This study examines the fidelity of the Meteorological Research Institute (MRI) atmospheric general circulation model (AGCM), ensemble runs forced with observed sea surface temperature (SST), in simulating Indian summer monsoon rainfall (ISMR), and its interannual variation. Despite the simple ensemble mean (SEM) capturing essential features of climatological ISMR pattern and its extreme ISMR anomalies, it still shows certain systematic bias in simulating mean seasonal variation of rainfall over the Asia-Pacific region. Concurrently, the ISMR interannual variability throughout the analysis period is not adequately represented. A bias-correction is applied to remove this bias by deriving weights for the member simulations for each Julian day separately at every grid point through multiple linear regression of their daily rainfall, against corresponding observation over a 23-year ‘training phase’ (out of the total 24-year analysis period). Thereafter, at every grid point, for each Julian day of the remaining 1-year ‘forecast phase’, the bias-removed ensemble mean (BREM) is computed as an optimal linear combination of weighted member simulations. In cross validation, each year in the analysis period is treated successively as the ‘forecast phase’, with the remaining 23 years included in the corresponding training phase. The methodology minimises the systematic bias in mean seasonal variation of rainfall, and BREM consequently improves upon SEM in simulating the mean ISMR pattern, and its interannual variability over the entire analysis period. The skill of BREM in forecasting the ISMR, and its intraseasonal variability is validated, for the severe monsoon drought of 2002. The effective removal of climatological bias brings out the realistic precipitation response in BREM to fluctuations in SST boundary forcing. As a result, BREM captures the seasonal rainfall anomaly pattern of 2002, and markedly improves its intraseasonal evolution. Apart from the basic skill of the AGCM ensemble system, pronounced equatorial SST impact in modulating the monsoon circulation during 2002, played a seminal role in the success of the methodology. The analysis also underlines the importance of mean seasonal variation, not only for capturing ISMR climatology, and its interannual variation but for improving its intraseasonal variability as well. With the aid of realistic SST forecasts, this methodology has the application potential for dynamical prediction of ISMR.
In the morning of 26 Nov 1997, a squall line in association with a tapering, or carrot-shaped cloud, formed along an ENE-WSW oriented cold front north of Taiwan, and moved toward the south-southeast. A segment embedded within the squall line subsequently evolved into a bow echo and made landfall over northern Taiwan. Being a very rare event and the first case in subtropical East Asia reported, this wintertime bow echo was examined in its environment, structure, and evolution mainly using the observations from the single-Doppler radar located at the Taiwan Taoyuan International Airport (TTIA). The bow echo developed in an environment of unusually large low-level vertical wind shear of 28 m s−1 over 0-3 km, with a moderate Convective Available Potential Energy (CAPE) of 1288 m2 s−2. The approaching baroclinic cold front was essential in providing not only a means to organize the convection, but also the vertical shear and thermodynamic conditions suitable for bow echo development. Two aspects of the present bow echo were quite different from those in the United States over mid-latitude continents: It occurred in cold season, and was accompanied by heavy rainfall, which was linked to its weaker rear inflow and slower propagation speed of about 16 m s−1. The bow echo had a length of 60-90 km, and a lifespan of about 4 h. After formation, it propagated rapidly eastward along the squall line and passed TTIA near 0600 LST, producing northwesterly surface gusts of 18.5 m s−1. The bow echo also exhibited many typical structural features; including a bulged apex, a reflectivity notch, the mid-level rear-to-front (RTF) inflow, reaching 25 m s−1, and a pair of cyclonic-anticyclonic bookend vortices. During the mature stage, the updraft tilted up-shear, and the RTF inflow jet was elevated (near 3-4 km) until very close to the gust front. The bookend vortices lasted for about 2-3 h, and were most evident at 2 km.
The Meteorological Research Institute of the Japan Meteorological Agency has developed a cloud-resolving nonhydrostatic 4-dimensional variational assimilation system (NHM-4DVAR), based on the Japan Meteorological Agency Nonhydrostatic Model (JMA-NHM), in order to investigate the mechanism of heavy rainfall events induced by mesoscale convective systems (MCSs). A horizontal resolution of the NHM-4DVAR is set to 2 km to resolve MCSs, and the length of the assimilation window is 1-hour. The control variables of the NHM-4DVAR are horizontal wind, vertical wind, nonhydrostatic pressure, potential temperature, surface pressure and pseudo relative humidity. Perturbations to the dynamical processes, and the advection of water vapor are considered, but these to the other physical processes are not taken into account. The NHM-4DVAR is applied to the heavy rainfall event observed at Nerima, central part of Tokyo metropolitan area, on 21 July 1999. Doppler radar's radial wind data, Global Positioning System's precipitable water vapor data, and surface temperature and wind data are assimilated as high temporal and spatial resolution data. The Nerima heavy rainfall is well reproduced in the assimilation and subsequent forecast, with respect to time sequence of 10-minute rainfall amount. The formation mechanism of the Nerima heavy rainfall is clarified from this study. A surface convergence line of horizontal winds was made of a southerly sea breeze and north-easterly winds over the Kanto plain around Nerima. Since the rise of temperature over the northern part of the Kanto plain was suppressed, due to a shield of clouds against sunshine, the difference of temperature between the convergence line and its northern side became large. Consequently, the wind convergence was enhanced around Nerima. An air with high equivalent potential temperature was lifted over this enhanced convergence line to generate cumulonimbi that caused the Nerima heavy rainfall.
Features of the South Atlantic convergence zone (SACZ) and the Baiu frontal zone (BFZ) simulated by an AGCM (T106L56: a primitive equation spectral model which has 56 &sigga;-levels and triangular spectral truncation at wave-number 106) are studied. The 24-year integration, from 1979 to 2002, by the model constrained by observed sea-surface temperature and sea-ice distribution, is used for this study. The detailed analysis is made for the typical case of SACZ and BFZ selected from data in 1985-1996. The South American monsoon circulation and the associated precipitation are reasonably reproduced. The precipitation zone of the SACZ, which extends southeastward from the southern part of Brazil to the South Atlantic, is sustained along the southwestern rim of the South Atlantic subtropical anticyclone during the southern summer. The Asian monsoon circulation and the associated precipitation are also reasonably reproduced. The precipitation zone of the BFZ, which extends northeastward from the southern part of China to Japan is formed along the northwestern rim of the North Pacific subtropical anticyclone in June. Many common features are found in the simulated SACZ and BFZ, in regard to the frontal structure and the associated synoptic- and meso-α-scale disturbances, and the relation to the respective subtropical anticyclone. However, an important difference is seen between their geographical environments. The cold south Atlantic in the poleward side of the SACZ provides the significant baroclinicity for SACZ, while the existence of warm land mass to the poleward side of the BFZ brings on weak baroclinicity of the BFZ. Another significant difference is seen in the large-scale environmental condition. The SACZ is influenced, in essence, by the trade winds and the subtropical anticyclone over the South Atlantic. The BFZ is strongly influenced by the moisture transport due to the Indian monsoon westerly and variation of the North Pacific subtropical anticyclone related to the convective activity over the western North Pacific.
The turbulence echo intensity observed by a wind-profiling radar is closely related to the vertical gradient of refractive index squared (M2), which largely depends on the venical humidity gradient in a moist atmosphere. We have developed a radar remote-sensing technique for determining humidity profiles by using the turbulence echo characteristics. The sign of M is determined so that the precipitable water vapor determined by the radar agrees with that derived from the GPS measurements. In this study we have combined the results collected with two co-located radars; the MU (Middle and Upper atmosphere) and Lower Troposphere Radar (LTR) operating at 46.5 MHz and 1.3 GHz frequencies, respectively, and humidity profiles determined at 0.3-7.5 km. The echo power profiles (signal-to-noise ratio, SNR) with the two radars are connected smoothly in a height range between 1.5 and 1.95 km, by considering reduction of the receiver sensitivity for the MU radar due to leakage of the transmission signal. The retrieved humidity profiles show detailed time-height variations, which agree well with the simultaneous Raman lidar and radiosonde measurements.
On January 18, 2001, a tornado formed over the Japan Sea near the Mikuni Town on the Hokuriku Coast of Japan. The “Mikuni Tornado” formed within a winter thundercloud, during a period of a cold air-outbreak, and heavy snowfall. The detailed structure of the tornado funnel cloud, and misocyclone with in the cloud are revealed from photographs, videotape, Doppler radar data, and the GPS sonde data. The tornado was generated 3 km offshore, close to the radar site. The tornado formed at the edge of a developing cell, had a single funnel, and was accompanied by a misocyclone near the cloud base; the position of the misocyclone corresponded well with a funnel cloud. The misocyclone had a diameter of 400-800 m and vorticity of 10−1 s−1, and coincided with the tornado funnel vortex in the cloud base. The funnel cloud had diameters of 150 m at the cloud base and 30 m near the surface, and a vorticity of 100 s−1. Although the funnel diameter at the cloud base changed remarkably during the life cycle of the tornado, the diameter near the surface remained largely unchanged. The funnel cloud began to dissipate after landing. The maximum wind speed of the tornado vortex is estimated to have been approximately 30 ms−1 (F0-scale), and the lifetime of the funnel cloud was 7 minutes. No damage or strong winds were recorded at the coastal radar site, which was located about 1 km from the tornado. The environmental feature that generated the Mikuni Tornado was vertical wind shear of the sub-cloud layer (3.5 × 10−2 s−1). The tornado was a non-supercell type tornado that formed under the uniform winter monsoon. This result indicates that small-scale snowclouds can produce tornadoes during periods of cold air outbreaks.
One possible impact of sea surface temperature (SST) anomalies in the Indian Ocean (IO) on the ongoing El Niño is investigated using an air-sea coupled general circulation model (CGCM). A suite of CGCM experiments by imposing the IO basin-wide warming (BW) and IO dipole/zonal mode (IODZM) are conducted to assess the feedback effects of the El Niño-related SST anomalies on the Pacific. While the IODZM during boreal fall does not have a significant impact on the Pacific sector, the BW during boreal winter enhances the surface easterlies over the equatorial Western Pacific (WP) during the maturedecay phase of El Niño. The strengthened easterlies act to enhance the SST cooling over the WP, thereby the zonal gradient of SST between the IO and WP is greater than the climatology. These enhanced WP easterlies induce an advanced transition to the La Niña phase, which is caused by the upwelling ocean Kelvin waves. These results imply that the BW in the IO, to some extent, can be hastening the El Niño to La Niña transition.
Intraseasonal variability (10-60 days) of sea surface temperature (SST) over the north Indian Ocean and its influence on regional precipitation variability over the Indian subcontinent are examined. SST, cloud liquid water and precipitation over the Indian Ocean of the Tropical Rainfall Measuring Mission (TRMM), precipitation of Climate Prediction Center Merged Analysis of Precipitation (CMAP), and low-level atmospheric parameters of National Center for Environmental Prediction (NCEP) II reanalysis are utilized for this study. Western Ghats (WG) in the southwest and the Ganges-Mahanadi Basin (GB) in the northeast of the Indian subcontinent are observed to be the regions of maximum precipitation with large standard deviations of the intraseasonal variability. Active (break) phases of precipitation occur in these regions by the northward propagation of positive (negative) precipitation anomalies over the Arabian Sea and the Bay of Bengal. Latitude-time plots during the active phase of the WG region shows that the positive SST anomalies over the Arabian Sea formed by suppressed surface latent heat flux and increased downward shortwave radiation flux lead the positive precipitation anomalies. Surface air temperature anomalies follow the SST anomalies and then destabilize the lower atmosphere between 1000 hPa and 700 hPa. These results indicate that, in the northward propagating dynamical surface convergence, underlying SST anomalies tend to form a favorable condition for convective activity and may sustain enhanced precipitation over the convergence region. This results in enhanced precipitation anomalies over the WG region that move further northeastward and merge with the northward propagating precipitation anomalies from the Bay of Bengal, enhancing the active phase of the GB region.
Unique long-term agro-meteorological measurements of Mongolian grasslands enabled us to investigate the relationship between the phenology of Stipa spp., one of the dominant perennial species (such as emergence, heading, flowering, maturity, and senescence), and moisture conditions for three stations representative of the major vegetation zones, during 1993 to 2002. The results showed that the emergence date relates neither to a specific temperature nor to an effective accumulative temperature, but to the presence of precipitation that occurs within five days prior to the emergence in most cases. For a northern-most wettest forest steppe region (Bulgan), the precipitation amount, and period of days from emergence to heading are significantly correlated (r = 0.93), while a southward typical steppe region (Arvaikheer) also exhibited a positive correlation (but not exceeding the 5% significance level). The positive correlation occurred at Bulgan most likely, because for drought years, Stipa spp. tended to switch a phenological stage from the vegetative growth (that is, a biomass increase), to reproductive phase (that is, seed production) earlier than for a normal year. One possible trigger for the switching is a decreased soil moisture, associated with a break of the rainy season.
A long-term global atmospheric reanalysis, named “Japanese 25-year Reanalysis (JRA-25)” was completed using the Japan Meteorological Agency (JMA) numerical assimilation and forecast system. The analysis covers the period from 1979 to 2004. This is the first long-term reanalysis undertaken in Asia. JMA's latest numerical assimilation system, and specially collected observational data, were used to generate a consistent and high-quality reanalysis dataset designed for climate research and operational monitoring and forecasts. One of the many purposes of JRA-25 is to enhance the analysis to a high quality in the Asian region. Six-hourly data assimilation cycles were performed, producing 6-hourly atmospheric analysis and forecast fields of various physical variables. The global model used in JRA-25 has a spectral resolution of T106 (equivalent to a horizontal grid size of around 120 km) and 40 vertical layers with the top level at 0.4 hPa. In addition to conventional surface and upper air observations, atmospheric motion vector (AMV) wind retrieved from geostationary satellites, brightness temperature from TIROS Operational Vertical Sounder (TOVS), precipitable water retrieved from orbital satellite microwave radiometer radiance and other satellite data are assimilated with three-dimensional variational method (3D-Var). JMA produced daily sea surface temperature (SST), sea ice and three-dimensional ozone profiles for JRA-25. A new quality control method for TOVS data was developed and applied in advance. Many advantages have been found in the JRA-25 reanalysis. Predicted 6-hour global total precipitation distribution and amount are well reproduced both in space and time. The performance of the long time series of the global precipitation is the best among the other reanalyses, with few unrealistic variations from degraded satellite data contaminated by volcanic eruptions. Secondly, JRA-25 is the first reanalysis to assimilate wind profiles around tropical cyclones reconstructed from historical best track information; tropical cyclones were analyzed properly in all the global regions. Additionally, low-level cloud along the subtropical western coast of continents is well simulated and snow depth analysis is also of a good quality. The article also covers material which requires attention when using JRA-25.