lag rainfall (used as a proxy for heating) by a specified time scale. The time scale determines (via a gravity wave space–time conversion factor) the size of the large-scale region represented by the periodic CRM domain, which can be of arbitrary size or dimensionality.

The sensitivity of rain rate to deep cooling and moistening, representing an upward displacement by a large-scale wave of first baroclinic mode structure, is positive. Near linearity is found for ±1 K perturbations, and the sensitivity is about equally divided between temperature and moisture effects. For a second baroclinic mode (vertical dipole) displacement, the sign of the perturbation in the lower troposphere dominates the convective response. In this dipole case, the initial sensitivity is very large, but quantitative results are distorted by the oversimplified large-scale dynamics parameterization, which only allows for deep baroclinic mode responses. Imposition of moderate wind shear (10 m s−1 over the troposphere) has no significant impact on rain rate.

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Brian E. Mapes

Abstract

A toy model of large-scale deep convection variations is constructed around a radiative–convective equilibrium climate, with an observed mean sounding as its thermodynamic basic state.

Vertical structure is truncated at two modes, excited by convective (one-signed) and stratiform (two-signed) heating processes in tropical deep convection. Separate treatments of deep and shallow convection are justified by observations that deep convection is more variable. Deep convection intensity is assumed to be modulated by convective available potential energy (CAPE), while occurrence frequency is modulated by the ratio of convective inhibition (CIN) to “triggering energy” K, a scalar representing the intensity of subgrid-scale fluctuations. Deep convective downdrafts cool and dry the boundary layer but also increase K. Variations of K make the relationship between convection and thermodynamic variables (CAPE, CIN, θ e) nonunique and amplify the deep convective response to temperature waves of small (∼1°C) amplitude.

For a parameter set in which CAPE variations control convection, moist convective damping destroys all variability. When CIN/K variations have dominant importance (the “inhibition-controlled” regime), a mechanism termed “stratiform instability” generates large-scale waves. This mechanism involves lower-tropospheric cooling by stratiform precipitation, which preferentially occurs where the already cool lower troposphere favors deep convection, via smaller CIN. Stratiform instability has two subregimes, based on the relative importance of the two opposite effects of downdrafts: When boundary layer θ e reduction (a local negative feedback) is stronger, small-scale waves with frequency based on the boundary layer recovery time are preferred. When the K-generation effect (positive feedback) is stronger, very large scales (low wavenumbers of the domain) develop. A mixture of these scales occurs for parameter choices based on observations. Model waves resemble observed waves, with a phase speed ∼20 m s−1 (near the dry wave speed of the second internal mode), and a “cold boomerang” vertical temperature structure.

Although K exhibits “quasi-equilibrium” with other convection variables (correlations > 0.99), replacing the prognostic K equation with diagnostic equations based on these relationships can put the model into wildly different regimes, if small time lags indicative of causality are distorted. The response of model convection to climatological spatial anomalies of θ e (proxy for SST) and K (proxy for orographic and coastal triggering) is considered. Higher SST tends broadly to favor convection under either CAPE-controlled or inhibition-controlled regimes, but there are dynamical embellishments in the inhibition-controlled regime. The Kelvin wave seems to be the preferred structure when the model is run on a uniform equatorial β plane.

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Brian E. Mapes

Abstract

A beat source with a vertical profile like that of observed tropical mesoscale convective systems (MCSs) is shown to cause, through inviscid gravity wave dynamics, upward displacement at low levels in a mesoscale region surrounding the heating. Typical values are ∼10%–30% area contraction at the surface everywhere within 270 km of the heating 6 h after it starts. As a result, conditions near an existing MCS (but beyond the area of MCS outflow) become more favorable for the development of additional convection. This theory predicts that cloud clusters should be gregarious. Infrared satellite imagery confirms that almost half of the cold cloudiness observed in a month over the oceanic warm pool region was contributed by just 14 objectively defined multiday “supercluters”.

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Marja Bister and Brian E. Mapes

Abstract

A cloud-resolving model is used to study the effects of a vertical temperature dipole on convective cloud development. Such dipole anomalies, with a warm-above-cool structure in the troposphere, are known to be forced by mesoscale convective systems (MCSs) in the Tropics. The experiments involve letting convection develop in perturbed initial soundings with open lateral boundary conditions. Convection is driven solely by surface fluxes. In the control run, a field of deep convection ensues. With a strong dipole anomaly that is warm in the upper troposphere, no clouds ascend beyond the middle troposphere. In this case, cumulus congestus clouds strongly moisten the midtroposphere with relative humidity increases by up to 24% by the end of the 6‐h simulation. With a half-strength anomaly, a mixed population results: mainly middle-topped congestus clouds, but with some intermittent deep cells. The partitioning between cloud types is somewhat sensitive to model resolution, with a change from 1- to 0.5-km grid spacing resulting in relatively more congestus clouds and fewer deep cells.

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Brian E. Mapes and Xiaoqing Wu

Abstract

Domain-average momentum budgets are examined in several multiday cloud-resolving model simulations of deep tropical convection in realistic shears. The convective eddy momentum tendency F, neglected in many global circulation models, looks broadly similar in two- and three-dimensional simulations. It has a large component in quadrature with the mean wind profile, tending to cause momentum profile features to descend. This component opposes, and exceeds in magnitude, the corresponding large-scale vertical advective tendency, which would tend to make features ascend in convecting regions. The portion of F in phase with the mean wind is isolated by vertically integrating F · u, yielding a kinetic energy tendency that is overwhelmingly negative. The variation of this energy damping with shear flow kinetic energy and convection intensity (measured by rain rate) gives a “cumulus friction” coefficient around −40% to −80% per centimeter of rain in 3D runs. Large scatter reflects the effects of varying convective organization. Two-dimensional runs overestimate this friction coefficient for the υ (out of plane) wind component and underestimate it for the u (in plane) component. Another 2D artifact is that 460-hPa-wavelength shear is essentially undamped, consistent with the descending jets reported by Held et al. in a free-running 2D cloud model.

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Brian E. Mapes and Jialin Lin

Abstract

A simple new analysis method for large single-Doppler radar datasets is presented, using data from several tropical field experiments. A cylindrical grid is chosen, to respect both the geophysical importance of altitude and the radar importance of range and azimuth. Horizontal and temporal fine structure are sacrificed, by compiling data as hourly histograms in 12 × 24 × 36 spatial grid cells of 15° azimuth × 8 km horizontal range × 500 m height, respectively. Mean Doppler radial velocity in each region is automatically unfolded (dealiased) using a simple histogram method, and fed into a velocity–azimuth display (VAD) analysis. The result is a set of hourly horizontal wind and wind divergence profiles, with associated error estimates, for circles of different radii centered on the radar.

These divergence profiles contain useful heating profile information in many weather situations, not just occasional cases of uniform widespread rainfall. Consistency of independent estimates for concentric circles, continuity from hour to hour, and good mass balance indicate high-quality results in one 48-h example sequence shown, from the East Pacific Investigations of Climate (EPIC 2001) experiment. Linear regression of divergence profiles versus reflectivity-estimated surface rain rates is used to illustrate the dominant systematic pattern: convective rain with low-level wind convergence evolves into stratiform rain with middle-level convergence, on a characteristic time scale of several hours. Absolute estimates of moisture convergence per unit of ZR calculated rainfall vary strongly among experiments, in ways that appear to indicate reflectivity calibration errors. This indicates that Doppler data may offer a useful and unique bulk constraint on rainfall estimation by radar.

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Hirohiko Masunaga and Brian E. Mapes

Abstract

The moist deep tropics are typically separated from the drier subtropics by a sharp horizontal gradient of moisture. The physical nature of this tropical margin is investigated by using A-Train satellite observations to reconstruct its composite mean quasi-meridional thermodynamic structure and processes. The margin is defined here as the most poleward position of a specified column water vapor (CWV) threshold along a satellite track. Multiple CWV thresholds are selected from 35 to 60 mm, bracketing the global tropics histogram minimum value of 48 mm. For all margin thresholds, CWV increases equatorward from the subtropics and eventually asymptotically approaches 48 mm far on the tropical side, apparently as a coincidence of composite averaging since values of 48 mm are infrequent as noted above. For all margin thresholds, precipitation peaks on the tropical side and then asymptotically approaches equatorward a value of 85 W m−2, equal to the evaporation asymptote. For the 48-mm threshold, total diabatic forcing of the air column (radiative heating plus surface latent and sensible heat fluxes) changes sign from positive on the tropical side to negative in the subtropics, with the main contrast in radiative heating, owing principally to the longwave effect of high clouds. An analytic two-vertical-mode model of equatorward-flowing air columns is fitted from the observations to elucidate the processes in a Lagrangian column transition. The model captures key features of the composite, and suggests that a key process in the abrupt moistening at the margin is bottom-heavy ascent growing upward beneath the deep subtropical subsidence.

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Brian E. Mapes, Ping Liu, and Nikolaus Buenning

Abstract

Two dominant high-frequency features of Northern Hemisphere summer climatology are examined in an atmosphere–land general circulation model (AGCM): the sudden onset of rains in south Asia, and the midsummer rainfall minimum in the tropical Americas. A control simulation succeeds in capturing these observed features fairly well. A slowed-calendar experiment is performed, to see whether these features are close to equilibrium with seasonally evolving forcings (orbital geometry and SST). The results indicate that some lag (disequilbrium) within the AGCM delays south Asian onset by about a month, from May in the experiment when seasonal forcing evolves extremely slowly to June in the normal, full-speed seasonal cycle. Disequilibrium also acts to delay and limit the amplitude of the Americas midsummer drought, and the associated intrusion of the Atlantic subtropical high into the Intra-Americas Seas’ region. It is hypothesized that early summer (centered on the solstice) temperature over mid- and high-latitude continents, which differs greatly between experiment and control, drives the low-latitude rainfall differences. A more mysterious pole-to-pole, annual-mean, zonal wave-1 difference is also found in the slowed-calendar experiment.

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Baohua Chen, Chuntao Liu, and Brian E. Mapes

Abstract

In this study, TRMM-observed precipitation in the tropics is decomposed according to the horizontal area of radar precipitation features, with special emphasis on large systems (rain area > 104 km2) that contribute roughly half of tropical rainfall. Statistical associations of rain-weighted radar precipitation feature (RPF) size distributions with atmospheric variables on the 1.5° grid of ERA-Interim data are explored. In one-predictor distributions, the association with total precipitable water vapor (TPWV) is the strongest, while relative humidity at low and midlevels and low-level wind shear are also positively related to large-RPF rain fraction. Standard CAPE and CIN variables computed from grid-mean thermodynamic profiles are only weakly related to the size of rain systems. Joint distributions over two variables are also reported. The relative importance of predictors varies over different regions. The eastern Pacific is distinctive for having large rain systems in environments with a moist boundary layer but a dry midtroposphere, with strong shallow wind shear and small CAPE. In contrast, the large-storm environment over the western Pacific is found to be moister in the whole troposphere, with relatively weaker wind shear and larger CAPE. Over tropical land, the Sahel and central Africa stand out as having a great fractional rainfall contributed by large RPFs. Their associated environment is characterized by lower TPWV but stronger shallow wind shear and larger CIN and CAPE, in comparison to the equatorial Amazon basin and the Maritime Continent. Based on these associations, statistical reconstructions of the geographical distribution of large-RPF rain fraction from grid-mean atmospheric predictors are attempted.

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Stefan N. Tulich and Brian E. Mapes

Abstract

Multiscale convective wave disturbances with structures broadly resembling observed tropical waves are found to emerge spontaneously in a nonrotating, two-dimensional cloud model forced by uniform cooling. To articulate the dynamics of these waves, model outputs are objectively analyzed in a discrete truncated space consisting of three cloud types (shallow convective, deep convective, and stratiform) and three dynamical vertical wavelength bands. Model experiments confirm that diabatic processes in deep convective and stratiform regions are essential to the formation of multiscale convective wave patterns. Specifically, upper-level heating (together with low-level cooling) serves to preferentially excite discrete horizontally propagating wave packets with roughly a full-wavelength structure in troposphere and “dry” phase speeds cn in the range 16–18 m s−1. These wave packets enhance the triggering of new deep convective cloud systems, via low-level destabilization. The new convection in turn causes additional heating over cooling, through delayed development of high-based deep convective cells with persistent stratiform anvils. This delayed forcing leads to an intensification and then widening of the low-level cold phases of wave packets as they move through convecting regions. Additional widening occurs when slower-moving (∼8 m s−1) “gust front” wave packets excited by cooling just above the boundary layer trigger additional deep convection in the vicinity of earlier convection. Shallow convection, meanwhile, provides positive forcing that reduces convective wave speeds and destroys relatively small-amplitude-sized waves. Experiments with prescribed modal wind damping establish the critical role of short vertical wavelengths in setting the equivalent depth of the waves. However, damping of deep vertical wavelengths prevents the clustering of mesoscale convective wave disturbances into larger-scale envelopes, so these circulations are important as well.

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