Intraseasonal variations (ISV) of climate from the central Indian Ocean to the western Pacific are investigated at high frequencies (20-50 day period) and low frequencies (50-100 day period) in association with El Niños, mainly using 7 years of zonal wind, 8 years of precipitation, and 31 years of outgoing long-wave radiation (OLR) data observed from satellites. Zonal wind, validated against Tropical Atmosphere Ocean (TAO) buoy measurements at 147°E, 0° and 156°E, 0°, indicated a larger portion of ISV was contained in high frequencies during the 1999-2000, 2000-0l, 2002-03, and 2003-04 seasons, as compared to the 1996-97 and 2001-02 seasons, with the latter seasons occurring a year before El Niño events. For 1996-97 and 2001-02, a particular frequency of 3 cycles per season (64-day period) explained a significant portion of the zonal wind ISV in a coherent band from the Indian Ocean to the western Pacific. Furthermore, it was found that the westerly wind signal in this region propagated eastward. Satellite-based precipitation was likewise examined, and showed a similar pattern in time and space. Consistently, OLR data indicated that the ratio of low-frequency ISV to high-frequency ISV was significantly correlated with the Nino 3.4 index at a one-year lead time. Low-frequency variance alone also showed a significant, albeit weaker, correlation, and high-frequency variance indicated no correlation. We suggest that high-frequency variance acts as noise, reducing the relationship between low-frequency ISV and El Niño/Southern Oscillation. Thus, a strong low-frequency variance in combination with a weak high-frequency variance (high ratio value) is important for understanding the initiation of El Niños. Another interesting feature observed from OLR was the area with the largest ratios of low-frequency to high-frequency ISV had shifted westward during the 30-year record. The westward shift suggests that the eastern Indian Ocean is playing a more important role in ISV-El Niño relations in recent years.
A global warming projection experiment was conducted on the Earth Simulator using a very high horizontal resolution atmospheric general circulation model, with 20-km grid size (the 20-km model). Such high horizontal resolution in a global climate model is unprecedented for a global warming projection. Experiments using the 20-km model were conducted by adopting the time-slice method, in which future changes in sea surface temperature (SST) were predicted by an atmosphere-ocean general circulation model (AOGCM) called MRI-CGCM2.3. The A1B emission scenario, proposed by the Intergovernmental Panel on Climate Change (IPCC), was assumed in the experiment. The model reproduces a realistic Baiu rain band under the present-day climate conditions in terms of geographical distribution and northward seasonal march. Experiments of the dependency of the horizontal resolution on the reproducibility of the Baiu rain band have revealed that the 20-km model generally exhibits higher performance than a model with a lower horizontal resolution. The future climate simulation shows that precipitation, and its intensity increases over the Yangtze River valley of China, the East China Sea, Western Japan, and the ocean to the south of the Japan archipelago. Conversely, precipitation and its intensity decrease over the Korean peninsula and Northern Japan. The termination of the Baiu season tends to be delayed until August. The future precipitation change is mainly attributable to the change in the horizontal transport of water vapor flux and its convergence associated with the intensification of a subtropical high. This can be interpreted as an atmospheric response to the El Niño condition of the ocean. The change in the wind field mainly contributes to the change in the water vapor flux in the case of the Baiu rain band.
Cumulonimbus clouds frequently develop over mountains, a plain, and the sea in the summer in association with thermally induced local circulations. On July 5, 2000, when the sea breeze from the Pacific Ocean blew over the Noubi Plain and arrived at the slope of the Ibuki Mountains where a valley wind circulation developed, a cumulonimbus cloud occurred over the slope of the Ibuki Mountains. In this paper, the structure and evolution of the cumulonimbus cloud are investigated using the data of Doppler radars. The direction of the environmental vertical wind shear was southeast, which is parallel to the slope of the Ibuki Mountains, when the cumulonimbus cloud occurred. The cumulonimbus cloud maintained forabout 2 hours. The cumulonimbus cloud consisted of groups of precipitating cells; “Primary Cell” and “Secondary Cells.” The former developed with tilting toward the downshear side and moved down the slope. The latter developed almost uprightly on the upshear side of the Primary cell. There were 6 groups of cells in the cumulonimbus cloud. The developing process and structure of group C, which was the most intense group, were investigated in detail. After Primary Cell C1, with tilting toward the downshear side, developed, Secondary Cells C2, C3 and C4 of group C developed on the upshear side (the Ibuki Mountains side) of cell C1. An outflow from cell C1 toward the upshear side of cell C1 lifted the low-level air. Cells C2, C3 and C4 developed almost uprightly on the upshear side of cell C1, where the convection of cell C1 weakened the venical wind shear. Cells C3 and C4 had maximum reflectivety of over 50 dBZ and the echo top of 15 km above sea level (ASL). Cells C3 and C4 developed explosively in the group C due to the horizontal convergence at the middle layer, which was strengthened by the outflow from cell C2, the northeasterly inflow toward cells C3 and C4, and the lifted low-level air on the Ibuki Mountains side.
Dominant modes of surface air temperature (SAT) interannual variations in Japanese summer climate are detected by a rotated empirical orthogonal function analysis, and statistical relations between the dominant SAT modes and atmospheric circulation are investigated by regression analyses. Relations to the large-scale atmospheric circulation patterns that had been already known, are also examined. As a result, two dominant modes are detected that correspond well to the large-scale atmospheric circulation patterns, and relations between the SAT modes and atmospheric circulation can be interpreted statistically. The first mode represents SAT variations in the central part of Japan. The second mode represents those in the northern part. Greater than 75% of SAT variations can be explained by these two leading modes. The first mode accompanies variations of sunshine duration, and is related with the strength of the Tibetan High. The second mode involves quasi-six-year periodicity, and accompanies an appearance of the shallow type of the Okhotsk High, with the cold northeasterly winds that is related with the convective activity around the Philippine Islands.
Aerosols (PM2.0), and associated precursor gases have been continuously monitored at Mt. Lemmon (2791 m ASL), Arizona, since September 1992. Month-long samples are collected on filters and chemically analyzed resulting in a decade-long record with over 100 data points for each species—among the longest such records currently available. The species determined include SO42−, NO3−, Cl−, NH4+, Ca2+, Mg2+, K+, Na+, elemental carbon (EC), organic carbon (OC), NH3(g), SO2(g), HCI(g) and HNO3(g). The data reveal long-term trends, seasonal variations, and correlations between species. PM2.0 (1.48 μg m−3, annual mean) is mainly comprised of SO42− (49% w/w), NHI4+ (16%), EC (11%) and OC (22%). The mean SO42−/NH4− equivalent ratio is 1:1 suggesting complete neutralization. Median PM2.0 was 1.33 μg m−3 (range = 0.17-4.32 μg m−3). Median EC was 0.14 μg m−3 (0.01-0.76), and median OC was O.29 μg m−3 (0.03-1.33). The annual mean trends of all species, with the exception of SO42−, SO2(g), NH4+ and NH3(g), appear to be increasing, but some trends may not be statistically significant. Long-term decreasing trends in SO2(g) and SO42−, reflect source controls implemented over the past decade, whereas HNO3(g) has been increasing, possibly due to increased NOX emissions associated with population growth in the region. The associated conversion of agricultural land to urban use might be leading to a decrease in NH3(g). Annual trends for EC (5.2 ± 2.7 ng m−3 y−1) and EC/OC ((1.5 ± 0.75) × 10−2 y−1) appear to be positive and significant, but there is no significant annual OC trend. There appears to be a significant secondary source of OC, presumably derived from photoxidation of biogenic hydrocarbons. There is no significant trend in the calculated annual mean extinction coefficient but the calculated single scattering albedo (ω) may be decreasing (−1.5 ± 1.1 × 10−3 y−1), possibly caused by increasing EC associated with forest fires and/or fossil fuel combustion. Depending on the value of the critical single scattering albedo, the aerosol might already be a net absorber, or it might only become so by the end of the century if current trends continue.
A typhoon bogus data assimilation (BDA) scheme is built in the MM5 3D-Var system with mainly three components: a) An algorithm to specify the typhoon bogus sea-level pressure (SLP) and wind profiles is constructed in the MM5 3D-Var system based on the typhoon report issued by the regional typhoon center and the 3D-Var background; b) The errors of the bogus observations are empirically assigned according to our knowledge of the typhoon SLP and wind distributions; c) The MM5 3D-Var system includes typhoon bogus observation operators. Since typhoon bogus observations can have large differences from the 3D-Var background fields, special treatment is made to allow the typhoon bogus data retained in the minimization procedure. Numerical experiments are conducted using Typhoon Rusa (2002) case in the Northwestern Pacific Ocean near landfall to the Korean Peninsula. It is indicated that the 3D-Var bogus algorithm works well and can improve the prediction of typhoon track and intensity. With MM5 3D-Var BDA, the initial typhoon vortex is balanced with the dynamical and statistical balance embedded in the 3D-Var system. It can generate a well-defined warm core structure in the typhoon initialization. Compared with bogussing in the 3D-Var background fields, MM5 3D-Var BDA results in less spin-down/up problems in the subsequent typhoon forecast. Sensitivity experiments show that assimilation of the only bogus SLP data produces too strong a typhoon intensity, while assimilation of the only bogus wind data produces much weaker intensity prediction than the observations. Assimilation of both bogus SLP and wind data obtains the best initialization and prediction for this typhoon case. Its landfall location, time and intensity are very close to the observation in the experiment.
In this paper the effects of radiation on a meso-scale precipitating system is investigated during a severe storm in South China on June 8, 1998. This was done by using the Pennsylvania State University (PSU)/National Center for Atmospheric Research (NCAR)/Meso-scale Model 5 Version 3 (MM5_V3) after the introduction of radiative transfer schemes that are able to treat water clouds, ice crystals, snow, and groupel. The results suggest that the rainfall patterns do not differ too much for the various radiation schemes used in the numerical calculations, but rather influence the rainfall intensity in the central areas. The radiative effects on precipitation are more significant during daytime, as compared to nighttime. The diurnal variation of rainfall is enhanced by the radiative processes. Computed precipitation intensities, and radiative cooling/heating rates, are dependent on the specific radiative transfer scheme used. The results suggest that model improvement of daytime cloud radiative processes is crucial for a better representation of such effects on meso-scale precipitating system.
This paper presents an inter-comparison of rainfall parameters (median volume diameter and rain rate) using C-band polarimetric radar, a 2D-video disdrometer and a 400 MHz profiler for the Baiu front event of 8-9 June 2005 in Okinawa, Japan. These instruments are part of the Okinawa Sub-Tropical Environment Remote Sensing Center, operated by the National Institute of Information and Communications Technology (NICT). The 2D-video disdrometer is used to derive the mean axis ratio of raindrops versus drop diameter, as well as the drop size distribution for the Baiu event. The data are then used to simulate various relations between polarimetric scattering parameters such as: specific attenuation (Ah), and specific differential attenuation (Adp), versus specific differential phase (Kdp) which are required to correct the measured reflectivity at horizontal polarization (Zh), and the differential reflectivity (Zdr) for rain attenuation. The 2D-video disdrometer data are also used to arrive at retrieval formulas for median volume diameter (D0) from radar Zdr and rain rate from radar Kdp. The intense Baiu event of 8-9 June 2005 was composed of heavy convective rain cells embedded in large areas of stratiform rain. The inter-comparison of D0 and rain rate (R) between instruments was conducted for 12 hours (03:00-07:00, 11:00-19:00 UTC on 8th June 2004). The C-band radar retrievals were found to be in excellent agreement with the 2D-video disdrometer for the entire period. The 400 MHz profiler retrievals of D0 and R were in good agreement with 2D-video disdrometer during the more steady rain periods, with more scatter observed during the heavier convective rain periods. These inter-comparisons demonstrate the accuracy of C-band polarimetric radar to retrieve important rainfall parameters, as well as the accurate correction for rain attenuation using differential propagation phase.
This study aims to investigate diurnal sea surface temperature (SST) rise in Mutsu Bay, and its relationship with the local atmospheric field. First, the characteristics of diurnal SST warming is revealed, using satellite and buoy data during 1995-2000. Satellite observations can clearly capture large diurnal SST rise that reaches a few degrees, and clarify the spatial distribution of the SST rise. The diurnal variation of the satellite-derived SST will reflect that of skin SST, which is defined as the temperature of the top layer with the thickness of a few tens micrometers. The analysis of the bulk SST observed with buoys at 1-m depth reveals that the frequency of large diurnal amplitude of SST (ΔSST) appearance becomes high from April to August, and the bulk ΔSST exceeds 1.0 K in some places in the bay for 71% of the days in July. Over the bay, surface air temperature in the daytime is higher than the bulk SST on average in April-August. From spring to summer the wind becomes weaker, and the easterly wind becomes frequent. The weak wind plays a role in promoting the diurnal SST rise, and the warm air-temperaturemay also secondarily contribute to large ΔSST. A model experiment is then performed to examine a local atmospheric circulation that develops around the bay under a clear and calm condition in summer, and to address the interaction between the circulation, and the diurnal SST warming. A local circulation forced by the land surface solar heating of this area produces the weak easterly surface wind and the subsidence of the air, and makes the air warmer over the bay. This is consistent with the observational results. In such a case, the diurnal rise of skin SST simulated by an ocean model exceeds 2 K all over the bay, and it is especially large in the northeastern area. If the diurnal skin SST rise is considered in the atmospheric model, although the circulation pattern does not change, the circulation becomes weaker and the surface air temperature increases. This result suggests that the diurnal SST variation will be important for the practical prediction of air temperature in coastal areas.
This study is designed to elucidate the impact of interannual variability of meteorological parameters on vegetation activity over Mongolia using 10-day composite NDVI (Normalized Difference Vegetation Index) data set and surface meteorological data (precipitation, temperature and snow depth) for 97 meteorological stations from 1993 to 2000. The analysis is made on vegetation in two developmental stages; the rapid-growth stage (almost June to July) and the mature stage (almost July to August). Positive correlations at 99% significant level between precipitation and vegetation activity are recognized for 29% and 42% of meteorological stations in the rapid-growth stage and the mature stage, respectively. Precipitation in June and July affects vegetation activity in both stages. The impact of air temperature on vegetation activity in the mature stage differs by season. The vegetation activity is negatively correlated with summer temperature over most area. Negative correlations are found over the western part of Mongolia with respect to temperature in early winter, and positive correlations are concentrated in the northeastern part of Mongolia with respect to temperature in mid-winter. Furthermore, there are five meteorological stations near the Khenty Mountains, with high correlation coefficients between snow depth and vegetation activity in the rapid-growth stage; however, the snow depth effect is limited to a narrow region. The possibility of prediction the vegetation activity in the two stages is examined using a multiple regression method, based on the above-mentioned results. Since correlation coeflicients between observed vegetation activity and estimated vegetation activity from the multiple regression equations are high satisfactorily, it is found that the prediction algorithm has a potential for the prediction of NDVI over Mongolia.
Hefei Doppler radar observation data over the downstream region of the Yangtze River during the Meiyu period from 2001 to 2003, were analyzed in order to reveal the predominant structural characteristics of meso-β-scale convective systems (MβCSs) around the Meiyu front. Convective and stratiform portions were separated from MβCSs using the bright-band fraction (BBF) method. The daily and yearly mean vertical profiles of radar reflectivity for the convective portion were calculated. Results showed that the vertical profile of the convective portion of MβCSs for 3 years was characterized by low altitude of radar reflectivity peaks (around 3 km), and large decrease of reflectivity with height above the melting level. To understand these characteristics of MβCSs, the convection of medium depth (CMD) is defined as a group of convective cells whose echo top height, with the reflectivity of 15 dBZ, is equal to or less than 8 km, and in which the reflectivity peak is below 4 km throughout their lifetime. To investigate the structural characteristics of MβCSs around the Meiyu front, observed MβCSs were categorized into slow-moving (≤ 3 m s−1) and south-of-front (SSF) type, slow-moving and along-the-front (SAF) type, fast-moving (≥ 7 m s−1) and along-the-front (FAF) type, and slow-moving and north-of-front (SNF) type; according to their movement speed, and their locations relative to the surface front. The predominant convection in the SSF type was the CMD, and it covered 51% of the convective area. The CMD and deep convections (DC) coexisted in the SAF, with the CMD covering 34% of convective area. The FAF type was organized from the DC, and the SNF type primarily consisted of the CMD. The environmental conditions under which the SSF type formed were characterized by a weak wind convergence (<2 × 10−5 s−1) near the surface, a low level of neutral buoyancy and humid atmosphere below the middle level. The large contribution of the CMD to convective rainfall amount in the SSF type, and its unnegligible contribution in the SAF type, indicate that the CMD has one of the main structures of the Meiyu frontal convective precipitation systems.
The relationship between convective activity over the Tibetan Plateau (TP) and meso-scale cloud systems in the Baiu/Meiyu frontal zone, are examined for the 1998 summer monsoon season by using GEWEX Asian Monsoon Experiment (GAME) reanalysis data and Geostationary Meteorological Satellite (GMS)-IR Tbb data. Diurnal cycle of the plateau-scale heat low associated with convective clouds is dominated, forming a plateau-scale convergence line. Moisture transport from south of TP is essential for developing convective clouds, and the convergence line. Occasionally, the convergence line extends to the eastern edge of the TP, which triggers a meso-scale cyclonic vortex over the head of the Yangtze River basin through the plateau edge cyclogenesis (PEC). This vortex induces a strong low level jet with moisture inflow to the east of it, which facilitates further development of the vortex to a meso-α scale cloud system embeddedin the Baiu/Meiyu front (BMF) over China. The occurrence of PEC is likely to be modulated by an interaction between mid-latitude westerly waves, with a quasi-biweekly time scale, passing over the TP, and a shorter-period oscillation of the monsoon trough over the Indian sub-continent, with around a 4-7 day period.
Annual maxima of daily windrun is modeled statistically with respect to year for nineteen locations spread throughout New Zealand, and for one location in the Cook Islands. The generalized extreme value distribution is fitted to each data set to describe both the mean and the variability for each location and to predict statistically their behavior in the coming decades. We find evidence of significant trend in wind extremes for eight of the locations in New Zealand considered: six of these were decreases with percentage change in the median of annual maximum windrun ranging from 16-41% over the period of records, while the remaining two were increases with percentage change ranging from 20-24%. We provide estimates of 10, 100 and 1000 year return levels for daily windrun, and describe how they vary with the latitude of the locations. We also establish statistical upper bounds for maximum daily windrun for some of the locations in New Zealand: four locations in the North Island and one in the South Island.
Frozen ground plays an important role in the energy and water cycle of cold regions, and affects the environment and agricultural practices in these regions. The effect of climate warming on soil frost is an important concern, but our present understanding of such effect is limited, due to the lack of long-term data covering a large region. This study analyzes a unique regional database of 20-year records from 1986-2005 of soil frost, combined with long-term climate data from 1955-2005. Annual maximum frost depths (Dmax) in the Township of Memuro (514 km2) in Tokachi, Hokkaido have decreased significantly in the last 20 years. The decrease in Dmax was caused by the development of thick snow cover in early winter that insulates the ground, not by the increase in air temperature. The Dmax is strongly correlated with a soil freezing index (F20), that integrates the combined effects of air temperature and snow cover. Using F20 as a surrogate of Dmax, it was shown that the decreasing frost depth was a regional phenomenon occurring over the Tokachi Plain, covering an area of several thousand square kilometers. The timing of a major decrease in F20 in the mid to late 1980’s coincided with sharp decreases of snowfall in the Hokuriku region of Japan and the amount of drift ice in the southern part of the Sea of Okhotsk, both of which are regarded as indicators of the strength of the East Asian winter monsoon activities.