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Chongxing Fan
and
Xianglei Huang

Abstract

In the absence of scattering, thermal contrast in the atmosphere is the key to infrared remote sensing. Without the thermal contrast, the amount of absorption will be identical to the amount of emission, making the atmospheric vertical structure undetectable using remote sensing techniques. Here we show that, even in such an isothermal atmosphere, the scattering of clouds can cause a distinguishable change in upwelling radiance at the top of the atmosphere. A two-stream analytical solution, as well as a budget analysis based on Monte Carlo simulations, are used to offer a physical explanation of such influence on an idealized isothermal atmosphere by cloud scattering: it increases the chance of photons being absorbed by the atmosphere before they can reach the boundaries (both top and bottom), which leads to a reduction of TOA upwelling radiance. Actual sounding profiles and cloud properties inferred from satellite observations within 6-h time frames are fed into a more realistic and comprehensive radiative transfer model to show such cloud scattering effect, under nearly isothermal circumstances in the lower troposphere, can lead to ∼1–1.5-K decrease in brightness temperature for the nadir-view MODIS 8.5-μm channel. The study suggests that cloud scattering can provide signals useful for remote sensing applications even for such an isothermal environment.

Open access
Jun-Ichi Yano
and
Robert S. Plant

Abstract

The importance of the convective life cycle in tropical large-scale dynamics has long been emphasized, but without explicit analysis. The present work provides it by coupling the convective energy cycle under the framework of Arakawa and Schubert’s convection parameterization with a shallow-water analog atmosphere. A careful derivation of the system is first presented, because it is rather missing in the literature. The squared frequency of linear convectively coupled waves is given by a squared sum of the dry gravity wave and the convective energy cycle frequencies, shortening the period of the convective cycle through the large-scale coupling. In a weakly nonlinear regime, the system follows an equation analogous to the Korteweg–de Vries equation, which exhibits a solitary wave solution, with behavior reminiscent of observed tropical westerly wind bursts.

Significance Statement

The present work suggests that a nonlinear description of a large-scale tropical system with an explicit convective life cycle may provide a simple model of tropical westerly wind bursts. At the same time, an important lesson to learn is that, if the focus of a study is on the global scale of the atmosphere, it is wise not to try to include a convective life cycle explicitly into the model. Such a configuration will simply be dominated by the short convective-scale variabilities, which one would wish to filter out.

Restricted access
Kerry Emanuel

Abstract

Large values of convective available potential energy (CAPE) are an important ingredient for many severe convective storms, yet there has been comparatively little research on how, physically, such large values arise or why they take on the observed values and climatology. Here we build on recently published observational and theoretical work to construct a simple, one-dimensional coupled soil–atmosphere model of preconvective boundary layer growth, driven by a single diurnal cycle of prescribed net surface radiation. Based on this model and previously published research, we suggest that high CAPE (>∼1000 J kg−1) results when air masses that have been significantly modified by passage over dry, lightly vegetated soils are advected over moist and/or moderately vegetated soils and then exposed to surface solar heating. Several diurnal cycles may be needed to raise the moist static energy of the boundary layer to levels consistent with high CAPE. The production of CAPE and erosion of convective inhibition (CIN) are strongly affected by the potential temperature of the desert-modified air mass, the level of near-surface soil moisture (and root-zone soil moisture if significant vegetation is present), the type of soil, and the characteristics of the vegetation. Consequently, CAPE production and severe convective weather may be significantly affected by regional-scale land-use changes and by climate change.

Significance Statement

The energy available for severe convective storms depends strongly on the properties of the underlying soil and vegetation and the temperature of air masses formed over dry terrain upstream. This implies that the severity of convective storms can be strongly affected by changes in land use and by climate change.

Open access
Marius Levin Thomas
and
Volkmar Wirth

Abstract

Banner clouds are clouds in the lee of steep mountains or sharp ridges on otherwise cloud-free days. Previous studies investigated various aspects of banner cloud formation in numerical simulations, most of which were based on idealized orography and a neutrally stratified ambient atmosphere. The present study extends these simulations in two important directions by 1) examining the impact of various types of orography ranging from an idealized pyramid to the realistic orography of Mount Matterhorn and 2) accounting for an ambient atmosphere that turns from neutral to stably stratified below the mountain summit. Not surprisingly, realistic orography introduces asymmetries in the spanwise direction. At the same time, banner cloud occurrence remains associated with a coherent area of strong uplift, although this region does not have to be located exclusively in the lee of the mountain any longer. In the case of Mount Matterhorn with a westerly ambient flow, a large fraction of air parcels rises along the southern face of the mountain, before they reach the lee and are lifted into the banner cloud. The presence of a shallow boundary layer with its top below the mountain summit introduces more complex behavior compared to a neutrally stratified boundary layer; in particular, it introduces a dependence on wind speed, because strong wind is associated with strong turbulence that is able to raise the boundary layer height and, thus, facilitates the formation of a banner cloud.

Open access
C. Todd Rhodes
,
Varavut Limpasuvan
, and
Yvan Orsolini

Abstract

Traveling planetary waves surrounding sudden stratospheric warming events can result from direct propagation from below or in situ generation. They can have significant impacts on the circulation in the mesosphere and lower thermosphere. Our study runs a series of ensembles initialized from the Whole Atmosphere Community Climate Model, version 4, nudged up to 50 km by 6-hourly Modern-Era Retrospective Analysis for Research and Application, version 2, reanalysis to compile a library of sudden stratospheric warming events. To our knowledge, we present the first composite or ensemble study that attempts to link direct propagation and in situ generation by evaluating the wave geometries associated with the overreflection perspective, a framework used to describe how planetary waves interact with critical and turning levels. The present study looks at the evolution of these interactions through the onset of sudden stratospheric warmings with an elevated stratopause (ES-SSWs). Robust and unique features of ES-SSWs are determined by employing an ensemble study that compares ES-SSWs with normal winters. Our study evaluates the production and impacts of westward-propagating, quasi-stationary, and eastward-propagating planetary waves surrounding ES-SSWs. Our results show that eastward-propagating planetary waves are generated within the westward stratospheric wind layer after ES-SSW onset which aids in restoring the eastward stratospheric wind. The interaction of quasi-stationary and westward-propagating waves with the westward stratospheric wind is explored from an overreflection perspective and reaffirms that westward-propagating planetary waves are produced from instabilities at the top of the westward stratospheric wind reversal.

Restricted access
Daniel J. Lloveras
,
Dale R. Durran
, and
James D. Doyle

Abstract

We use convection-permitting idealized simulations of moist midlatitude cyclones to compare the growth of synoptic-scale perturbations derived from an adjoint model with the growth of equal-energy-norm, monochromatic-wavelength perturbations at the smallest resolved scale. For initial magnitudes comparable to those of initial-condition uncertainties in present-day data assimilation systems, the adjoint perturbations produce a “forecast bust,” significantly changing the intensity and location of the cyclone and its accompanying precipitation. In contrast, the small-scale-wave perturbations project strongly onto the moist convection, but the upscale growth from the random displacement of individual convective cells does not significantly alter the cyclone’s development nor its accompanying precipitation through 2–4-day lead times. Instead, the differences in convection generated at early times become negligible because the development of subsequent convection is driven by the mostly unchanged synoptic-scale flow. Reducing the perturbation magnitudes by factors of 10 and 100 demonstrates that nonlinear dynamics play an important role in the displacement of the cyclone by the full-magnitude adjoint perturbations, and that the upscale growth of small-magnitude, small-scale perturbations is too slow to significantly change the cyclone. These results suggest that a sensitive dependence on the synoptic-scale initial conditions, analogous to that of the Lorenz (1963) system, may be more relevant to 2–4-day midlatitude-cyclone forecast busts than the upscale error growth in the Lorenz (1969) model.

Restricted access
Stephen T. Garner

Abstract

Three-level and thee-layer models of tropical cyclones (TCs) have provided a more conceptual view of TC dynamics than conventional numerical models. They have been purpose-built, with special treatments of boundary layers and/or convection. We show that a further simplification with minimal parameterization and a seamless connection to higher resolution captures TCs about as well. The framework of radiative–convective equilibrium avoids ambiguities from temporal and spatial boundaries. For the TCs, the minimal grid provides one level for outflow and one level for most of the inflow. A version with 10 levels is used for comparison. For the same average pressure intensity, the wind field is slightly broader around the three-level vortices, with stronger subsidence in the core and 25% more mass and moisture flux. However, thermodynamic efficiency, mechanical efficiency, and TC counts are about the same. Across runs with different surface temperatures and cooling rates, global energy scaling makes reasonable predictions of the maximum velocity allowing for variations in the effective forcing/dissipation area and surface humidity. TC count is inconsistent with theories for size as a function of Coriolis parameter. An overturning circuit is isolated within a composite vortex and analyzed using energy and entropy budgets to mirror analytical models. Effective radiation and dissipation temperatures are less extreme than often assumed in such models, yielding a smaller thermodynamic efficiency near the global value of ∼0.1. The pressure deficit arises mostly from inflow enthalpy increase, as expected, but dissipation reduces the contribution from an outflow pressure increase. The influence of ambient CAPE makes up most of the difference.

Restricted access
Ewan Short
,
Todd P. Lane
,
Craig H. Bishop
, and
Matthew C. Wheeler

Abstract

Diurnal processes play a primary role in tropical weather. A leading hypothesis is that atmospheric gravity waves diurnally forced near coastlines propagate both offshore and inland, encouraging convection as they do so. In this study we extend the linear analytic theory of diurnally forced gravity waves, allowing for discontinuities in stability and for linear changes in stability over a finite-depth “transition layer.” As an illustrative example, we first consider the response to a commonly studied heating function emulating diurnally oscillating coastal temperature gradients, with a low-level stability change between the boundary layer and troposphere. Gravity wave rays resembling the upper branches of “Saint Andrew’s cross” are forced along the coastline at the surface, with the stability changes inducing reflection, refraction, and ducting of the individual waves comprising the rays, with analogous behavior evident in the rays themselves. Refraction occurs smoothly in the transition-layer solution, with substantially less reflection than in the discontinuous solution. Second, we consider a new heating function which emulates an upper-level convective heating diurnal cycle, and consider stability changes associated with the tropical tropopause. Reflection, refraction, and ducting again occur, with the lower branches of Saint Andrew’s cross now evident. We compare these solutions to observations taken during the Years of the Maritime Continent field campaign, noting better qualitative agreement with the transition-layer solution than the discontinuous solution, suggesting the tropopause is an even weaker gravity wave reflector than previously thought.

Significance Statement

This study extends our theoretical understanding of how forced atmospheric gravity waves change with atmospheric structure. Gravity wave behavior depends on atmospheric stability: how much the atmosphere resists vertical displacements of air. Where stability changes, waves reflect and refract, analogously to when light passes from water to air. Our study presents new mathematical tools for understanding this reflection and refraction, demonstrating reflection is substantially weaker when stability increases over “transition layers,” than when stability increases suddenly. Our results suggest the tropical tropopause reflects less gravity wave energy than previously thought, with potential design implications for weather and climate models, to be assessed in future work.

Restricted access
Jungmin M. Lee
,
Cheng Tao
,
Walter M. Hannah
,
Shaocheng Xie
, and
David C. Bader

Abstract

Many climate models exhibit a dry and warm bias over the central United States during the summer months, including the Energy Exascale Earth System Model (E3SM) and its Multiscale Modeling Framework (MMF) configuration. Understanding the causes of this bias is important to shine a light on this common model error and reduce the uncertainty in future projections. In this study, we use E3SMv2 and E3SM-MMF to assess how parameterized and resolved convection affect temperature and precipitation biases over the Southern Great Plains site of the Atmospheric Radiation Measurement program. Both configurations overestimate near-surface temperature and underestimate precipitation at the ARM SGP site. The bias is associated with a lack of low-level clouds during days without precipitation and too much incoming solar radiation causing the surface to warm. Low-level cloud fraction in E3SM-MMF during the nonprecipitating days is lower in comparison to E3SMv2 and observation, consistent with the larger warm bias. We also find that the underestimated precipitation can be characterized as “too frequent, too weak” in E3SMv2 and “too rare, too intense” in E3SM-MMF. These deficiencies conspire to sustain the warm and dry bias over the central United States.

Open access
Puja Roy
,
Robert M. Rauber
, and
Larry Di Girolamo

Abstract

This study investigates the evolution of temperature and lifetime of evaporating, supercooled cloud droplets considering initial droplet radius (r 0) and temperature ( T r 0 ), and environmental relative humidity (RH), temperature (T ), and pressure (P). The time (t ss) required by droplets to reach a lower steady-state temperature (T ss) after sudden introduction into a new subsaturated environment, the magnitude of ΔT = T T ss, and droplet survival time (t st) at T ss are calculated. The temperature difference (ΔT) is found to increase with T , and decrease with RH and P. ΔT was typically 1–5 K lower than T , with highest values (∼10.3 K) for very low RH, low P, and T closer to 0°C. Results show that t ss is <0.5 s over the range of initial droplet and environmental conditions considered. Larger droplets (r 0 = 30–50 μm) can survive at T ss for about 5 s to over 10 min, depending on the subsaturation of the environment. For higher RH and larger droplets, droplet lifetimes can increase by more than 100 s compared to those with droplet cooling ignored. T ss of the evaporating droplets can be approximated by the environmental thermodynamic wet-bulb temperature. Radiation was found to play a minor role in influencing droplet temperatures, except for larger droplets in environments close to saturation. The implications for ice nucleation in cloud-top generating cells and near cloud edges are discussed. Using T ss instead of T in widely used parameterization schemes could lead to enhanced number concentrations of activated ice-nucleating particles (INPs), by a typical factor of 2–30, with the greatest increases (≥100) coincident with low RH, low P, and T closer to 0°C.

Significance Statement

Cloud droplet temperature plays an important role in fundamental cloud processes like droplet growth and decay, activation of ice-nucleating particles, and determination of radiative parameters like refractive indices of water droplets. Near cloud boundaries such as cloud tops, dry air mixes with cloudy air exposing droplets to environments with low relative humidities. This study examines how the temperature of a cloud droplet that is supercooled (i.e., has an initial temperature < 0°C) evolves in these subsaturated environments. Results show that when supercooled cloud droplets evaporate near cloud boundaries, their temperatures can be several degrees Celsius lower than the surrounding drier environment. The implications of this additional cooling of droplets near cloud edges on ice particle formation are discussed.

Open access