The atmospheric water vapor transport and moisture budget associated with two decadal summer rainfall shifts in 1978/79 and 1992/93 over East China were investigated using observational precipitation and the European Centre for Medium-Range Weather Forecasts (ECMWF) 40 Years Re-Analysis (ERA-40) dataset. After 1978/79, summer precipitation increased abruptly in the Yangtze-Huaihe River valley (YH) but decreased in South China (SC) and North China (NC). Associated with this rainfall shift, southerly water vapor transport over East China was weakened; an anticyclonic moisture circulation anomaly along with decreasing moisture convergence existed in SC; abnormal water vapor from western SC converged in YH with that from western NC, then turned eastward, instead of northward to NC. After 1992/93, rainfall over SC increased dramatically. This is closely related to two abnormal anticyclonic moisture circulations to the south and the north when their northwesterly and southwesterly outflows converged over SC. During these two regime shifts, it was the variation of meridional water vapor flux, located mainly in the lower troposphere, which played an important role in the rainfall anomalies over YH, SC, and NC. The water vapor transport anomalies were mainly controlled by the disturbance wind field instead of the disturbance moisture field.
We assessed the capability of the regional climate model (RCM) approach in simulating the monthly 50th and 99th percentiles of marine wind speed and significant wave height on the Japanese coast during tropical cyclone season (June-October) of 2002-2004. To simulate the marine winds, we performed dynamical downscaling of the Japanese Reanalysis (JRA-25) using three RCMs. We found that, by applying the Brier skill score, coastal wind speed of the RCMs is closer to the observed, when compared to the JRA-25. The better performance of the RCM approach was found to come from a successful reduction of the bias rather than that of the root mean square error. The RCMs, however, simulate erroneously low (high) marine wind speed northeast of Japan (along the Pacific coast and Kuroshio Extension). The erroneous marine wind speed was found to be related with the sea surface temperature bias associated with the coarse horizontal resolution of the JRA-25. We employed the dynamically-downscaled marine winds to force a wave model (WaveWATCH-III), and found that the marine winds of the RCMs are more effective than the JRA-25 in reproducing significant wave height on the Japan Sea coast.
This study investigates the mesoscale interaction of typhoon circulation and southwesterly flow during the passage of Typhoon Morakot (2009) over Taiwan using radar observations. Single Doppler radar analysis characterized typhoon features with remarkably distant rainbands, and identified the strong southwesterly monsoonal flow with a maximum speed of more than 45 m s-1 in the southern flank of distant rainbands. Dual-Doppler synthesized winds elucidated a confluent mechanism on the northern side of the rainbands with a maximum convergence over 1.5 × 10-3 s-1. Velocity azimuth display (VAD) winds showed the intensification of southwesterly flow at low levels. The southwesterly monsoonal flow initiated about 6 hrs before typhoon landfall, and then became prominently involved with typhoon circulation. Also, the radial component with respect to the typhoon center was enhanced and became comparable with the tangential one (～30 m s-1) about 7.5 hrs after landfall. The variation in intensity of radial components can be regarded as a unique precursor for the extension of influence of the southwesterly monsoonal flow onto the typhoon circulation. Furthermore, the strong convergence showed that the interaction between the southwesterly flow and typhoon circulation might contribute to the development of rainbands, as well as the intensification of the inward radial flow embedded within typhoon.
Cloud properties associated with tropical convection are analyzed for 11 models participating in Cloud Feedback Model Intercomparison Project Phase 1 (CFMIP1) in comparison with International Satellite Cloud Climatology Project (ISCCP) and other satellite observations and reanalysis datasets. Cloud properties are analyzed for different regimes of large-scale circulation field sorted by monthly mean of pressure coordinated vertical velocity at 500 hPa as an index of large-scale circulation. The present analysis is focused on warm oceanic regions with sea surface temperatures above 27°C where convection is active. The warm oceanic regions cover the vertical motion regimes ranging from strong ascent to weak descent. The ISCCP simulator outputs are used to evaluate cloud properties in the models. Cloud amount of optically thick high-clouds with optical thicknesses (τ) ≧ 3.6 and cloud-top pressure (CTP) ≦ 440 hPa is overestimated in the strong ascent regime while that of optically thin high-clouds with τ < 3.6 is underestimated for all the regimes. Cloud amount of optically thick low-clouds with CTP ≧ 680 hPa is overestimated in the weak vertical motion regime as well in some models. The relevance of cloud amount bias to cloud radiative effect bias is discussed. Observations show that optically thick clouds in the strong ascent regime often have tops around 180-310 hPa. In many models, the cloud top often reaches higher altitude compared to the observations. The tendency can especially be seen in the models adopting the moisture accumulation type scheme presumably due to excessively deep convection. Comparison of upward motion strength among the models and reanalyses suggests that cumulus parameterization performs better when entrainment rate is varied with large-scale environmental fields to reduce the convection deepness where necessary.
Based on updated quality controlled daily records, extreme events were defined using temperature indices proposed by the Commission for Climatology/Climate Variability and Predictability project’s Expert Team on Climate Change Detection, Monitoring and Indices, and temporal trends during the 1970-2009 period were examined in Thailand. Results revealed that Thailand has indeed experienced significant country-wide warming over the last four decades, and extreme events associated with both the cold and warm extremes of daily minimum and maximum temperature distributions have changed accordingly. Trends in temperature indices showed much spatial trend coherence and widespread significant warming, generally consistent with the regional evidence recently documented in other Asia-Pacific Network countries. Significant upward trends in temperature extreme indices were found in the annual number of warm nights and days, the annual occurrence of warm spells, tropical nights, summer days, and the highest and lowest values of daily minimum temperatures. In contrast, significant downward trends were observed in the annual number of cold nights and days, the annual occurrence of cold spells, diurnal temperature range and annual extreme temperature range. Changes in these temperature extreme indices were consistent with a significant shift in the temperature distribution toward warmer conditions over the recent decades. To address changes in Thailand’s temperature more clearly, however, the plausible underlying mechanisms, including urbanization effects, need to be further studied.
The withdrawal of Baiu, the end of early summer rainy period over Japan, is marked by a stepwise northward migration of the westerly jet in the upper troposphere. The Coupled Model Intercomparison Project Phase3 (CMIP3) dataset was utilized to investigate possible changes in the seasonal development under a warmer climate. To obtain realistic model simulations, we evaluated model performance by comparing simulated and observed westerly jets for the present-day climate, selecting the top five models. Future climate projections using these models show that the westerly jet will be strengthened to the south of the jet axis, and the amount of precipitation in late July will be increased over the main land of Japan. These findings indicate that, by the end of the 21st century, withdrawal of the Baiu season will be delayed. The CMIP3 models also allowed changes in larger-scale atmospheric circulation to be predicted. Most of the CMIP3 models project that the Asian jet will be intensified on its equatorward side. Decreased upper tropospheric divergence in the western North Pacific and simultaneous shrinking of the Tibetan high suggest that the simulated change in the westerly jet is closely connected to the weakening of monsoonal circulation in Asia.
Recent studies that characterize central tropical Pacific warming (often referred to as El Niño Modoki) and its teleconnections to East Asia show coherent teleconnection patterns during boreal summer. Observed increases in the frequency of El Niño Modoki and a lack of understanding regarding its impacts on precipitation over the Korean peninsula motivate our work. In this study, we investigate the regional precipitation patterns over South Korea associated with El Niño and El Niño Modoki events during the June-September season. The results show significant and consistent increases in the seasonal precipitation totals and maximum precipitation, as well as extreme rainy days during the decay of El Niño Modoki. In contrast, there is a reduction of moisture fluxes into South Korea in the case of the eastern Pacific El Niño. The analysis presented here provides an improved understanding of the potential impacts of the tropical Pacific sea surface temperature on warm-season (June-September) water availability and hydrometeorological extremes over the Korean peninsula.
Spiral bands are characteristic meso-beta-scale structures of typhoons in their mature stages. Observational study shows that spiral bands cause strong rainfall. The spiral bands are classified into two types: inner and outer rainbands. The inner rainband is formed near the typhoon center. In this study, we focus on the precipitation process in the inner rainband within the typhoon. Two neighboring spiral bands are often observed near the typhoon center. Previous studies have shown the mechanism of intensifying rainfall in the inner-side spiral band of two neighboring inner rainbands that frequently form in this region. However, the intensification of the outer-side spiral band of two neighboring inner rainbands has not been widely reported. Therefore, to clarify the mechanism of intensifying rainfall in the spiral bands, we focus on cloud microphysical processes and perform a numerical experiment using a cloud-resolving model. We show that cold rain processes are important for the intensification of precipitation in the spiral band. In particular, production and growth of graupel are the most effective processes for the intensification of precipitation in the spiral band.
A multi-cellular storm over the Zoshigaya area of Tokyo, Japan (5 August 2008) was observed by two X-band dual-polarization radars, and this paper aims to investigate the structure of the precipitation cores within its individual precipitation cells. The precipitation cell and core are defined here on the basis of liquid water content (LWC). The storm comprised 20 precipitation cells, each with a precipitation core. Of these, 17 cells were characterized by a single precipitation core (single-core cells) and lasted for less than 30 minutes (5-25 minutes). In contrast, the other three cells consisted of several auxiliary precipitation cores (multi-core cells) that were produced in succession and descended to the ground each lasting approximately 15 minutes, the cells themselves were relatively long-lived (≥40 minutes). Single-core cells developed via updrafts driven by low-level convergence that persisted for approximately 10 minutes before being converted to a downdraft by precipitation loading. Without the supporting updraft, the precipitation core fell to the ground 5-25 minutes after its first appearance. In contrast, replacement of precipitation cores in multi-core cells during their mature stage was driven by periodic strong updrafts associated with a low-level southeasterly inflow that supplied warm, moist air to the precipitation cell. The results of a statistical analysis of precipitation cell and core are presented. The rainfall amount from each precipitation cell was proportional to the cell’s lifetime, with a slope of 0.89 for the relationship and a correlation coefficient of 0.95. The average updraft and downdraft velocities of multi-core cells (7.9 and 4.7 m s-1, respectively) were stronger than in the single-core cells (5.2 and 3.7 m s-1, respectively). The average liquid water content of precipitation cores in single-core cells was 4.0 g m-3, whereas the multi-core cells averaged 5.3 g m-3. The average formation heights of single- and multi-core cells were 4.7 km and 4.6 km, respectively. The intensity and formation height of the precipitation cores are approximately proportional to each other.
We studied the temporal and spatial characteristics of extreme typhoon rainfall in Taiwan using Central Weather Bureau hourly precipitation data from 21 surface stations during the past 51 years (1960-2010). Extreme rainfall is defined as 95th percentile intensity of total rain events, or equivalently, rain events greater than 9 mm hr-1 which contribute 40% to the total rain amount in Taiwan. It was found that approximately 70% (20%) of extreme rain is in the typhoon season (Mei-Yu) from July to October (from May to June). There are significant variations of typhoon extreme rainfall over the annual and decadal time scales, with larger extreme rainfall values and events in the periods of 1960-1976 and 1994-2010, and less in the 1977-1993 period. The recent 1994-2009 period has the most extreme rainfall and events, as well as, inter-annual variability. In contrast, there are strong inter-annual variations of Mei-Yu extreme rainfall, but no significant decadal variations. The averaged typhoon rain intensity, however, is about the same, being 19 mm hr-1 in all these three periods. Our analysis indicates that the typhoon extreme rainfall spatial pattern is phased locked with the Central Mountain Range, Taiwan. In general, the amount of extreme rainfall was related to the typhoon translation speed and duration time, but not typhoon intensity. Slower speeds and longer duration time lead to larger extreme rainfall values. Our analysis also indicate that the mean duration time of Taiwan landfall typhoons with northern tracks (tracks north of 23 degrees latitude) is about 3 hours longer than that of southern track typhoons in the last 51 years, and is more likely to produce three times as much extreme rainfall. The interactions of summer or winter monsoons with typhoons are also important factors that may contribute to the extreme rainfall in Taiwan. Examples of extreme rainfall due to typhoon circulation interaction with summer and winter monsoon flows are presented. Monsoon water vapor supply, typhoon slow translation speed, and mesoscale convection due to typhoon-monsoon flow interactions are the key factors in extreme precipitation events.
A heavy rainfall event occurred along the Ibuki-Suzuka Mountains situated to the west of the Nobi Plain, Japan on September 2-3, 2008. This event was caused by a stationary precipitation band that formed along the mountains with a north-south alignment. This study examines the maintenance mechanism of the precipitation band and describes the characteristics of high rainfall intensity in the band. For this purpose, data from the Japan Meteorological Agency (JMA) radar, JMA wind-profiler radar, and a dual-Doppler analysis from two X-band polarimetric Doppler radars are used. The band stagnated for 13 hours from 12 Japan Standard Time (JST; 9 hours ahead of UTC) on September 2. Its length and width were approximately 100 and 20 km, respectively. Low-level warm and moist southeasterly winds with equivalent potential temperature greater than 355 K below 1 km impinged on the eastern slope of the mountains and continuously developed precipitation cells. These cells propagated northward by southerly winds above 1 km and contributed to the formation of the precipitation band. The maintenance of the precipitation band can be attributed to a persistent vertical wind shear; that is, the low-level southeasterly and mid-level southerly winds. The characteristics of high rainfall intensity in the precipitation band is examined by dual-Doppler analysis. Low-level southerly winds with high equivalent potential temperature converged over a microscale wedge-shaped valley that opens southward between the Ibuki-Suzuka Mountains and its branch, the Yoro Mountains aligned north-northwest to south-southeast. The existence of graupel particles near the melting level and a high amount of large raindrops below it, depicted by the polarimetric radar, are possible causes for the increased precipitation in the region.
Recent studies have emphasized the important role of moisture in altering tropical cyclone (TC) vortex structure. Latent heat released in outer rainbands induces change in the secondary circulation, and exerts negative impact on TC inner core intensity. This study is to further explore the TC structural behavior with the presence of vertical wind shear. Typhoon Talim (2005) was simulated using the Weather Research and Forecasting (WRF) model, and sensitivity experiments were conducted by artificially modifying the amount and distribution of moisture around TC vortex. With the presence of an easterly vertical wind shear, the simulated Typhoon Talim developed quasi-stationary outer rainbands that concentrate in the southwestern (downshear left) sector. Air from the north (upstream side of the outer rainbands) traveled faster into TC core than air from the other directions, thus Typhoon Talim was more sensitive to moisture variations in the north than in the south. With enhanced moisture supply into outer rainbands, simulated TCs grow larger in size. However, their inner core intensity and strength are weakened because latent heat released in outer rainbands induces updrafts and reduces mid- to low-level radial inflow that advects absolute angular momentum into inner core. On the contrary, TCs simulated with reduced moisture supply become smaller in size since drier environment inhibits convection in outer core region. The relatively convection-free outer core region favors the formation of strong radial inflow that accelerates the inner core spin-up process. This causes TCs to contract while their inner core strength and intensity increase. Although moisture in outer core region imposes a negative effect on inner core intensification, it contributes to the maintenance of outer core strength and TC size by inducing more convection in the outer core region. Thus, abundant moisture supply in TC outer core region is critical to the growth of horizontal extent of TC primary circulation.
Typhoon Saomai (2006) was one of the most severe typhoon landfalls in China from 1956 to 2010. The rapid intensification process of Typhoon Saomai is simulated with the advanced research version of the Weather Research and Forecasting (ARW) modeling system using different cloud microphysical parameterization schemes. The horizontal spacing of the finest nested mesh is 1.5 km. The intensity, precipitation, and inner-core structures of the simulated typhoons are verified against the observations. The performances of various cloud microphysical parameterization schemes are compared. It is found that varying the microphysics scheme generates little sensitivity in track, but results in pronounced deviations in intensity and inner-core structures. The results indicate the condensation and depositional growth of graupel or snow of the suitable cloud microphysical parameterization scheme enhances the diabatic heat releasing in the inner core region. The released diabatic heating determines the intensity and inner-core structures of typhoon. Furthermore, a positive feedback associated with the diabatic heating plays an important role in the intensification of the simulated storm with a suitable cloud microphysical parameterization scheme.
This study uses a cloud-resolving model to examine the impact of sedimentation of cloud ice on the cloud-top height and the precipitation intensity of typical precipitation systems in East Asia, including a typhoon, snow clouds over the Sea of Japan, and the Baiu front. The fall velocity of cloud ice is assumed to be 0.1 m s-1. When the sedimentation process of cloud ice is included in the model, the horizontal distribution and frequency of the cloud-top height show significantly better agreement with satellite observations. Furthermore, cloud ice with sedimentation concentrates at a lower level than that without sedimentation, and converts to snow and graupel by microphysical growth processes. More solid water substances located in the thin layer above the 0°C level contribute to intensification of precipitation at the surface by several percent, especially in the convective area.
In the present study, a stationary convective system persisting for approximately six hours over the Asagiri Highland adjoining Suruga Bay, Japan on 28 July 2010 was investigated primarily using X-band multi-parameter radar observation data. Over the Asagiri Highland with the moist southerly wind from Suruga Bay associated with thermally induced local circulations, 54 precipitating cells were generated continuously. The anvil extending southeastward from the existing precipitating cells did not prevent new precipitating cells from continuous appearance. Thus, the convective system was maintained for approximately six hours. Then, small advection of the transient cells contributed to the formation of a convective system with a horizontal scale of approximately 20 km. Among them, the precipitating cells that appeared over the gradual slope toward Mt. Kenashi, which is located in the western part of Asagiri Highland, moved toward the northwest part of Mt. Fuji. The precipitating cells were supplied with sufficient moisture because the direction was normal to that of moist southerly wind blowing from Suruga Bay. In one of those precipitating cells, two cores were identified. One was associated with a shallow convection below the melting level, and the other with a deep convection. They rained on almost the same position in the precipitating cell. The precipitating cell with such cores brought a localized heavy rainfall by efficiently converting the moisture transported from Suruga Bay into rain.
The physical characteristics of rain are reflected by the shape of the raindrop size distribution (RDSD). Specifically, the RDSD is the result of different precipitation formation processes. We measured the RDSD at the surface in heavy rainfall during SoWMEX/TiMREX (2008) in Taiwan. The heavy rainfall was characterized by a squall line accompanied by trailing stratiform precipitation, and it was partitioned into three regions based on radar reflectivity patterns: convective line, stratiform, and reflectivity trough. The convective line was further partitioned into the convective center, leading edge, and trailing edge using a threshold rainfall intensity of 20 mm h-1. The leading edge, which belongs to the convective line, had upward motion from the surface and contained many small drops. The leading edge was characterized by a small median volume diameter (D0) and a linear shape of gamma RDSD. In the convective center, a strong updraught rose to the top of the cell with time, and many large drops over 4 mm were observed. The convective center, which had large D0 and normalized intercept parameter (NW), was characterized by an upward convex shape of gamma RDSD. The range of raindrop diameter decreased toward the trailing edge, with no updraught or only weak updraughts at high altitudes. The trailing edge had large shape and slope parameters, along with a more upward convex shape of gamma RDSD. A bright band was observed in the stratiform region with continual downward motion from the bright band to the surface, even though the intensity became weak. In the stratiform region, D0 and NW were small and the gamma RDSD had an upward convex shape. The different RDSDs in each region of a maritime squall line suggest the existence of different cloud microphysical processes described by the change of RDSD parameters: the coalescence process induces an increase of D0 and shape parameter and a decrease of NW, while the break-up process induces a decrease of D0 and an increase of NW. In comparison with other maritime storms, the convective center has small log10NW and large Dm value. And the convective edge region is positioned between convective center and stratiform region in the Dm-log10NW scatter plot.