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Studies have implicated the importance of longwave (LW) cloud‐radiative forcing (CRF) in facilitating or accelerating the upscale development of tropical moist convection. While different cloud types are known to have distinct CRF, their individual roles in driving upscale development through radiative feedback is largely unexplored. Here we examine the hypothesis that CRF from stratiform regions has the greatest positive effect on upscale development of tropical convection. We do so through numerical model experiments using convection‐permitting ensemble WRF (Weather Research and Forecasting) simulations of tropical cyclone formation. Using a new column‐by‐column cloud classification scheme, we identify the contributions of five cloud types (shallow, congestus, and deep convective; and stratiform and anvil clouds). We examine their relative impacts on longwave radiation moist static energy (MSE) variance feedback and test the removal of this forcing in additional mechanism‐denial simulations. Our results indicate the importance stratiform and anvil regions in accelerating convective upscale development. Plain Language Summary: Infrared or longwave radiation and its interaction with clouds is important in the formation of tropical storms. Given the different shapes and distributions of distinct cloud types, we hypothesize that they interact with longwave radiation differently, and therefore exert different impacts on the organization of tropical convection. This issue has largely been unexplored. To address this gap, we tested our hypothesis by analyzing numerical model simulations of the formation of two tropical cyclones. Further, we developed a new cloud classification scheme based on cloud properties that identifies five distinct cloud types. Using this classification, we examined the impact of radiative interactions with different cloud types on the development of tropical storms by turning off this feedback in specific cloud types. Our results indicate that light‐raining regions, such as stratiform and anvil clouds, contribute dominantly to longwave cloud‐radiative trapping and the moistening of convective regions. This is due to both these cloud types' strong greenhouse trapping effect and their extensive areal coverage, which spreads this effect over large regions of a developing storm. Key Points: A new column‐by‐column cloud microphysical classification scheme is developed for application with numerical modelsRadiative feedback due to stratiform and anvil clouds is a leading driver of tropical convective upscale developmentThe local radiative forcing by deep convective regions is similar in magnitude to stratiform but its impact is limited by its smaller area [ABSTRACT FROM AUTHOR]

Ensemble‐based data assimilation of radar observations across inner‐core regions of tropical cyclones (TCs) in tandem with satellite all‐sky infrared (IR) radiances across the TC domain improves TC track and intensity forecasts. This study further investigates potential enhancements in TC track, intensity, and rainfall forecasts via assimilation of all‐sky microwave (MW) radiances using Hurricane Harvey (2017) as an example. Assimilating Global Precipitation Measurement constellation all‐sky MW radiances in addition to GOES‐16 all‐sky IR radiances reduces the forecast errors in the TC track, rapid intensification (RI), and peak intensity compared to assimilating all‐sky IR radiances alone, including a 24‐hr increase in forecast lead‐time for RI. Assimilating all‐sky MW radiances also improves Harvey's hydrometeor fields, which leads to improved forecasts of rainfall after Harvey's landfall. This study indicates that avenues exist for producing more accurate forecasts for TCs using available yet underutilized data, leading to better warnings of and preparedness for TC‐associated hazards in the future. Plain Language Summary: Track, intensity, and rainfall are fundamental elements of all forecasts and warnings associated with tropical cyclones (TCs). Over the last few decades, the forecast community has significantly improved TC track forecasts. Notable improvements in TC intensity forecasts have recently been achieved using high‐resolution models and remote‐sensing observations over the inner‐core region of TCs. This study builds on these earlier efforts by investigating the impacts of microwave (MW) observations on the forecast accuracy of TC track, intensity, and rainfall. Because MW radiances are sensitive to water vapor, liquid water, and ice, assimilating these observations into numerical TC forecasts is expected to improve estimates of the liquid water and ice within TCs, leading to better rainfall forecasts. These expectations are borne out in our study of Hurricane Harvey. Results indicate that incorporating currently available yet underutilized observations into numerical TC forecasts can further improve warnings of, and preparedness for, TC‐associated hazards in the future. Key Points: Satellite all‐sky infrared (IR) and microwave (MW) radiances are assimilated to assess their impacts on forecasts for Hurricane HarveyAlong with IR radiances, MW radiances improve the track and intensity forecasts for HarveyMW radiance assimilation leads to better analyses of the hydrometeor fields and more accurate rainfall forecasts [ABSTRACT FROM AUTHOR]

This study describes a new mechanism governing the diurnal variation of vertical motion in tropical oceanic heavy rainfall zones, such as the intertropical convergence zone. In such regions, the diurnal heating of widespread anvil clouds due to shortwave radiative absorption enhances upward motion in these upper layers in the afternoon. This radiatively driven ascent promotes an afternoon maximum of anvil clouds, indicating a diurnal cloud‐radiative feedback. The opposite occurs at nighttime: While rainfall exhibits a dominant peak at night‐early morning, the boundary layer rooted upward motion and latent heating tied to this peak are forced to be more bottom heavy by the nighttime anomalous radiative cooling at upper levels. This mechanism therefore favors the stratiform top‐heavy heating mode during daytime and suppresses it nocturnally. These diurnal circulation signatures arise from microphysical‐radiative feedbacks that manifest on the scales of organized deep convection, which may ultimately impact the daily mean radiation budget. Plain Language Summary: Observations show that the rainfall in tropical thunderstorms over the oceans peaks at night‐early morning. This is because solar radiation leads to warming in the upper troposphere in rainy and cloudy regions, which suppresses the convective overturning motions that cause the rainfall. Given the nocturnal peak in rainfall, we intuitively expect the upper‐level outward spreading anvil clouds of thunderstorms to maximize at night, when thunderstorms are strongest. While anvil clouds indeed peak at night, they additionally peak in the midafternoon, which cannot be explained by this nocturnal convection. In this study we show through global model simulations that the afternoon peak of anvil clouds over tropical oceans is caused by the upper‐cloud daytime maximum of radiative warming. Although this radiative warming suppresses convective motions (and hence rainfall), it causes a circulation response in the upper troposphere that manifests in locally increased upward motion. This upward motion enhances the production and spreading of anvil clouds in the afternoon. Key Points: Tropical upward motion exhibits a bimodal diurnal cycle with a top‐heavy shape during the day and a bottom‐heavy shape at nightThe diurnal radiative heating cycle within anvil clouds is critical to upper‐level diurnal circulation changeUpper‐level lifting due to daytime cloud‐shortwave heating causes an afternoon peak in anvil clouds, indicating a feedback [ABSTRACT FROM AUTHOR]

Observations of the diurnal cycle in tropical cyclones (TCs) systematically indicate a ∼12‐hr offset between peak rainfall rate and the maximum height of anvil clouds in the TC cloud canopy. This phasing conflicts with archetypal models of organized deep convection, which suggest a tight coupling between rainfall, vertical cloud growth, and anvil clouds. We show that this phasing owes to the bimodal diurnal evolution of the transverse circulation, which peaks nocturnally from low–midlevels, and during daytime in the upper troposphere. The bottom‐heavy nocturnal circulation state is driven by latent heating from nocturnally invigorated deep convection, while the top‐heavy daytime state is the thermally direct circulation response to strong shortwave‐cloud warming in the optically thick TC cloud canopy. This daytime upper‐level circulation response manifests in a lifting of the maximum height of the TC outflow and, in turn, a lifting and invigoration of the upper‐level anvil clouds of the TC cloud canopy. Plain Language Summary: We typically expect the rainfall, deeply growing thunderstorm clouds, and outward‐spreading anvil clouds in tropical cyclones (e.g., hurricanes) to closely coincide in time—that is, when rainfall peaks, we expect deep clouds to be vigorously growing, and anvil clouds in the upper‐level cloud canopy to be spreading outward and at their greatest height. Previous observational studies, however, indicate that in the 24‐hr diurnal cycle, anvil clouds behave distinctly from the deep thunderstorm clouds that produce the rain: while rainfall and deep clouds peak overnight, the height and spreading of the anvil clouds peaks during the day. This study indicates that this distinct behavior of anvil clouds owes to the interaction between solar radiation and ice clouds. Namely, the abundant ice crystals in anvil clouds absorb much of the incoming solar radiation, leading to strong warming there. This daytime warming, in turn, enhances the upward motion at the height of these anvil clouds, causing the storm's upper‐level outflow to lift. The lifting of outflow promotes the anvil clouds to lift and spread. This lifting of anvil clouds in the outflow maximizes in the early afternoon, even though rainfall is suppressed during daylight hours. Key Points: Solar heating of optically thick canopy clouds lifts outflow in the afternoonThis afternoon outflow lifting explains the afternoon peak in cloud top heightThe diurnal variation of insolation does not impact the timing of spontaneous cyclogenesis in RCE [ABSTRACT FROM AUTHOR]

Although the importance of the diurnal cycle in modulating clouds and precipitation has long been recognized, its impact on the climate system at longer timescales has remained elusive. Mounting evidence indicates that the diurnal cycle may substantially affect leading climate modes through nonlinear rectification. In this study, an idealized cloud-resolving model experiment is executed to isolate a diurnal timescale feedback in the shallow cumulus regime over the tropical warm pool. This feedback is isolated by modifying the period of the diurnal cycle (or removing it), which proportionally scales (or removes) the diurnal thermodynamic forcing that clouds respond to. This diurnal forcing is identified as covarying cycles of static stability and humidity in the lower troposphere, wherein the most unstable conditions coincide with greatest humidity each afternoon. This diurnal forcing yields deeper clouds and greater daily-mean cumulus heating than would otherwise occur, in turn reducing large-scale subsidence from day to day according to the 'weak temperature gradient' approximation. This diurnal forcing therefore manifests as a timescale feedback by accelerating the onset of deep convection. The longwave cloud-radiation effect is found to amplify this timescale feedback, since the resulting invigoration of clouds (increased upper-cloud radiative cooling, with suppressed cooling below) scales with cloud depth (i.e., optical thickness), and hence with the magnitude of diurnal forcing. These findings highlight the pressing need to remedy longstanding problems related to the diurnal cycle in many climate models. Given the evident sensitivity of climate variability to diurnal processes, doing so may yield advances in climate prediction at longer timescales. [ABSTRACT FROM AUTHOR]

An idealized cloud-resolving model experiment is executed to study the prominent cumulus diurnal cycle in suppressed regimes over the tropical warm pool. These regimes are characterized by daytime cumulus invigoration and cloud-layer moistening connected with enhanced diurnal cycles in shortwave radiative heating (SW) and sea surface temperature (SST). The relative roles of diurnally varying SW and SST in this cumulus diurnal cycle are assessed, wherein radiation is modeled and SST is prescribed. Large-scale subsidence is parameterized using the spectral weak temperature gradient (WTG) scheme, such that large-scale vertical motion ( wwtg), and hence subsidence drying, is modulated by diurnal changes in diabatic heating. A control simulation exhibits daytime cumulus invigoration that closely matches observations, including midday cloud-layer moistening. This cumulus invigoration is composed of two distinct modes: (1) a midday nonprecipitating ('forced') mode of predominately shallow clouds, driven by the peak in SST and surface fluxes as the mixed layer deepens and dries; and (2) a precipitating late-afternoon ('active') mode characterized by deeper clouds in connection with a more moist cloud layer. This cloud-layer moistening is driven by the daytime relaxation of wwtg subsidence, which is prompted by the midday peak in SW. The transition from the surface flux-driven forced mode to the active precipitating mode is accompanied by a transition from relatively small-scale boundary layer circulation cells to larger cells that are highly modulated by cold pools, consistent with observations. When the diurnal cycle is removed, clouds are persistently shallower with virtually no rainfall, emphasizing the inherent nonlinearity of the cumulus diurnal cycle. [ABSTRACT FROM AUTHOR]

The hypothesis that the islands of the Maritime Continent (MC) enhance total rainfall and time‐mean upward motion is tested using a convection‐permitting regional model. Sensitivity experiments with the islands removed greatly diminish both rainfall and upward motion, supporting the hypothesis. We examine the individual factors in this enhancement, isolating the impacts of the diurnal cycle from those of basic‐state (i.e., constant) forcing of orography and the land surface. We find that the basic‐state forcing by land is the only factor that substantially enhances total island rainfall, specifically through the enhancement of mean surface heat fluxes. The diurnal cycle and orographic forcing, however, substantially enhance rainfall in the seas surrounding the islands. Moreover, the diurnal cycle is found to be essential for promoting mesoscale circulations on the spatial scales of the islands, which are critical to both the upscale growth of deep convection and the most extreme rainfall rates. Plain Language Summary: Long‐term mean rainfall is greater in the Maritime Continent (MC) than its neighboring oceans, yet an explanation for this has long eluded us. In this study, we have examined this issue through experiments with a high‐resolution numerical model, wherein we remove either the islands or the daily (diurnal) cycle by imposing daily‐mean solar heating to test its impact. The results indicate that the enhancement of total rainfall over the islands is the result of the low heat capacity of islands compared to oceans and is not impacted by the diurnal cycle. With such low heat capacity, solar energy received at the island surface is rapidly transferred to the atmosphere, in turn enhancing rainfall. Yet the findings also indicate that the diurnal cycle is vital for triggering stronger thunderstorms and higher local rain rates in the MC region. Key Points: Island rainfall enhancement is critical to time‐mean upward motion in the Maritime ContinentThis enhancement is dominated by basic‐state island heating, with diurnal processes only enhancing total rainfall in the surrounding seasThe diurnal cycle enhances the organization of deep convection and extreme rain rates through its coupling with mesoscale circulation [ABSTRACT FROM AUTHOR]

The Radiative-Convective Equilibrium Model Intercomparison Project (RCEMIP) is an intercomparison of multiple types of numerical models configured in radiative-convective equilibrium (RCE). RCE is an idealization of the tropical atmosphere that has long been used to study basic questions in climate science. Here, we employ RCE to investigate the role that clouds and convective activity play in determining cloud feedbacks, climate sensitivity, the state of convective aggregation, and the equilibrium climate. RCEMIP is unique among intercomparisons in its inclusion of a wide range of model types, including atmospheric general circulation models (GCMs), single column models (SCMs), cloud-resolving models (CRMs), large eddy simulations (LES), and global cloud-resolving models (GCRMs). The first results are presented from the RCEMIP ensemble of more than 30 models. While there are large differences across the RCEMIP ensemble in the representation of mean profiles of temperature, humidity, and cloudiness, in a majority of models anvil clouds rise, warm, and decrease in area coverage in response to an increase in sea surface temperature (SST). Nearly all models exhibit self-aggregation in large domains and agree that self-aggregation acts to dry and warm the troposphere, reduce high cloudiness, and increase cooling to space. The degree of self-aggregation exhibits no clear tendency with warming. There is a wide range of climate sensitivities, but models with parameterized convection tend to have lower climate sensitivities than models with explicit convection. In models with parameterized convection, aggregated simulations have lower climate sensitivities than unaggregated simulations., (©2020. The Authors.)