Journal of the Meteorological Society of Japan. Ser. II
Online ISSN : 2186-9057
Print ISSN : 0026-1165
ISSN-L : 0026-1165
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Article : Special Edition on Global Precipitation Measurement (GPM): 5th Anniversary
  • Shinta SETO, Toshio IGUCHI, Robert MENEGHINI, Jun AWAKA, Takuji KUBOTA ...
    2021 Volume 99 Issue 2 Pages 205-237
    Published: 2021
    Released: April 19, 2021
    [Advance publication] Released: December 16, 2020
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    In this paper, precipitation rate retrieval algorithms for the Global Precipitation Measurement mission's Dual-frequency Precipitation Radar (DPR) are developed. The DPR consists of a Ku-band radar (KuPR; 13.6 GHz) and a Ka-band radar (KaPR; 35.5 GHz). For the KuPR, an algorithm similar to the Tropical Rainfall Measuring Mission's Precipitation Radar algorithm is developed, with the relation between precipitation rate R and massweighted mean diameter Dm (R−Dm relation) replacing the relation between the specific attenuation k and effective radar reflectivity factor Ze. The R−Dm relation can also be applied to the KaPR and dual-frequency algorithms. In both the single-frequency and dual-frequency algorithms, the forward retrieval method is applied with an assumed adjustment factor for the R−Dm relation (ε) and the results are evaluated to select the best value of ε. The advantages of the dual-frequency algorithm are the availability of the dual-frequency surface reference technique and the ZfKa method, which is a method to use the attenuation-corrected radar reflectivity factor Zf of KaPR, to select ε as well as the possibility to selectively use measurements from KuPR or KaPR. This paper also describes the derivation of the scattering table and the R−Dm relation as well as the procedure for non-uniform beam filling correction in detail. The outputs are then statistically analyzed to demonstrate algorithm performance.

Review Article
  • Matthew H. HITCHMAN, Shigeo YODEN, Peter H. HAYNES, Vinay KUMAR, Susan ...
    2021 Volume 99 Issue 2 Pages 239-267
    Published: 2021
    Released: April 14, 2021
    [Advance publication] Released: December 02, 2020
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    The history of observational studies regarding the influence of the stratospheric quasi-biennial oscillation (QBO) on the tropical and subtropical upper troposphere and lower stratosphere (UTLS) is reviewed. QBO phases of westerly (QBO W) and easterly (QBO E) winds are defined in the lower stratosphere. During 1960–1978, radiosonde data revealed QBO modulation of the UTLS with a warm anomaly during QBO W in the tropics and cool anomalies near 30°S and 30°N. This pattern agreed with theory of the QBO mean meridional circulation (MMC), which predicted a coherent, antiphased response between the tropics and subtropics. During 1978–1994, satellite observations of aerosol and temperature confirmed the existence of the QBO MMC. During 1994–2001, global data sets enabled analysis of zonal mean QBO variations in tropopause temperature. In 2001, National Centers for Environmental Prediction reanalyses for the 42-yr period 1958–2000 revealed seasonal and geographical variations in QBO W–E tropopause temperature, pressure, and zonal wind, which are presented here. An update using the 38-yr Modern-Era Retrospective Analysis for Research and Applications, Version 2, and the 40-yr European Centre for Medium Range Weather Forecasts Reanalysis (ERA-Interim) data sets provides a more complete view of seasonal and geographical variation.

    The QBO range in tropical tropopause values is ∼ 0.5–2 K, ∼ 100–300 m, and ∼ 1–3 hPa, being colder and higher during QBO E, especially during boreal winter and spring. The QBO temperature signal tends to be larger near regions where deep convection is common. The QBO signal in the southern subtropics is enhanced during austral winter. During QBO W, the subtropical westerly jet is enhanced, while the Walker circulation is weaker, especially during boreal spring. A new climatology of the QBO MMC is presented. QBO E may enhance convection by reducing both static stability and wind shear in the UTLS.

Article : Special Edition on Global Precipitation Measurement (GPM): 5th Anniversary
  • Hong-Wen JIAN, Wei-Ting CHEN, Peng-Jen CHEN, Chien-Ming WU, Kristen L. ...
    2021 Volume 99 Issue 2 Pages 269-285
    Published: 2021
    Released: April 15, 2021
    [Advance publication] Released: December 02, 2020
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    This study investigates the synoptic-scale flows associated with extreme rainfall systems over the Asian–Australian monsoon region (90–160°E and 12°S–27°N). On the basis of the statistics of the 17-year Precipitation Radar observations from Tropical Rainfall Measurement Mission, a total of 916 extreme systems, with both the horizontal size and maximum rainfall intensity exceeding the 99.9th percentiles of the tropical rainfall systems, are identified over this region. The synoptic wind pattern and rainfall distribution surrounding each system are classified into four major types: vortex, coastal, coastal with vortex, and none of above, with each accounting for 44, 29, 7, and 20 %, respectively. The vortex type occurs mainly over the off-equatorial areas in boreal summer. The coast-related types show significant seasonal variations in their occurrence, with high frequency in the Bay of Bengal in boreal summer and on the west side of Borneo and Sumatra in boreal winter. The none-of-the-above type occurs mostly over the open ocean and in boreal winter; these events are mainly associated with the cold surge events. The environment analysis shows that coast-related extremes in the warm season are found within the areas where high total water vapor and low-level vertical wind shear occur frequently. Despite the different synoptic environments, these extremes show a similar internal structure, with broad stratiform and wide convective core (WCC) rain. Furthermore, the maximum rain rate is located mostly over the convective area, near the convective–stratiform boundary in the system. Our results highlight the critical role of the strength and direction of synoptic flows in the generation of extreme rainfall systems near coastal areas. With the enhancement of the low-level vertical wind shear and moisture by the synoptic flow, the coastal convection triggered diurnally has a higher chance to organize into mesoscale convective systems and hence a higher probability to produce extreme rainfall.

Article
  • Yousuke YAMASHITA, Masayuki TAKIGAWA, Daisuke GOTO, Hisashi YASHIRO, M ...
    2021 Volume 99 Issue 2 Pages 287-308
    Published: 2021
    Released: April 15, 2021
    [Advance publication] Released: December 02, 2020
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    Atmospheric transport of aerosols such as black carbon (BC) affects the absorption/scattering of solar radiation, precipitation, and snow/ice cover, especially in areas of low human activity such as the Arctic. The resolution dependency of simulated BC transport from Siberia to the Arctic, related to the well-developed low-pressure systems in September, was evaluated using the Nonhydrostatic Icosahedral Atmospheric Model–Spectral Radiation Transport Model for Aerosol Species (NICAM-SPRINTARS) with fine (∼ 56 km) and coarse (∼ 220 km) horizontal resolutions. These low-pressure systems have a large horizontal scale (∼ 2000 km) with the well-developed central pressure located on the transport pathway from East Asia to the Arctic through Siberia. In recent years, the events analysis of the most developed low-pressure system indicated that the high-BC area in the Bering Sea observed by the Japanese research vessel Mirai on September 26–27, 2016, moved to the Arctic with a filamental structure from the low's center to the behind of the cold front and ahead of the warm front in relation to its ascending motion on September 27–28, 2016. The composite analysis for the developed low-pressure events in September from 2015 to 2018 indicated that the high-BC area was located eastward of the low's center in relation to the ascending motion over the low's center and northward/eastward area. Since the area of the maximum ascending motion has a small horizontal scale, this was not well simulated by the 220-km experiment. The study identified that the BC transport to the Arctic in September is enhanced by the well-developed low-pressure systems. The results of the transport model indicate that the material transport processes to the Arctic by the well-developed low-pressure systems are enhanced in the fine horizontal resolution (∼ 56 km) models relative to the coarse horizontal resolution (∼ 220 km) models.

  • Naveen CHANDRA, Prabir K. PATRA, Jagat S. H. BISHT, Akihiko ITO, Taku ...
    2021 Volume 99 Issue 2 Pages 309-337
    Published: 2021
    Released: April 15, 2021
    [Advance publication] Released: December 04, 2020
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    Supplementary material

    Methane (CH4) is an important greenhouse gas and plays a significant role in tropospheric and stratospheric chemistry. Despite the relevance of methane (CH4) in human-induced climate change and air pollution chemistry, there is no scientific consensus on the causes of changes in its growth rates and variability over the past three decades. We use a well-validated chemistry–transport model for simulating CH4 concentration and estimation of regional CH4 emissions by inverse modeling during 1988–2016. The control simulations are conducted using seasonally varying hydroxyl (OH) concentrations and assumed no interannual variability. Using inverse modeling of atmospheric observations, emission inventories, a wetland model, and a δ13C-CH4 box model, we show that reductions in emissions from Europe and Russia since 1988, particularly from oil–gas exploitation and enteric fermentation, led to decreased CH4 growth rates in the 1990s. This period was followed by a quasi-stationary state of CH4 in the atmosphere during the early 2000s. CH4 resumed growth from 2007, which we attribute to increases in emissions from coal mining mainly in China and the intensification of ruminant farming in tropical regions. A sensitivity simulation using interannually varying OH shows that regional emission estimates by inversion are unaffected for the mid- and high latitude areas. We show that meridional shift in CH4 emissions toward the lower latitudes and the increase in CH4 loss by hydroxyl (OH) over the tropics finely balance out, keeping the CH4 gradients between the southern hemispheric tropical and polar sites relatively unchanged during 1988–2016. The latitudinal emissions shift is confirmed using the global distributions of the total column CH4 observations via satellite remote sensing. During our analysis period, there is no evidence of emission enhancement due to climate warming, including the boreal regions. These findings highlight key sectors for effective emission reduction strategies toward climate change mitigation.

  • Kazuto TAKEMURA, Takeshi ENOMOTO, Hitoshi MUKOUGAWA
    2021 Volume 99 Issue 2 Pages 339-356
    Published: 2021
    Released: April 14, 2021
    [Advance publication] Released: December 02, 2020
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    This study examines the predictability of an enhanced monsoon trough, which is accompanied by a largescale cyclone in the lower troposphere, south of Japan seen in late August 2016. The monsoon trough is found to be enhanced by a meandering of the Asian jet and a consequent southwestward intrusion of upper-level high potential vorticity associated with a Rossby wave breaking east of Japan. Japan Meteorological Agency's operational one-month ensemble prediction during the forecast period of a week underestimates the intensity of the Rossby wave breaking and fails to predict the enhanced monsoon trough. A simple sensitivity analysis based on ensemble singular vectors indicates that initial perturbations over the Bering Sea and near the Asian jet entrance region can efficiently grow and propagate toward the region to the south of Japan, contributing to maximize the perturbations of the enhanced monsoon trough. The time evolution of the perturbations propagating toward the region to the south of Japan is consistent with that of the ensemble spread during the forecast period. Perturbed hindcast experiments were conducted with the initial perturbations obtained from the simple sensitivity analysis. The monsoon trough to the south of Japan in the perturbed experiment is significantly more enhanced than the unperturbed experiment, supporting the simple sensitivity analysis. These results indicate a crucial contribution of the initial perturbations associated with the Rossby wave breaking and near the Asian jet entrance region to the limited predictability of the enhanced monsoon trough in late August 2016.

  • Kyeong-Seok MIN, Kazuhisa TSUBOKI, Mayumi K. YOSHIOKA, Yukie MORODA, S ...
    2021 Volume 99 Issue 2 Pages 357-377
    Published: 2021
    Released: April 28, 2021
    [Advance publication] Released: December 04, 2020
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    A stationary line-shaped precipitation system (SLPS), which is one type of mesoscale convective systems, is a typical heavy rain-producing weather system formed during warm seasons in Japan. Although the Kinki district, western Japan, is known as a frequent occurrence region for SLPSs, their formation mechanisms in the region have not been sufficiently elucidated yet because of their complex formation processes. Using observational data and high-resolution numerical experiments, in this study, we investigated a SLPS event that occurred on 1 September 2015. We also conducted numerical sensitivity experiments regarding the orography and initial time.

    The observational data showed that the relative humidity at lower levels was high during the SLPS event. The southwesterly was dominant at middle levels over the Kinki district during the formation of the SLPS. The formation of the SLPS was associated with neither a mesoscale low-pressure system nor a synoptic-scale cold front, demonstrating that these were not necessary conditions for the formation of the SLPS.

    In the numerical experiments, we found that the SLPS was formed in a low-level convergence zone of the westerly with the warm and moist south-southwesterly from the Kii Channel. New convective cells formed over the north of Awaji Island and are propagated northeastward by the middle-level southwesterly. This cell formation process was repeated and resulted in the formation of the SLPS. The sensitivity experiments for the orography around the occurrence area of the SLPS indicated that the orography was not a significant factor for the formation of the SLPS in this event. The orography can modify the location of the SLPS.

Article : Special Edition on Global Precipitation Measurement (GPM): 5th Anniversary
  • Tatsuya SEIKI
    2021 Volume 99 Issue 2 Pages 379-402
    Published: 2021
    Released: April 12, 2021
    [Advance publication] Released: December 08, 2020
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    Supplementary material

    This study proposes a method of detecting three-dimensional hail distribution by using the Global Precipitation Measurement (GPM) Dual-frequency Precipitation Radar (DPR) products in combination with the atmospheric temperature from a reanalysis product. In this study, the hail class contains hailstones, high-density graupel, and small frozen droplets. The radar reflectivity at the Ku-band (ZKu) and dual-frequency ratio (DFR) values are examined for hydrometeor classification at the five atmospheric temperature ranges in comparison to the ground radar product in a test hailstorm case. A simple model assuming binary collision for the riming process, which represents a significant reduction in the number concentration with the conservation of the mass concentration, explains well the hail signals on the scatterplot of ZKu and the DFR. This study determines the thresholds of the ZKu and DFR values at each temperature range based on the simple model. Furthermore, this study evaluates the thresholds in 74 hailstorm cases and proposes two filters to remove melting snow and rain contamination below the freezing level. The 5-year dataset of the GPM-DPR observations shows that hail is widely distributed over oceanic convergence zones as well as over continental convective regions. Most oceanic hail layers are found to be thin (i.e., less than 1500 m) and confined near the freezing level. Therefore, such hail signals have been potentially missed by ground observations. An additional filter removes such thin hail layers and effectively works to detect only deep hailstorms, specifically over continental regions.

Article
  • Kiyotaka SHIBATA, Ayano SAI
    2021 Volume 99 Issue 2 Pages 403-422
    Published: 2021
    Released: April 28, 2021
    [Advance publication] Released: January 13, 2021
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    The surface meteorological data in Japan, beginning around the 1880s, archived by the Japan Meteorological Agency are analyzed focusing on the long–term trends and variations in humidity and temperature. It is found that the annual–mean temperature trend exhibits statistically significant warming of 1.0–2.5°C century−1 for most stations, while the annual–mean relative humidity shows significantly decreasing trend of −2 % to −12 % century−1 for most stations with small seasonality. On the other hand, the annual–mean mixing ratio trend displays a different spatial distribution compared to the temperature or relative humidity trend. In this study, three types of trends exist: significantly positive and negative values, and virtually zero. Significantly negative trends of about −0.2 g kg−1 to −0.3 g kg−1 century−1 are located approximately in the Pacific side of Honshu from the middle Tohoku through Shikoku to the eastern Kyushu. Significantly positive trends of about 0.2 g kg−1 to 0.4 g kg−1 century−1 are observed over Hokkaido, the western Japan along Sea of Japan, the western Kyushu, and the remote islands including Okinawa. The overall pattern is similar for other seasons except for most of the remote islands in winter. Empirical orthogonal function (EOF) analysis indicates that the linear trends in the annual–mean temperature and relative humidity can be almost explained by the nearly uniform persistent warming and drying of EOF–1 components. On the other hand, for the annual–mean mixing ratio, EOF–2 is almost identical with the linear trend component, although the fraction of EOF–2 (14 %) is much smaller than that of EOF–1 (49 %). In recent years from 1960 to 2018 the mixing ratio and temperature trends are very different from those in the longer period from the 1880s. The mixing ratio trend and the temperature trend increase on average from 0.0 g kg−1 to 0.5 g kg−1 century−1 and from 1.5°C to 2.5°C century−1, respectively

Article : Special Edition on Global Precipitation Measurement (GPM): 5th Anniversary
  • Victorien De MEYER, Rémy ROCA
    2021 Volume 99 Issue 2 Pages 423-436
    Published: 2021
    Released: April 22, 2021
    [Advance publication] Released: January 14, 2021
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    The theory of extreme precipitation has matured over the last decade and stipulates that the intensity of the extreme precipitation is balanced with the surface humidity. The changes in surface humidity can further be approximated by the changes in surface temperature. The analytically derived scaling coefficient based on the Clausius–Clapeyron derivative is ∼ 6 % K−1 in the tropics. While frequently confronted with observations over land, the theory has so far only been marginally evaluated against precipitation data over the ocean. Using an ensemble of satellite-based precipitation products and a suite of satellite-based sea-surface temperature (SST) analyses at 1°-1day resolution, extreme scaling is investigated for the tropical ocean (30°S–30°N). The focus is on the robust features common to all precipitation and SST products. It is shown in this study that microwave constellation-based precipitation products are characterized by a very robust positive scaling over the 300–302.5-K range of 2-day-lagged SST. This SST range corresponds to roughly 60 % of the amount of tropical precipitation. The ensemble mean scaling varies between 5.67 ± 0.89 % K−1 and 6.33 ± 0.81 % K−1 depending on the considered period and is found to be very close to the theoretical expectation. The robustness of the results confirms the suitability of the current generation of constellation-based precipitation products for extreme precipitation analysis. Our result further confirms the extreme theory for the entire tropical ocean. Yet, the significant differences in the magnitude of the extreme intensity of the products require dedicated validation efforts.

Article
  • Shipra JAIN, Rattana CHHIN, Ruth M. DOHERTY, Saroj K. MISHRA, Shigeo Y ...
    2021 Volume 99 Issue 2 Pages 437-448
    Published: 2021
    Released: April 22, 2021
    [Advance publication] Released: January 13, 2021
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    Equilibrium climate sensitivity (ECS) is defined as the change in the global mean surface air temperature due to the doubling or quadrupling of CO2 in a climate model simulation. This metric is used to determine the uncertainty in future climate projections, and therefore, the impact of model changes on ECS is of large interest to the climate modeling community. In this paper, we propose a new graphical method, which is an extension of Gregory's linear regression method, to represent the impact of model changes on ECS, climate forcing, and climate feedbacks in a single diagram. Using this visualization method, one can (a) quantify whether the model or process change amplifies, reduces, or has no impact on global warming, (b) evaluate the percentage changes in ECS, climate forcing, and climate feedbacks, and (c) quantify the ranges of the uncertainties in the estimated changes. We demonstrate this method using an example of climate sensitivity simulations with and without interactive chemistry. This method can be useful for multimodel assessments where the response of multiple models for the same model experiment (e.g., usage of interactive chemistry compared with the prescribed chemistry as shown here) can be assessed simultaneously, which is otherwise difficult to compare and comprehend. We also demonstrate how this method can be used to examine the spread in ECS, climate forcing, and climate feedbacks with respect to the multimodel mean (or one benchmark model) for multimodel frameworks such as Coupled Model Intercomparison Project Phase 5 or for different ensemble members in a large ensemble of simulations conducted using a single model.

Notes and Correspondence
  • Shuhei MAEDA, Kazuto TAKEMURA, Chiaki KOBAYASHI
    2021 Volume 99 Issue 2 Pages 449-458
    Published: 2021
    Released: April 14, 2021
    [Advance publication] Released: January 13, 2021
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    This study analyzes the modulation of planetary waves associated with the Eurasian (EU) pattern—one of the dominant teleconnection patterns seen over northern Eurasia in the boreal winter—through composite analyses using the Japanese 55-year reanalysis dataset to reveal its dynamic mechanism, including wave-mean flow interaction.

    From the viewpoint of deviation from climatological flow, the EU pattern is known as a stationary Rossby wave teleconnection type with an equivalent barotropic structure and action centers over northern Europe, mid-western Siberia, and Japan. However, from the viewpoint of deviation from the zonal average, the EU pattern modulates planetary wave activities, which include the East Asian winter monsoon as one component.

    In the positive phase of the EU pattern, corresponding to the enhanced Asian monsoon, the upward and eastward propagation of the planetary wave from Central Eurasia to the North Pacific in the troposphere is enhanced, compared with that of the climatology. The baroclinic energy conversion from the zonal mean to the deviation from that over East Asia contributes to the amplified planetary wave. The enhanced upward and eastward propagating planetary wave converges in the upper troposphere, thereby causing anomalous extratropical direct circulation and cold outflow toward the lower mid-latitude troposphere. These results indicate that the EU pattern is one of the global dynamic modes related primarily to planetary wave activities.

Article
  • Kazuto TAKEMURA, Hitoshi MUKOUGAWA
    2021 Volume 99 Issue 2 Pages 459-472
    Published: 2021
    Released: April 14, 2021
    [Advance publication] Released: January 13, 2021
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    This study assesses the predictability of an enhanced monsoon trough south of Japan in late August 2016, which is accompanied by Rossby wave propagation over Eurasia and a consequent anticyclonic Rossby wave breaking east of Japan, with a relaxation technique using an atmospheric general circulation model. Three types of the relaxation experiments are conducted, with nudging the model forecast in the upper troposphere toward reanalysis, for regions of the Rossby wave breaking east of Japan, the Rossby wave propagation over Eurasia, and both the regions from Eurasia to the east of Japan. All types of the relaxation experiments show improved reproducibility of the enhanced monsoon trough, which the operational one-month ensemble prediction in Japan Meteorological Agency failed to predict. Compared with a result of a control experiment, the relaxation experiments show the more amplified Rossby wave propagation over Eurasia and Rossby wave breaking east of Japan, as seen in the reanalysis. The upper-level wave amplification contributes to the improved reproducibility of the enhanced monsoon trough, through that of southwestward intrusion of upper-level high potential vorticity airmass toward the southeast of Japan. The results of relaxation experiments indicate primary and secondary contributions from corrected forecast errors of the Rossby wave breaking east of Japan and the Rossby wave propagation over Eurasia to the predictability of the monsoon trough, respectively. Their relative contributions to the enhanced monsoon trough are consistent with a result of ensemble-based simple sensitivity analysis shown in a related previous study.

Article : Special Edition on Global Precipitation Measurement (GPM): 5th Anniversary
  • Morihiro SAWADA, Kenichi UENO
    2021 Volume 99 Issue 2 Pages 473-496
    Published: 2021
    Released: April 22, 2021
    [Advance publication] Released: January 29, 2021
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    Heavy precipitation events were identified during the cold seasons of 2014–2019 using two-day accumulated precipitation data at 137 stations of the Japan Meteorological Agency. The mechanisms for producing heavy precipitation regarding the structure of an occluding extratropical cyclone were analyzed using the products of the Dual-frequency Precipitation Radar (DPR) onboard the Global Precipitation Measurement (GPM) core satellite and trajectory analysis on the European Centre for Medium-range Weather Forecasts atmospheric reanalysis data. Upper-ranked events with heavy precipitation were predominantly caused by extratropical cyclones and were in mature stages. In the top 50 ranked events, three south-coast cyclones were nominated, and the relationships between developing the mesoscale precipitation system and airstreams were intensively diagnosed. Hourly precipitation changes at stations that recorded heavy precipitation were primarily affected by a combination of the warm conveyor belt (WCB), cold conveyor belt (CCB), and dry intrusion (DI). Wide-ranging stratiform precipitation east of the cyclone center was composed of low-level WCB over the CCB and upper WCB, and convective clouds around the cyclone center were associated with the upper DI over the WCB that provided an extreme precipitation rate at the surface, including the formation of a band-shaped precipitation system. Convective cloud activities also contributed to moist air advection over the stationary stratiform precipitation areas recognized as the upper WCB. DPR products also identified deep stratiform precipitation in the cloud-head area behind the cyclone center with mid-level (near-surface) latent heat release (absorption) with increased potential vorticity along the CCB, making feedback intensification of the cyclone possible.

Notes and Correspondence
  • Peter VOELGER, Peter DALIN
    2021 Volume 99 Issue 2 Pages 497-504
    Published: 2021
    Released: April 26, 2021
    [Advance publication] Released: January 13, 2021
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    A case study of the occurrence of polar stratospheric clouds (PSCs) on February 13th, 2017, in northern Sweden is reported in this paper. For the first time, a quasistationary edge of a bright and extended PSC layer (∼ 600-km long) on the eastern side of the Scandinavian mountain range was photographed and registered using lidar observations. Both lidar measurements and model simulations demonstrated that atmospheric conditions were fairly unchanged for several hours during the presence of the PSC. Strong winds across the Scandinavian mountain range were responsible for triggering the formation of mountain lee waves in the Kiruna area, which induced the formation of the quasistationary long and straight edge of the PSCs.

Article : Special Edition on Years of the Maritime Continent (YMC)
  • Lujia FENG, Tengfei ZHANG, Tieh-Yong KOH, Emma M. HILL
    2021 Volume 99 Issue 2 Pages 505-536
    Published: 2021
    Released: April 26, 2021
    [Advance publication] Released: January 15, 2021
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    Supplementary material

    Using data from the Sumatran GPS Array in Indonesia–a hero network in tectonic and earthquake studies–we study the summer intra-seasonal variability of precipitable water vapor (PWV) over Sumatra in years without strong inter-annual variability. Unlike most other studies that used external meteorological data to derive PWV from Global Positioning System (GPS) signal delays, we use the zenith wet delay (ZWD) time series estimated from a regular geodetic-quality processing routine as a proxy for PWV variations without using auxiliary meteorological data. We decompose the ZWD space-time field into modes of variability using rotated Empirical Orthogonal Function (EOF) analysis and investigate the mechanisms behind the two most important modes using linear regression analysis both with and without lags. We show that the summer intra-seasonal variability of daily ZWD over Sumatra in 2008, 2016, and 2017 was dominated by the South Asian Summer Monsoon and further influenced by dry-air intrusions associated with Rossby waves propagating in the Southern Hemisphere midlatitudes. Both active South Asian monsoons and dry-air intrusions contribute to the dryness over Sumatra during northern summer. Our results indicate an intra-seasonal connection between the South Asian and western North Pacific Summer Monsoons: when the South Asian monsoon is strong, it pumps atmospheric water vapor over the eastern Indian Ocean to feed into the western North Pacific monsoon. We also show a tropical-extratropical teleconnection where PWV over the southern Maritime Continent can be modulated by the activity of eastwardtraveling Rossby waves in the southern midlatitudes. Our case study demonstrates the use of regional continuously operating GPS (cGPS) networks for investigating atmospheric processes that govern intra-seasonal variability in atmospheric water vapor.

Article
  • Tsung-Han LI, Yuqing WANG
    2021 Volume 99 Issue 2 Pages 537-554
    Published: 2021
    Released: April 26, 2021
    [Advance publication] Released: January 15, 2021
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    This study examines the role of boundary layer dynamics in tropical cyclone (TC) intensification using numerical simulations. The hypothesis is that although surface friction has a negative effect on TC intensification due to frictional dissipation (direct effect), it contributes positively to TC intensification by determining the amplitude and radial location of eyewall updrafts/convection (indirect effect). Results from a boundary layer model indicate that TCs with a larger surface drag coefficient (CD) can induce stronger and more inwardly penetrated boundary layer inflow and upward motion at the top of the boundary layer. This can lead to stronger and more inwardly located condensational heating inside the radius of maximum wind with higher inertial stability and is favorable for more rapid intensification.

    Results from full-physics model simulations using TC Model version 4 (TCM4) demonstrate that the intensification rate of a TC during the primary intensification stage is insensitive to CD if CD is changed over a reasonable range. This is because the increased/reduced positive contribution by the indirect effect of surface friction to TC intensification due to increased/reduced CD is roughly offset by the increased/reduced negative (direct) dissipation effect due to surface friction. However, greater surface friction can significantly shorten the initial spinup period through stronger frictional moisture convergence and Ekman pumping and thus expedite moistening of the innercore column of the TC vortex but is likely to lead to a weaker storm in the mature stage.

  • Tsung-Han LI, Yuqing WANG
    2021 Volume 99 Issue 2 Pages 555-573
    Published: 2021
    Released: April 26, 2021
    [Advance publication] Released: January 15, 2021
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    In Part I of this series of studies, we demonstrated that the intensification rate of a numerically simulated tropical cyclone (TC) during the primary intensification stage is insensitive to the surface drag coefficient. This leads to the question of what is the role of the boundary layer in determining the TC intensification rate given sea surface temperature and favorable environmental conditions. This part attempts to answer this question based on a boundary layer model and a full-physics model as used in Part I. Results from a boundary layer model suggest that TCs with a smaller radius of maximum wind (RMW) or of lower strength (i.e., more rapid radial decay of tangential wind outside the RMW) can induce stronger boundary layer inflow and stronger upward motion at the top of the boundary layer. This leads to stronger condensational heating inside the RMW with higher inertial stability and is thus favorable for a higher intensification rate. Results from full-physics model simulations indicate that the TC vortex initially with a smaller RMW or of lower strength has a shorter initial spinup stage due to faster moistening of the inner core and intensifies more rapidly during the primary intensification stage. This is because the positive indirect effect of boundary layer dynamics depends strongly on vortex structure, but the dissipation effect of surface friction depends little on the vortex structure. As a result, the intensification rate of the simulated TC is very sensitive to the initial TC structure.

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