Conditional Instability Of The Second Kind Research Articles

Evidence of nonlinear Walker circulation feedbacks on extreme El Niño Pacific diversity: Observations and CMIP5 models

AbstractThe Walker circulation (WC) is essential for the formation and diversity of El Niño events. However, the nonlinear WC feedback during extreme Central and Eastern El Niño episodes (C and E episodes, respectively) has received little attention. This study used observational datasets and the Atmospheric Model Intercomparison Project (AMIP) and historical simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Eight out of 21 historical models that simulate the El Niño‐Southern Oscillation (ENSO) nonlinearity also simulate the nonlinear Bjerknes feedback in C and E episodes. The opposite does not necessarily occur. However, the underestimation of E might limit the empirical determination. Moreover, few historical models simulate the shallow conditional instability of the second kind (CISK) mechanism. Positive C episodes feature an eastward shift in the ascending branch of the Pacific Walker cell (PWC), while shallow convection prevails over the far‐eastern Pacific (FEP). Positive E events feature two anomalous ascending branches located over the central‐western Pacific (170°W) and FEP (80°W). Positive anomalies in sea surface temperature over the FEP induce the second ascending branch. The positive stratification anomaly in the central Pacific Ocean, which is associated with overestimated Ekman feedback, limits the eastward displacement of the first ascending branch of the PWC. The net surface heat flux determines the duration of growth of the two ascending branches of the PWC during C and E events. Because of their coarse resolution, the historical models underestimate the positive stratification anomaly in the FEP, causing the quick demise of the second ascending branch.

Localization and Invigoration of Mei‐yu Front Rainfall due to Aerosol‐Cloud Interactions: A Preliminary Assessment Based on WRF Simulations and IMFRE 2018 Field Observations

AbstractAerosol‐cloud interactions remain a major source of uncertainty in our understanding and modeling of the Earth's hydrological cycle. Based upon a diagnostic and modeling analysis utilizing the latest field measurements from the Integrative Monsoon Frontal Rainfall Experiment (IMFRE) 2018, this paper reports the effects of aerosols on the cloud properties along the Mei‐yu front over the Middle Reaches of Yangtze River in China. Numerical experiments with the Weather Research and Forecasting (WRF) model suggest that increasing aerosol number concentration reduces surface precipitation by ~8.8% and delays the onset of rainfall by ~30 min. Furthermore, warm clouds are suppressed but the convective cores are slightly intensified. This corresponds to an overall aerosol effect of “localization and invigoration” of the Mei‐yu rainfall and thus an elevated probability of short‐term heavy rainfall. The signals of “convective invigoration” with a bulk scheme in this study are relatively weak compared to those simulated by bin microphysics. The increased aerosol concentration strengthens Mei‐yu front and changes local morphology of the front, consistent with earlier studies demonstrating positive effects of convective heating on the genesis and maintenance of Mei‐yu front via conditional instability of the second kind (CISK) and diabatic generation of potential vorticity. Also discussed are the uncertainties of bulk microphysics in simulating aerosol‐cloud interactions, which may shed light on the design of future field campaigns to further understand the impact of aerosol‐cloud interactions on weather and climate over China in boreal summer.

Enhanced Persistence of Equatorial Waves via Convergence Coupling in the Stochastic Multicloud Model

Abstract Recent observational and theoretical studies show a systematic relationship between tropical moist convection and measures related to large-scale convergence. It has been suggested that cloud fields in the column stochastic multicloud model compare better with observations when using predictors related to convergence rather than moist energetics (e.g., CAPE) as per Peters et al. Here, this work is extended to a fully prognostic multicloud model. A nonlocal convergence-coupled formulation of the stochastic multicloud model is implemented without wind-dependent surface heat fluxes. In a series of idealized Walker cell simulations, this convergence coupling enhances the persistence of Kelvin wave analogs in dry regions of the domain while leaving the dynamics in moist regions largely unaltered. This effect is robust for changes in the amplitude of the imposed sea surface temperature (SST) gradient. In essence, this method provides a soft convergence coupling that allows for increased interaction between cumulus convection and the large-scale circulation but does not suffer from the deleterious wave–conditional instability of the second kind (CISK) behavior of the Kuo-type moisture-convergence closures.

An important mechanism sustaining the atmospheric "water tower" over the Tibetan Plateau

Abstract. The Tibetan Plateau (TP), referred to as the "roof of the world", is also known as the "world water tower" because it contains a large amount of water resources and ceaselessly transports these waters to its surrounding areas. However, it is not clear how these waters are being supplied and replenished. In particular, how plausible hydrological cycles can be realized between tropical oceans and the TP. In order to explore the mechanism sustaining the atmospheric "water tower" over the TP, the relationship of a "heat source column" over the plateau and moist flows in the Asian summer monsoon circulation is investigated. Here we show that the plateau's thermal structure leads to dynamic processes with an integration of two couplings of lower convergence zones and upper divergences, respectively, over the plateau's southern slopes and main platform, which relay moist air in two ladders up to the plateau. Similarly to the CISK (conditional instability of the second kind) mechanism of tropical cyclones, the elevated warm–moist air, in turn, forces convective weather systems, hence building a water cycle over the plateau. An integration of mechanical and thermal TP forcing is revealed in relation to the Asian summer monsoon circulation knitting a close tie of vapor transport from tropical oceans to the atmospheric "water tower" over the TP.

Resolving the upper-ocean warm layer improves the simulation of the Madden–Julian oscillation

Here we show that coupling a high-resolution one-column ocean model to an atmospheric general circulation model dramatically improves simulation of the Madden–Julian oscillation (MJO) to have realistic strength, period, and propagation speed. The mechanism for the simulated MJO involves both frictional wave-convective conditional instability of the second kind (Frictional wave-CISK) and air–sea convective intraseasonal interaction (ASCII). In particular, better resolving the fine structure of upper ocean temperature, especially the warm layer, produces more vigorous atmosphere–ocean interaction and strengthens intraseasonal variations in both SST and atmospheric circulation. This helps organize and strengthen deep convection, inducing a stronger Kelvin-wave like perturbation and frictional near-surface convergence to the east. In addition, the warmer SST ahead of the MJO also acts to destabilize the boundary layer and enhance frictional convergence. These lead to a more realistic eastward-propagating MJO. A suite of sensitivity experiments were performed to show the robustness of the mechanisms and to demonstrate: (1) that mean state differences are not the main contributors to the improved simulation of our coupled model; (2) the role of SST variability in enhancing frictional convergence and intraseasonal variations in precipitation, and (3) that the simulation is significantly degraded when the first ocean model layer is thicker than 10 m. Our coupled model results are consistent with observations and demonstrate a simple but effective means to significantly improve MJO simulation and potentially also forecasts.

Roles of the Brazilian Plateau in the Formation of the SACZ

The role of the Brazilian Plateau (BP) in maintaining the South Atlantic convergence zone (SACZ) has been examined by statistical analysis and numerical experiments. Statistical analysis using 27 years of data showed that the SACZ is most intense when it is over the BP. In this case, low-level cyclonic circulation appears over the southwestern part of the BP and forms westerly flow, which intensifies low-level convergence along the SACZ with northeasterly flow from the Amazon and northerly flow along the western edge of the South Atlantic subtropical high. A vorticity budget analysis indicates that precipitation over the BP that accompanies stretching maintains the cyclonic circulation. Sensitivity experiments using a regional atmospheric model for two different cases indicate that precipitation over the BP plays a dominant role as an atmospheric heat source in maintaining the cyclonic circulation and the SACZ. In model experiments in which rain was stopped around the BP but the topography was kept, the cyclonic circulation disappeared, and the SACZ shifted southward away from its original position. In comparison with a control run, precipitation over the BP was weakened and the SACZ shifted southward in experiments in which the BP was removed or its complex, multiple-valley terrain was smoothed out. The results of this study support the ideas suggested in previous studies (i.e., that the BP has an anchor effect on the SACZ): Precipitation is intensified over the complex terrain of the BP, and mechanisms of conditional instability of the second kind occur between the precipitation over the BP and the cyclonic circulation to its southwest, which intensifies moisture convergence over the plateau.

冬季から春季にかけて東シナ海上を伝播する気象津波の発生源に関する解析

Abiki is a kind of meteotsunami (a large amplified seiche) reached to west Kyushu coastal area. This study is aimed to clarify the characteristics of the atmospheric disturbance forcing the tsunami-like ocean long wave. The sea-level pressure disturbance observed and modeled over the East China Sea had its source over the southeastern China mountains and was then propagated by a jet stream toward western Japan with a help of both wave-duct and wave-CISK (conditional instability of the second kind) mechanisms. The backward trajectory analysis also showed that the atmospheric gravity wave propagated with its phase speed of aroung 30 ms-1, and it took 8-12 hours from atmospheric wave source to Kyushu coastal area. Hence, the forecast of the midtroposhere instability can be useful for the meteotsunami prediction.

Atmospheric pressure-wave bands around a cold front resulted in a meteotsunami in the East China Sea in February 2009

Abstract. A meteotsunami hit southwest Kyushu on 25 February 2009, with an estimated maximum amplitude of 290 cm, which was higher than that recorded for the 1979 Nagasaki event. This study investigated mesoscale meteorological systems over the East China Sea during the time leading up to the February 2009 event using a Weather Research and Forecast model. The disturbance in the sea-level pressure originated from a gravity wave over southeastern China. The sea-level pressure disturbance observed and modelled over the East China Sea had its source over the southeastern China mountains and was then propagated by a jet stream toward western Japan with the help of both wave-duct and wave-CISK (conditional instability of the second kind) mechanisms. Two synoptic systems supported the momentum convergence and the formation of band-shaped unstable layers in the mid-troposphere. The high-latitude trough extended from eastern Siberia and there was subtropical high pressure over the western Pacific Ocean. The phase speed of the atmospheric wave was as high as 25–30 m s−1, corresponding to the phase speed of long ocean waves on the East China Sea. Improvements in determining the amplitude and timing of the disturbance remain for future work.

Baroclinic Waves with Parameterized Effects of Moisture Interpreted Using Rossby Wave Components

Abstract A theoretical framework is developed for the evolution of baroclinic waves with latent heat release parameterized in terms of vertical velocity. Both wave–conditional instability of the second kind (CISK) and large-scale rain approaches are included. The new quasigeostrophic framework covers evolution from general initial conditions on zonal flows with vertical shear, planetary vorticity gradient, a lower boundary, and a tropopause. The formulation is given completely in terms of potential vorticity, enabling the partition of perturbations into Rossby wave components, just as for the dry problem. Both modal and nonmodal development can be understood to a good approximation in terms of propagation and interaction between these components alone. The key change with moisture is that growing normal modes are described in terms of four counterpropagating Rossby wave (CRW) components rather than two. Moist CRWs exist above and below the maximum in latent heating, in addition to the upper- and lower-level CRWs of dry theory. Four classifications of baroclinic development are defined by quantifying the strength of interaction between the four components and identifying the dominant pairs, which range from essentially dry instability to instability in the limit of strong heating far from boundaries, with type-C cyclogenesis and diabatic Rossby waves being intermediate types. General initial conditions must also include passively advected residual PV, as in the dry problem.

Destructive meteotsunamis along the eastern Adriatic coast: Overview

The paper overviews meteotsunami events documented in the Adriatic Sea in the last several decades, by using available eyewitness reports, documented literature, and atmospheric sounding and meteorological reanalysis data available on the web. The source of all documented Adriatic meteotsunamis was examined by assessing the underlying synoptic conditions. It is found that travelling atmospheric disturbances which generate the Adriatic meteotsunamis generally appear under atmospheric conditions documented also for the Balearic meteotsunamis (rissagas). These atmospheric disturbances are commonly generated by a flow over the mountain ridges (Apennines), and keep their energy through the wave-duct mechanism while propagating over a long distance below the unstable layer in the mid-troposphere. However, the Adriatic meteotsunamis may also be generated by a moving convective storm or gravity wave system coupled in the wave-CISK (Conditional Instability of the Second Kind) manner, not documented at other world meteotsunami hot spots. The travelling atmospheric disturbance is resonantly pumping the energy through the Proudman resonance over the wide Adriatic shelf, but other resonances (Greenspan, shelf) are also presumably influencing the strength of the meteotsunami waves, especially in the middle Adriatic, full of elongated islands and with a sloping bathymetry. The generated long ocean waves are hitting funnel-shaped bays or harbours of large amplification factors, resulting in meteotsunami waves with heights up to 6 m at the very end of bays or harbours. Within models the mechanism is fairly well understood, but it is extremely difficult to reproduce these events (and presently almost impossible to forecast) as the meteotsunami generating process is highly variable at both temporal and spatial scales. The final part of the paper discusses the possibilities for further research of the Adriatic meteotsunamis, and meteotsunamis in general, including the basis of a meteotsunami warning system which should be able to capture potentially dangerous travelling atmospheric disturbances in real-time.

An Enhanced Moisture Convergence–Evaporation Feedback Mechanism for MJO Air–Sea Interaction

Abstract Simulations using an atmospheric model forced with observed SST climatology and the same atmospheric model coupled to a slab-ocean model are used to investigate the role of air–sea interaction on the dynamics of the MJO. Slab-ocean coupling improved the MJO in Australia’s Bureau of Meteorology atmospheric model over the Indo-Pacific warm pool by reducing its period from 70–100 to 45–70 days, thereby showing better agreement with the 30–80-day observed oscillation. Air–sea coupling improves the MJO by increasing the moisture flux in the lower troposphere prior to the passage of active convection, which acts to promote convection and precipitation on the eastern flank of the main convective center. This process is triggered by an increase in surface evaporation over positive SST anomalies ahead of the MJO convection, which are driven by the enhanced shortwave radiation in the region of suppressed convection. This in turn generates enhanced convergence into the region, which supports evaporation–wind feedback in the presence of weak background westerly winds. A subsequent increase in low-level moisture convergence acts to further moisten the lower troposphere in advance of large-scale convection in a region of reduced atmospheric pressure. This destabilizing mechanism is referred to as enhanced moisture convergence–evaporation feedback (EMCEF) and is utilized to understand the role of air–sea coupling on the observed MJO. The EMCEF mechanism also reconciles traditionally opposing ideas on the roles of frictional wave–conditional instability of the second kind (CISK) and wind–evaporation feedback. These results support the idea that the MJO is primarily an atmospheric phenomenon, with air–sea interaction improving upon, but not critical for, its existence in the model.

A Diagnostic Case Study of Mei-yu Frontogenesis and Development of Wavelike Frontal Disturbances in the Subtropical Environment

Abstract During 6–7 June 2003, a mei-yu front over southern China, with active mesoscale convective systems (MCSs) along it and to its south, intensified rapidly in 24 h to possess a strong cyclonic circulation with relative vorticity centers reaching 1.7 × 10−4 s−1 and a low-level jet (LLJ) of 22.5 m s−1 at 850 hPa. Moreover, both the mass and wind fields and convection developed a wavelike structure at meso-α scale along the front. Using mainly gridded analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF), the present study documented the evolution of this case and diagnosed the mechanisms responsible for its rapid intensification and the development of mesoscale disturbances using methods including the piecewise potential vorticity (PV) inversion. Results indicate that this mei-yu front was characterized by strong horizontal shear and moderate temperature gradient, and its development was not baroclinic in nature. The initiation of a wavelike structure along the frontal shear zone was consistent with barotropic instability. The growth of mesoscale frontal disturbances (cyclogenesis) was a result of the nonlinear mechanism similar to the conditional instability of the second kind (CISK) in which the frontal PV centers and cumulus convection reinforce each other through a positive feedback process. As the MCSs were both intense and persistent, the latent heating was highly efficient and caused significant enhancement of the mei-yu frontal system, even at latitudes below 25°N. The vorticity budget analysis indicates that the front was maintained mainly by a stretching effect, while its gradual eastward extension and slow southward migration were linked to horizontal advection and a tilting effect, respectively. Induced by the MCSs, the transverse circulation south of the front was exceptionally strong, and the LLJ developed within its lower branch of northward-directed ageostrophic flow through Coriolis torque.

A View on Tropical Cyclones as CISK

The author's view on tropical cyclones as the conditional instability of the second kind (CISK) is presented. Many theoretical and numerical studies of tropical cyclones have discussed the original CISK of Ooyama (1964, 1969) and Charney and Eliassen (1964) in which frictional convergence plays an important role. In this paper, the author emphasizes that this CISK is applied primarily to the eyewall circulation in intense tropical cyclones, and describes other types of CISK, which are appropriate to explain many of tropical disturbances and tropical depressions, and the formation and early development stages of tropical cyclones. In addition, the importance of resolving mesoscale organized convection in coarse-resolution numerical models is emphasized.

High-resolution simulation and analysis of the mature structure of a polar low over the sea of Japan on 21 January 1997

This paper presents a high-resolution simulation of a remarkable polar low observed over the Sea of Japan on 21 January 1997 by using a 5-km mesh non-hydrostatic model MRI-NHM (Meteorological Research Institute Non-Hydrostatic Model). A 24-hour simulation starting from 0000 UTC 21 January 1997 successfully reproduced the observed features of the polar low such as the wrapping of western part of an initial E-W orientation vortex, the spiral-shaped bands, the cloud-free “eye”, and the warm core structure at its mature stage. The “eye” of the simulated polar low was relatively dry, and was associated with a strong downdraft. A thermodynamic budget analysis indicates that the “warm core” in the “eye” region was mainly caused by the adiabatic warming associated with the downdraft. The relationship among the condensational diabatic heating, the vertical velocity, the convergence of the moisture flux, and the circulation averaged within a 50 km×50 km square area around the polar low center shows that they form a positive feedback loop, and this loop is not inconsistent with the CISK (Conditional Instability of the Second Kind) mechanism during the developing stage of the polar low.

Contrasting Characteristics between the Northward and Eastward Propagation of the Intraseasonal Oscillation during the Boreal Summer

Abstract This study investigates the structural and evolutionary characteristics of the eastward- and northward-propagating intraseasonal oscillation (ISO) in the Indian Ocean and western Pacific during the boreal summer. Along the equator, the near-surface moisture convergence located to the east of the deep convection region appears to result in the eastward propagation of the ISO, consistent with the frictional wave–CISK (conditional instability of the second kind) mechanism proposed in previous studies. The eastward propagation is characterized by sequentially downstream development of deep convection occuring mainly in certain regions such as 60°, 95°, 120°, and 145°E, and the date line. The northward propagation of deep convection can be attributed to the low-level moisture convergence located to the north. This convergence is a deep structure extending from the surface to the middle troposphere. Near-surface convergence appears only after the systems approach the landmass in the north. It is sugges...

Potential Vorticity Diagnostics of a Mei-Yu Front Case

Abstract The present study selects the mei-yu frontal case of 12–13 June 1990 over southeastern China, and performs potential vorticity (PV) diagnostic analysis to discuss the mechanism responsible for the intensification and maintenance of frontal vorticity. The mei-yu front had typical characteristics at the western section, and was shallow with weak baroclinity but strong horizontal shear vorticity. For this particular case, results of piecewise PV inversion indicate that latent heat release associated with both deep convections and stratiform clouds was responsible for the frontal vorticity, and the apparent frontogenesis near 0000 UTC 13 June was driven almost entirely by an outbreak of deep convections along the front, through the conditional instability of the second kind (CISK) mechanism proposed by Cho and Chen. The positive feedback between the mei-yu frontogenesis and cumulus latent heating, in which the front provided low-level convergence and helped organize the convection while latent heatin...

Propagation and initiation mechanisms of the Madden‐Julian oscillation

An observational study of the propagation and onset mechanisms of the Madden‐Julian oscillation (MJO) is performed using cyclostationary empirical orthogonal function analysis and a Kelvin‐Rossby wave decomposition method for the NOAA outgoing longwave radiation and the NCEP/NCAR Reanalysis data. In these analyses, two different regions of surface convergence are observed ahead of MJO convection by different wind components as previously found in a SST‐coupled GCM simulation. At the leading edge of enhanced convection, the friction‐induced meridional convergence appears during the developing phase when the enhanced convection is located over the Indian Ocean and the Maritime Continents. Another region of surface convergence is identified just east of an enhanced convection core, and this tends to pull the convective core to the east. The surface convergence is formed by zonal wind convergence, and Kelvin and Rossby waves both play a comparable role in the formation of the convergence zone. Therefore the frictional Kelvin‐Rossby wave‐CISK (conditional instability of the second kind) is regarded as a primary factor for the eastward propagation of the MJO. Over the western Indian Ocean, moistening in the boundary layer occurs ∼2 weeks earlier than the beginning of a new MJO cycle. This moistening accompanies low‐level convergence by encircling Kelvin waves which propagated from the region of enhanced convection during the previous cycle, as well as by Rossby waves generated from the region of reduced convection over the Indian Ocean. This earlier formation of the boundary‐layer moisture convergence provides a favorable environment for triggering new convection. This interaction of Kelvin and Rossby waves also accounts for the suppression of convection from the western side of the convection core. For example, over the Indian Ocean, anomalous surface divergence appears at the western edge of the enhanced convection due to both circumnavigating Kelvin waves from the region of reduced convection of the previous cycle and Rossby waves from the region of the enhanced convection itself. Thus the MJO is a self‐maintaining and self‐generating form of tropical variability through the interaction between convection and large‐scale circulation in the presence of boundary‐layer dynamics.

The Observed Structure of Convectively Coupled Kelvin Waves: Comparison with Simple Models of Coupled Wave Instability

Abstract Observations of the horizontal and vertical structure of convectively coupled Kelvin waves are presented and are compared with the predicted structures of moist Kelvin (or gravity) waves in three simple models of coupled wave instability: wave–conditional instability of the second kind (CISK), wind-induced surface heat exchange (WISHE), and stratiform instability. The observations are based on a linear regression analysis of multiple years of ECMWF reanalysis and station radiosonde data. Results suggest that both the wave-CISK and stratiform instability theories successfully predict many important features of observed moist Kelvin waves, but that unrealistic aspects of these models limit their ability to provide comprehensive explanations for the dynamics of these waves. It is suggested that an essential component of any theory for moist Kelvin waves is the second baroclinic mode heat source associated with stratiform precipitation.

Kinematic Characteristics of a Mei-yu Front Detected by the QuikSCAT Oceanic Winds

Abstract Based on conventional surface observations and NASA Quick Scatterometer (QuikSCAT) data, a heavy rainfall event that occurred in the Taiwan mei-yu season was chosen to further study the kinematic characteristics of the accompanying surface front. With the help of the QuikSCAT oceanic surface winds, it was found that the location and propagation of a mei-yu front over the ocean to the east of Taiwan during 10–12 June 2000 are better represented by the frontal wind shift line, which was located approximately on the leading edge of the baroclinic zone. The mesoscale system with cyclonic circulation embedded within the frontal zone was clearly shown in the wind field and kinematic parameters (horizontal divergence and vorticity) as well as satellite clouds and rainfall estimations. The conditional instability of the second kind (CISK) process was suggested to be responsible for the intensification of the mei-yu front and the frontal disturbance over the ocean. Under the influence of island topography...

The Impact of Surface Flux– and Circulation-Driven Feedbacks on Simulated Madden–Julian Oscillations

Abstract The intent of this study is to measure the total contribution of individual wind- and moist flux–induced feedback mechanisms on the generation and maintenance of simulated Madden–Julian oscillations (MJOs). This task is accomplished through the use of an idealized GCM that is also employed to examine the impact of sea surface temperatures on the robustness of the MJO signal. Among the mechanisms investigated are Conditional Instability of the Second Kind (CISK), Wind Induced Surface Heat Exchange (WISHE), and the feedback due to specific humidity differences between the surface and the boundary layer. A series of numerical experiments was conducted in which one or more of these feedbacks were suppressed. The resulting structure and periodicity of the simulated mode was analyzed using eigenvector and spectral analysis methods. The relative phases of surface wind, turbulent flux, and boundary layer moisture anomaly fields were then contrasted. It was found that the model produced a far stronger sig...