The Global Precipitation Measurement (GPM) Dual-Frequency Precipitation Radar (DPR) has operated in full scan (FS) mode in both the Ku-band and Ka-band since May 2018. This FS mode can enable about 245 km full swath dual frequency processing for the first time, whereas previous algorithms enabled dual frequency processing in the narrower inner swath, having about 125 km width, only. This paper describes the classification (CSF) module in newly developed DPR level 2 V06X experimental algorithms corresponding to FS mode. The CSF module classifies precipitation as three major types: stratiform, convective, and other, and provides bright-band (BB) information.
One-month statistics show that a marked jump occurs in the Ka-band precipitation type counts at the edges of the inner swath due to the different sensitivity of Ka-band radar in Ka-band Matched Scan (Ka-MS) mode compared with that in Ka-band High Sensitivity (Ka-HS) mode. However, statistics on precipitation type counts indicate that dual frequency processing works well not only in the inner swath but also in the outer swath. Statistics on BB counts show a significant improvement in BB detection, especially in the outer swath when dual frequency processing is performed.
In addition, the V06X Ku-band algorithm solves two problems related to the CSF module: (a) appearance of a very large near surface precipitation rate of stratiform precipitation reclassified by the slope method, and (b) a rare case of misjudging BB peak as an upper part of surface echo.
The data structures of GPM DPR algorithms were drastically changed in V06X. The new data structures introduced in V06X will be used in V07A and later. In this regard, the V06X CSF algorithms outlined in this paper will be a prototype of the future versions of each CSF algorithm.
This study proposes an algorithm detecting atmospheric blocking by extracting topological features of geopotential height data at 500 hPa. The algorithm uses topological flow data analysis (TFDA) providing a unique symbolic representation, named the partially cyclically ordered rooted tree (COT) representation, and a discrete graph structure, called a Reeb graph, to each structurally stable Hamiltonian vector field based on the mathematical theory of topological classifications for streamline patterns. It recognizes blocking events more simply and effectively using fewer meteorological parameters than conventional algorithms. Furthermore, the algorithm can determine morphological types of blocking events, an Omega shape or a dipole pattern, whereas no effective algorithm has been available so far. The identified blocking events and their morphological types are consistent with synopticians’ subjective judgments.
A non-Gaussian probability distribution function (PDF) and a new displacement correction method using PDF pseudo-regimes for precipitation were introduced to the dual-scale neighboring ensemble-based variational assimilation scheme (EnVar) for achieving improved assimilation of all-sky microwave imager (MWI) brightness temperatures (TBs) into a cloud-resolving model (CRM).
We evaluated the fits of the precipitation forecast perturbations of various disturbances with the existing non-Gaussian PDF models and selected a mixed lognormal distribution for the precipitation PDF model. Then we introduced rain-free and rainy PDF regimes to EnVar. We developed a new method for correcting precipitation displacement that introduces pseudo rain-free, rainy, and heavy-rain regimes and approximated their PDFs as regional averages of the PDFs around the target point. We estimated the horizontal scales of averaging based on the similarity of precipitation forecast perturbations. These methods improved the bias and normality of TB differences between observation and the first guess.
We conducted assimilation experiments using all-sky MWI TB observations for Typhoon Etau (T1518). Results show that the precipitation analysis using the EnVar employed herein was more similar to the global satellite map for precipitation (GSMaP) retrievals than those using a conventional EnVar. The introduction of a mixed lognormal PDF strengthened the precipitation analysis of heavy-rain areas around the typhoon and near a front. The usage of PDF pseudo-regimes considerably reduced the precipitation displacement error of the analysis. The EnVar employed herein improved the CRM forecasts for precipitation distribution up to 12 h and the typhoon position and central surface pressure for more than 24 h. The forecast analysis cycle of EnVar improved the CRM forecasts for heavy rain around the typhoon center up to 6 h and a heavy-rain band associated with the typhoon for more than 24 h when compared with the EnVar using a single-time TB observation.
Precipitation statistics from Global Precipitation Measurement Core Observatory Dual-Frequency Precipitation Radar (GPM DPR) are underestimated due to systematic bias depending on the scanning angle. Over five years of GPM DPR KuPR Version 06A data, the precipitation anomaly is −7 % and −2 % over land and ocean, respectively. This study improves the estimation of low-level precipitation-rate profiles and the detection of shallow storms (with top heights of ≤ 2.5 km), using reference datasets of near-nadir measurements.
First, the low-level precipitation profile (LPP) is updated using an a priori near-nadir database generated from structural-characteristics related variables of the precipitation and environmental parameters. The LPP correction increases precipitation over areas where downward-increasing precipitation profiles are dominant below 2 km, such as at high elevations and at middle and high latitudes. Globally, the LPP correction increases precipitation by 5 %. Second, the effect on precipitation data of missing shallow storms is estimated using the angle-bin difference in the detectability of storms with a top height of ≤ 2.5 km. The effect of the shallow precipitation deficiency (SPD) is comparable in magnitude to that of the LPP correction. A priori lookup tables for the SPD correction, constrained by the clutter-free bottom level and spatially averaged shallow precipitation fractions, are constructed so that the correction applies to gridded statistics at 0.1° and three-month scales. The SPD correction enhances precipitation by 50 % over specific low-rainfall oceans in the sub-tropics and at high latitudes, where shallow precipitation dominates. From these two corrections, precipitation increases by 8 % and 11 % over land and ocean, respectively. At latitudes between 60°N and 60°S, the difference in KuPR compared with satellite-gauge blended products is reduced from −17 % to −9 %, whereas that compared with gauge-based products is reduced from −19 % to −15 % over land.
An unprecedented cold wave swept through most parts of East Asia in January 2016, leading to record–breaking low temperatures and widespread snowstorms in many regions. Our analysis indicated that this East Asian cold wave was triggered by a series of consecutive extreme events in the Northern Hemisphere from late 2015 to early 2016. (1) On 28 December 2015, a severe cyclonic storm emerged in the North Atlantic, and a downstream blocking high formed in Europe through the downstream development process. The strong southerly jet stream between the storm and its downstream–blocking high steered the storm into the Arctic Circle, transported enormous warm and moist air masses, establishing warm conveyor belts, which led to an extraordinary Arctic warming event in late 2015; (2) This Arctic warming event in late 2015 resulted in a distinct Arctic dipole pattern resembling the negative phase of the Arctic Oscillation in early–mid-January 2016; and (3) The dipole pattern induced eastward propagation of the Rossby wave and led to the occupation of two downstream blocking highs in Urals and western North America. These two blocking highs, together with the low pressure between them, resulted in an inverted omega–shaped circulation pattern (IOCP) over the Siberia–North Pacific region. In addition, the IOCP distinctly modulated the meridional circulation cell along the East Asia–Siberia regions, which generated negative vorticity and anticyclonic advection to the Siberian region, ultimately intensifying the Siberian high. The IOCP and the associated enhanced Siberian high eventually induced the outbreak of a mega-cold wave in East Asia in January 21-25 2016.
This article describes humidity data correction based on an intercomparison between two manufacturers’ radiosondes with the assessment using precipitable water vapor (PWV) derived from Global Navigation Satellite System (GNSS) signals. In addition, we propose a method to determine whether the same correction procedure can be appliedif such intercomparison cannot be conducted.
During the intensive observation called Years of the Maritime Continent (YMC) - Boreal Summer Monsoon (BSM) study in 2018 (YMC-BSM 2018), intercomparison of radiosonde between Lockheed Martin LMS6 and Vaisala RS41-SGP was conducted at Laoag, Ilocos Norte, Philippines from late July to early August 2018. While their mean difference of relative humidity (RH) was better than 5%, dry bias was confirmed for LMS6 only during clear sky daytime soundings based on comparing PWV with that derived from GNSS signals. To use different radiosonde data with the same research-quality, we developed a correction table for LMS6 RH data.
While a direct intercomparison between different radiosondes and independently developed observational tools such as a GNSS-receiver is ideal to evaluate data quality, it cannot always be performed. We obtained LMS6 radiosonde data at different site at Yap Island, Federated States of Micronesia from another field campaign, YMC-BSM 2020, where any intercomparison could not be conducted. To decide whether the same correction procedure obtained from YMC-BSM 2018 can be applied to those data, we assessed their similarity based on the relationship between specific humidity from surface meteorological station data that were obtained independently before launch and radiosonde specific humidity averaged over 300 m from the initial radiosonde measurement point. This method allowed us to confirm the same behavior between Laoag data in 2018 and Yap data in 2020; thus, we applied our correction method to RH data in YMC-BSM 2020.
Stratospheric profiles of the mean age of air estimated from cryogenic air samples acquired during a field campaign over Indonesia, the Coordinated Upper-Troposphere-to-Stratosphere Balloon Experiment in Biak (CUBE/Biak), are investigated by employing the boundary impulse evolving response (BIER) method and Lagrangian backward trajectories, with the aid of an atmospheric general circulation model-based chemistry transport model (ACTM). The ACTM provides realistic meteorological fields at one-hour intervals by nudging toward the European Centre for Medium-Range Weather Forecasts Reanalysis-Interim (ERA-Interim). Since the BIER method is capable of taking unresolved diffusive processes into account, while the Lagrangian method can distinguish the pathways the air parcels took before reaching the sample site, the application of the two methods to the common transport field simulated by the ACTM is useful in assessing the CO2- and SF6-derived mean ages. The reliability of the simulated transport field has been verified by the reproducibility of the observed CO2, SF6, and water vapor profiles using the Lagrangian method. The profile of CO2 age is reproduced reasonably well by the Lagrangian method with a small young bias being consistent with the termination of trajectories in finite length of time, whereas the BIER method overestimates the CO2 age above 25 km altitude possibly due to high diffusivity in the transport model. In contrast, the SF6 age is only reproducible in the lower stratosphere, and far exceeds the estimates from the Lagrangian method above 25 km altitude. As air parcels of mesospheric origin are excluded in the Lagrangian age estimation, this discrepancy, together with the fact that the observed SF6 mole fractions are much lower than the trajectory-derived values in this height region, supports the idea that the stratospheric air samples are mixed with SF6-depleted mesospheric air, leading to overestimation of the mean age.
In this study, we assess the prediction skill of the Boreal Summer Intra-Seasonal Oscillation (BSISO) mode of one-month simulations using a global Non-hydrostatic Icosahedral Atmospheric Model (NICAM) with explicit cloud microphysics and a grid spacing of 14 km. The simulations were run as a series of hindcast experiments every day of August from 2000 to 2014. A total of 465 simulations were run with a 13950-day integration. Using forecast skill scores for statistical measurements, it was found that the model showed an overall BSISO prediction skill of approximately 24 days. The prediction skill tended to be slightly higher (~ 2 days) when BSISO events began in the initial phases 7 to 1, which corresponded to the re-initiation phase of the BSISO, during which a major convective center over the Philippine Sea decayed and a new convective envelope began aggregating over the western Indian Ocean. The phase speed and the evolution of the amplitude of the BSISO were well simulated by the model with a clear northwest–southeastward tilted outgoing longwave radiation (OLR) structure over the Maritime Continent and the western Pacific. However, the propagation speed was slower during phases 6 and 7, and the amplitude of the BSISO largely decayed during phases 8–1, which is likely to have been associated with the stagnant behavior of the convective cells over the Philippines sea. Based on a regression coefficient analysis using the moist static energy, the stagnation of the propagation over the Philippines was found to be largely attributable to the small southerly background bias in the model over the Philippines. The bias in the large-scale circulation was likely to have been associated with the bias in the moisture field and the background monsoonal circulation. We concluded that the model physics controlling the background fields are important factors in improving BSISO prediction skill.
Specific attenuation and equivalent radar reflectivity in a melting layer were measured using a dual Ka-band radar system. The system consists of two identically designed Ka-radars. When the two radars are arranged to face each other and a precipitation system comes between the two radars, they observe the system from opposite directions. The radar echoes suffer from rain attenuation, which appears symmetrically in both radar echo profiles. By differentiating measured radar reflectivity with range, the specific attenuation (k) can be estimated. After obtaining the specific attenuation, the equivalent radar reflectivity (Ze) is estimated. Melting layer observations were conducted on a slope of Mt. Zao, Japan. In the melting layer, the specific attenuation and the equivalent radar reflectivity vary considerably along the radio path. The relationship between k and Ze showed interesting characteristics which appears in a loop-shape on a k-Ze diagram. A simple theoretical study using the Rayleigh and Mie scattering theories for melting snow spheres showed that the basic loop-shape is resulted from the change in permittivity of precipitation particles during melting. The loop-shape is greatly expanded by change of the particle size. The Mie effect which is significant for large precipitation particles slightly modifies the loop-shape by reducing backscattering cross sections. The results also explain that the shelf-like profile instead of the peak-like in Ze.
Previous studies have suggested that the recent increase in tropical extreme deep convection, in particular over Asia and Africa during the boreal summer, has occurred in association with a cooling in the tropical lower stratosphere. The present study is focused on the Sahel region of West Africa, where an increased occurrence of extreme precipitation events has been reported over recent decades. The results show that the changes over West Africa since the 1980s involve a cooling trend in the tropical lower stratosphere and tropopause layer, combined with a warming in the troposphere. This feature is similar to that which might result from increased greenhouse gas levels, but is distinct from the interannual variation of precipitation associated with the transport of water vapor from the Atlantic Ocean. It is suggested that the decrease in the vertical temperature gradient in the tropical tropopause region enhances extreme deep convection over the Sahel, where penetrating convection is frequent, whereas tropospheric warming suppresses the shallower convection over the Guinea Coast. The essential feature of the recent changes over West Africa is therefore the depth of convection, rather than the total amount of surface precipitation.
This study examined the performance of Eulerian models in simulating dispersion fields at two coastal monitoring stations in the vicinity of a pollutant source (3.2 and 17.5 km distant) under the situation of the Fukushima 2011 nuclear accident using two horizontal resolutions (3 km and 250 m). A 250-m grid simulation was newly performed for the examination and was able to reproduce the wind and concentration fields in detail over complex terrain. The 3-km grid model could not reproduce the details of the winds and plumes around the Fukushima Daiichi Nuclear Power Plant but occasionally yielded a higher performance with a lower undetected error rate than the 250-m grid model due to the large numerical diffusion of the former. A disadvantage of Eulerian dispersion models is expected to be the artificial numerical diffusion in the advection process near emission sources. The artificial numerical diffusion increases the false alarm ratio (number of strikeouts while swinging) but fortunately decreases the undetected error rate (number of strikeouts while looking). This characteristic is appropriate for environmental emergency response (EER) systems. Furthermore, the 250-m grid model result was robustly improved by a plume augmentation (i.e., max pooling) process, which enlarged the plume widths and masked short time lags and small plume drifts. Plume augmentation was advantageous to the high-resolution model to improve statistical scores, which is beneficial for EER systems.
The assimilation of cloudy and rainy microwave observations is under investigation at Météo-France with a method called ‘1D-Bay+3D/4D-Var’. This method consists of two steps: (i) a Bayesian inversion of microwave observations and (ii) the assimilation of the retrieved relative humidity profiles in a 3D/4D-Var framework. In this paper, two estimators for the Bayesian inversion are used: either a weighted average (WA) or the maximum likelihood (ML) of a kernel density function. Sensitivity studies over the first step of the method are conducted for different degrees of freedom: the observation error, the channel selection and the scattering properties of frozen hydrometeors in the observation operator. Observations over a two-month period of the Global Precipitation Measurement (GPM) Microwave Imager (GMI) on-board the GPM-Core satellite and forecasts of the convective scale model Application of Research to Operations at Mesoscale (AROME) have been chosen to conduct these studies. Two different meteorological situations are analysed: those predicted cloudy in AROME but clear in the observations and, on the contrary, those predicted clear in AROME but cloudy in the observations.Main conclusions are as follows. First, low observational errors tend to be associated with the profiles with the highest consistency with the observations. Second, the validity of the retrieved profiles varies vertically with the set of channels used. Third, the radiative properties used in the radiative transfer simulations have a strong influence on the retrieved atmospheric profiles. Finally, the ML estimator has the advantage of being independent of the observation error but is less constrained than the WA estimator when few frequencies are considered. While the presented sensitivities have been conducted to incorporate the scheme in a data assimilation system, the results may be generalized for geophysical retrieval purposes.
In this study, we compare the accuracy of five representative similarity metrics in extracting sea level pressure (SLP) patterns for accurate weather chart classification: correlation coefficient, Euclidean distance (EUC), S1-score (S1), structural similarity (SSIM), and average hash. We use a large amount of teacher data to statistically evaluate the accuracy of each metric. The evaluation results reveal that S1 and SSIM have the highest accuracy in terms of both average and maximum scores. Their accuracy does not change even when non-ideal data are used as the teacher data. In addition, S1 and SSIM can reproduce the subjective resemblance between two maps better than EUC. However, EUC reproduces the central position of the signal in a sample case. This study can serve as a reference for identifying the most useful similarity metric for the classification of SLP patterns, especially when using non-ideal teacher data.
This study shows that the impact of El Niño-Southern Oscillation (ENSO) Modoki on boreal summer tropical cyclone (TC) formation over the western North Pacific (WNP) has experienced decadal changes during the past few decades. The correlation between the ENSO Modoki index (EMI) and TC frequency over the WNP is weak between 1975-1989, becomes strong and significant between 1990-2004, and becomes weak again between 2005-2019. Over the eastern part of the WNP, ENSO Modoki enhanced TC formation during 1990-2004 but did not significantly impact on the TC formation during 1975-1989 and 2005-2019. The difference in correlation strength primarily results from changes in large-scale features related to ENSO Modoki among the three sub-periods (1975-1989, 1990-2004 and 2005-2019). El Niño Modoki from 1990-2004 was characterized by a tripole SST pattern with maximum SST anomalies in the equatorial central Pacific, while from 1975-1989 and 2005-2019, the maximum SST anomalies were located over the subtropical northeastern Pacific. The two primary environmental variables likely leading to these observed relationships between ENSO Modoki and TCs were mid-level moisture (RH600) and low-level vorticity (VOR850). From 1990-2004, TC formation was enhanced both south of 20°N and north of 20°N. The increase in TC activity during El Niño Modoki south of 20°N was likely tied to greater RH600 and north of 20°N to larger cyclonic VOR850. In contrast, ENSO Modoki's impacts on both VOR850 and RH600 were weak from 1975-1989 and 2005-2019.
This paper focuses on the uncertainty of summer precipitation estimations produced by Global Satellite Mapping of Precipitation (GSMaP) over Mongolia, a region that has complex terrain and sparse weather observation networks. We first compared average summer precipitation over Mongolian territory as reported by several precipitation products. Although the interannual variability of the product was comparable, the amount of recorded precipitation differed among the various products. The rain-gauge-based analysis reported the lowest amount of precipitation, while the satellite-based GSMaP_MVK reported the highest amount. Our results represent a first estimate of the characteristic differences among the various precipitation-monitoring products, including GPM-based products, as they relate to climatic and hydro-meteorological assessments in Mongolia. We then made a detailed comparison using a case study in which a heavy rainfall event was captured by the Global Precipitation Measurement (GPM) mission's core observatory near Ulaanbaatar in July 2016. In this case, gauged and ungauged GSMaP estimates of the precipitation over the mountain area differed substantially between algorithm versions 6 and 7. An intercomparison of atmospheric numerical modeling, the GPM core observatory, and rain gauge observation showed that the rain gauge calibration of GSMaP effectively moderates the large error of the ungauged GSMaP data. The source of the significant ungauged GSMaP error is likely to be the rain rate estimates in version 7 of the algorithm. However, GSMaP gauge-calibrated estimates of the precipitation over mountainous areas may be affected by a potential underestimation of gauge analysis due to the missing localized precipitation occurring in the large gaps of the routine observation network. We expect that these findings will be helpful for developers seeking to further improve the GSMaP algorithm.
The evolution of a heavy rainfall event occurred on 19 August 2014 in northern Taiwan is investigated with observed data and analyses from a newly-developed system named IBM_VDRAS, which is based on a four-dimensional Variational Doppler Radar Assimilation System (VDRAS) with the capability to assimilate radar observations and surface station data over a complex terrain by adopting the Immersed Boundary Method (IBM). This event possesses different precipitating processes and track from those frequently observed in that region.
From the surface observations and the high spatiotemporal resolution analysis fields generated by IBM_VDRAS, it is found that the rainfall process started with the initiation of two individual convective cells triggered through the interaction between land-sea breeze and terrain in two different cities (Taoyuan and Taipei). The outflow of one of the convective cells developed in Taoyuan City at an earlier time merged with another convective system which grew in Taipei Basin, and provided favorable conditions to intensify the latter. The enhanced major convective cell moved into the Taipei City metropolitan area and produced 80 mm of precipitation within approximately 2.5 h. The kinematic, thermodynamic, and microphysical fields of the convective cells are analyzed in details to explain the mechanisms which helped to maintain the structure of the rainfall system. Sensitivity experiments of quantitative precipitation forecast (QPF) show that the terrains prevent the location of major rainfall from shifting outside of Taipei Basin. By assimilating surface data, the model can better predict the position of the rainfall.
In this study, we examine the resolution dependence of the convective spectrum in Community Atmospheric Model version 5 (CAM5) simulations, focusing on the transition from shallow to deep convection and the associated cloud-radiative effect (CRE) change. We first apply the bin method (percentile binning) on precipitation intensity to obtain the convective spectrum in the tropics. The same approach is also used in the column-integrated moist static energy (MSE) budget analysis. The binning results show that over the light-rain regime, the convective structure is dominated by shallow convection, functioning to destabilize the atmosphere by importing column-integrated MSE. The heavy-rain regime shows the coexistence of deep and shallow convection, which inclines to stabilize the atmosphere by exporting the column-integrated MSE. Moreover, we also note that the longwave (LW) component of CRE (LWCRE) is more sensitive to the change of model spatial resolution than the shortwave (SW) component of CRE (SWCRE), characterized by a stronger response in the coarser resolution run over the heavy-rain regime. The resolution dependence of convective spectrum and CRE changes presented in this study highlights the importance of scale-aware cumulus parameterization design in climate models, which is not yet implemented in CAM5.
This study examined the roles of heat fluxes from the Kuroshio Current in enhancing a frontal convective rainband associated with an extratropical cyclone that brought record-breaking heavy rainfall to Miyake Island, Japan, in January 2017. A simulation of the rainband (control experiment) and sensitivity experiments without the sensible and latent heat fluxes from the Kuroshio Current were conducted using a regional cloud-resolving model. The rainband developed along a non-classic front (outer front), which formed to the north of a warm front associated with the cyclone. The control experiment reproduced the intensity and migration of the rainband well. As the rainband developed, the heat fluxes from the Kuroshio Current became evident around the cold conveyor belt of the cyclone to the south of the rainband. The latent heat fluxes were ~2.3 times larger than the sensible heat fluxes. Comparisons between the control and sensitivity experiments demonstrated that the heat fluxes, especially the latent heat fluxes, enhanced the rainband. The sensible heat fluxes slightly intensified convective instability in the lower troposphere, whereas the latent heat fluxes significantly increased the near-surface moisture content and the convective instability. A frontal updraft along the outer front released the increased convective instability, which intensified the moisture convergence, condensation, and updraft, enhancing the rainband. The findings show that the heat fluxes from the Kuroshio Current, especially the latent heat fluxes, enhanced the heavy rainfall-producing rainband by increasing the moisture content and the convective instability.
The boreal summer intraseasonal oscillation (BSISO) is among the most pronounced subseasonal variability in the tropics during boreal summer. Compared to its wintertime counterpart, the so-called Madden-Julian oscillation (MJO), the BSISO convection displays more complicated spatio-temporal evolution, characterized by northward propagation over the northern Indian Ocean and western North Pacific as well as eastward propagation along the equator. It exerts a strong influence on a broad range of tropical weather and climate phenomena such as tropical cyclogenesis, monsoon onset and active/break cycles, among others. Our fundamental understanding of the BSISO has steadily advanced: so far various aspects of the BSISO have been described and several theories aiming to explain its northward propagation have been proposed. Yet, our skill to simulate the BSISO by general circulation models remains unsatisfactory, though it has been improved. This paper reviews some fundamental aspects of the BSISO from the viewpoint of observation, theory, and modeling.
This paper examined southwesterly flows and rainfall around the Taiwan area during the mei-yu seasons from 1979 to 2018. The occurrence percentage of the southwesterly flow events in southern Taiwan was highly correlated with 6-h accumulated rainfall and heavy precipitation in Taiwan, while those in northern Taiwan showed little correlation. Low pressure to the north of Taiwan and high pressure to the south exerted a large northward pressure gradient force across the Taiwan area, favoring the formation of southwesterly flows and rainfall in southern Taiwan. During an active year of southwesterly flow events, the Pacific high weakens and moisture is transported along two paths in the early mei-yu season: one from the Bay of Bengal and the other from the south of the Pacific high. The moisture-laden air results in high equivalent potential temperature near Taiwan, which in turn creates a large equivalent potential temperature gradient to the north of Taiwan. This setting favors the activity of mei-yu fronts and produces a low pressure environment. The pressure gradient thus increases, supporting the formation of southwesterly flows. The southwesterly flows then help to transport more moisture toward the Taiwan area, resulting in heavy rainfall as well as a further increase of equivalent potential temperature. This kind of positive feedback produces more fronts, stronger southwesterly flows, and heavier rainfall during the mei-yu season. The study also suggests that the meridional component of vertically integrated water vapor transport over the South China Sea and the Philippines in the early mei-yu season can be used to predict the occurrence of southwesterly flows and heavy rain for the entire mei-yu season.
Observational and model studies suggest that the stratosphere exerts a significant influence on the tropical troposphere. The corresponding influence, through dynamical coupling, of the stratosphere on the extratropical troposphere has over the last 15-20 years been intensively investigated, with consequent improvement in scientific understanding which is already being exploited by weather forecasting and climate prediction centres. The coupling requires both communication of dynamical effects from stratosphere to troposphere and feedbacks within the troposphere which enhance the tropospheric response. Scientific understanding of the influence of the stratosphere on the tropical troposphere is far less developed. This review summarises the current observational and modelling evidence for that influence, on timescales ranging from diurnal to centennial. The current understanding of potentially relevant mechanisms for communication and for feedbacks within the tropical troposphere and the possible implications of the coupling for weather and climate prediction are discussed. These include opportunities for model validation and for improved subseasonal and seasonal forecasting and the effects, for example, of changes in stratospheric ozone and of potential geoengineering approaches. Outstanding scientific questions are identified and future needs for observational and modelling work to resolve these questions are suggested.
Mesoscale patterns in atmospheric convection (between the inner scale of convecting-layer depth and the outer scales of domain constraints) are fascinating and ubiquitous. This review asks whether some aspects of that form (normalized for a given amount of convective activity) play a meaningful role or function in the total flow, especially in its more-predictable larger scales. Do some mesoscale features deserve to be called organization in its stronge sense, acting like multi-cellular organs in an organism? After enumerating hypotheses from null (mesoscale arrangement doesn't matter) to various detailed ideas (rectification of nonlinear processes with spatial agglomeration, size-dependent top-heaviness of heating, vertical momentum flux effects, adjustment roles, and the character of stochastic noise), a tabular framework for categorizing form-function research is offered. Function measures are divided into micro (mere quantification of budget terms averaged over mesoscale patches) vs. macro (roles played through time in larger-scale phenomena). Tools and approaches are arrayed from literal and explicit (case observations) to conceptualized (models, ranging from theory to numerical to statistical depictions), on timelines both historical (contacting case observations in some way) and synthetic (theory, simulation, and composites). Efforts are further distinguished by whether their inferences are associative (derived from conditional sampling of either form or function) or causal (involving controlled experimentation). Literature examples are surveyed, albeit incompletely, and future research strategies are suggested across this tabular landscape or framework. One spotlighted result is an apparent internal optimum in the horizontal geometry continuum between isotropic horizontal two-dimensionality and horizontally one-dimensional squall lines. Form-function questions could help justify, orient, and capitalize scientifically on the field's costly multiscale activities (requiring both coverage and resolution) in both observational and modeling realms. Data assimilation is a motivating application, and also a potentially powerful research tool for achieving greater synthesis. An observant human sensibility remains crucial for discovering and interpreting form-function relationships, at the very least to design more salient algorithms in the age of big data and computing.
Future change of Rossby wave breaking (RWB) frequency over the middle North Pacific (MNP) in August and the related features of large-scale atmospheric circulation are examined using large-ensemble simulations of current and future climates with a global circulation model. Correlation analysis indicates that the RWB frequency over the MNP in the current climate can show a relationship to El-Niño Southern Oscillation as reanalysis. The RWB frequency in the future climate shows significant decreases over the MNP, compared to that in the current climate. The large-scale atmospheric circulation in the upper troposphere in the future climate indicates a significant weakening of the Asian summer monsoon circulation and the consequent southward shifted Asian jet. The decreased RWB frequency over the MNP is associated with the modulated Asian jet, through reduced diffluence and deceleration of the jet in the basic-state over the region. Rossby wave propagation over Eurasia and the North Pacific in mid-latitudes is also clearly reduced in the future climate, consistent with the decreased RWB frequency over the MNP. Correlation and histogram analyses of the current and future experiments indicate that the significantly decreased RWB frequency over the MNP is associated with significantly suppressed convective activities east of the Philippines in the future climate. The diagnosis using ω-equation further shows the dynamical impact of the decreased RWB frequency on the suppressed convective activities, through the weakened extension of the Mid-Pacific trough and the consequent weakening of dynamically induced ascent east of the Philippines.