We studied the spectral dependencies of aerosol light-absorption over the East China Sea region using sky radiometer data collected at Cape Hedo (26.87°N, 128.25°E), Okinawa, Japan from 2006 to 2008. Absorption Angstrom Exponent (AAE) was around 1 for most of the observation data, indicating black carbon (BC) as the dominant light-absorbing aerosol. However, high values of AAE were often found in the spring season. Aerosols with such high AAE were characterized by comparatively high optical thickness (τ), low single scattering albedo (ω), and an increase of ω with wavelength (λ). Additionally, dust aerosols were considerably high when such high AAEs were observed. Such high AAEs could not be explained by considering only BC and dust as absorbing aerosols, but could be well explained by including the role of brown carbon aerosols. This finding highlights a complex scenario of light-absorbing aerosols in the spring season in this region, and suggests the necessity of considering the role of such brown carbon aerosols on a light-absorption phenomenon for adequately understanding aerosol climatology. The study also discussed that AAE of dust aerosol should be used with care with the known size distribution and spectral values of the imaginary refractive index.
Detailed structures and variability of lower tropospheric temperature inversions are investigated, using two sets of high-resolution (50 m in height and 3 hours in time) rawinsonde data obtained by the GAME-T enhanced rawinsonde observations conducted over the inland Indochina Peninsula in March 1997 and January 2000. In both observation periods, the strong inversions were observed at 3-4 km height. The causes of variations in their occurrence time and height are discussed by comparing them with meteorological conditions. Five processes are found to be responsible for the inversion variations: 1) cold-air advection accompanied by a cold-surge event, 2) cloud-top radiative cooling, 3) adiabatic heating due to subsidence, 4) shallow convection at or near the observation site, and 5) diurnal heating process due to boundary-layer growth.
The joint Japan-Mongolia-USA project DUVEX (Dust-Vegetation Interaction Experiment) was designed to develop a biogeophysical model which can simulate dust emission and ecosystem processes over the vegetated land surface. Dust emission processes have been investigated mostly on bare land, and there is very little information about vegetated land. Thus, intensive observations were conducted of a dust event that occurred on the Mongolian steppe on 24 April 2008. Meteorological and dust elements (e.g., saltation flux, visibility, dust concentration) and land-surface parameters (e.g., roughness length, vegetation cover, and the ground-based normalized difference vegetation index) were measured. During the event (from 13:00 to 18:00 LST on 24 April), the threshold wind speed at 1.54 m height, which is the minimum wind speed inducing saltation of particles ranging from 30 to 667 μm in diameter, was 8.9 m s-1 on a land surface with 7.2% vegetation cover with dead brown leaves, a small roughness length (0.0058 m), and a very dry sandy soil at 0-5 mm depth (water content, 0.002 g g-1). For comparison with previous studies, the threshold wind speed value was converted to the values at the heights in each study by using the logarithmic law of wind profile. Our value is close to the SYNOP-derived values for the same area, but larger than ground-observed and SYNOP-derived values for East Asian deserts.
Uncertainty in the values of air-sea exchange coefficients has a detrimental effect on tropical cyclone (TC) modeling. Since a TC is one of the most destructive disasters, a method is required to reduce such uncertainty with respect to scientific progress and disaster prevention. In this study, we investigate the feasibility of specifying air-sea exchange coefficients in the high-wind regime of a mature TC by an identical twin experiment using the adjoint data assimilation method. The forward integration is executed by an intermediate cloud-resolving atmosphere-ocean coupled model, while the datasets for the backward integration are sampled as in multiple aircraft missions. Our results show that the air-sea exchange coefficients are successfully improved toward the “True” values. The updated air-sea exchange coefficients yield persistent improvements in the maximum wind speed, the radius of maximum wind, the radius of strong updraft, and in the distribution of water vapor. Without adjustment of the exchange coefficients, the analysis field of the inner-core is contaminated, even if the initial state is modified by the adjoint method.
Using sea surface height (SSH) data derived from recent satellite observations, we present the observed dispersion relationship of mixed Rossby Gravity (Yanai) waves in the Pacific Ocean. A wavenumber-frequency spectral analysis of SSH fields shows prominent spectral peaks along the dispersion curves of first and second baroclinic mode oceanic Yanai waves. Also, salient features of tropical instability waves (TIWs) with a period of ∼17 days can be effectively isolated based on a cross-correlation analysis of sea surface temperature (SST) and SSH time series that are filtered in wavenumber-frequency domain. These statistical representations of oceanic Yanai waves and TIWs are important for the evaluation of numerical model simulations of these waves and for improving our understanding of the physics of 17-day TIWs.
We applied three Bayesian approaches with two metrics (bias and trend) to the sensitivity study of the projection of surface air temperature (SAT) changes over Japan for winter and summer due to global warming. In addition, we investigated the differences in projected probability density functions (PDFs) produced by the Bayesian approaches with the two metrics and the arithmetic ensemble mean (AEM) using available simulations from the Coupled Model Inter-comparison Project phase 3. The magnitude of model weights differs among the three Bayesian approaches with the same metrics, although the models with large weights are identical. However, models with large weights differ between the two metrics. Similar projected SAT changes of the models with large weights result in narrower PDFs of SAT changes than those of AEM. The peaks and means of the Bayesian-weighted PDFs are almost identical to those of the AEM's PDF. However, in some cases, 5, 75, and 95 percentiles of the PDFs differ distinctly among the three Bayesian approaches. Therefore, impact researchers should investigate the differences in the PDFs of SAT changes due to Bayesian approaches and metrics.
We have been performing radiative budget measurements at a ground-based observatory located on Fukue Island. In this study, we analyzed the data from instruments installed in the observatory and the data observed by MODIS, the satellite-borne imager, to evaluate cirrus cloud radiative forcing (CRF). The result shows that CRF took a large positive value as 127.98 W m-2 at the top of the atmosphere (TOA) and a small positive value as 13.2 W m-2 at the surface. As a result, it was found that optically very thin cirrus clouds exert a strong warming effect on the Earth's system and a weak warming effect on the Earth's surface when aerosols are not taken into account.
Drought has become widespread throughout the Northern Hemisphere since the mid-1950s, affecting the Mongolian steppe and pastureland used for livestock. Given this background, we investigated the relationship between modeled root-zone soil moisture (Wm) and vegetation activity based on Normalized Difference Vegetation Index (NDVI) data for the Mongolian steppe during the period 1982-2005. In general, interannual change in NDVI coincided with that in Wm. NDVI showed a stronger correlation with Wm (r = 0.91) than with precipitation (P) (r = 0.65). A strong positive correlation was found between seasonal changes in NDVI and above-ground biomass (r = 0.94). A comparison between years with high and low NDVImax revealed that the significant difference in P led to a significant time-lagged (about a half month) difference in Wm and finally to that in NDVI with time lags of about one month. In addition, NDVImax value of a given year was correlated with the Wm value for the current year (r2 = 0.53), and was more strongly correlated with the combination of the current year Wm and the preceding year NDVImax of (r2 = 0.55). This result suggests that on the interannual basis, the vegetation activity is primarily controlled by the current year soil moisture and slightly affected by underground structures stored in the root system.
A mesoscale data assimilation experiment was performed for the Beijing 2008 Olympics Research and Development Project (B08RDP) under the World Weather Research Program (WWRP) conducted during the period around the Beijing 2008 Olympic Games. In this experiment, the Japan Meteorological Agency (JMA) hydrostatic mesoscale 4D-Var analysis system was modified and utilized to produce accurate initial fields over the China area. In addition to the conventional observations, precipitation data observed by rain-gauges were assimilated as well as data produced from a nowcasting system of the Australian Bureau of Meteorology. The analysis system with rainfall data outperformed other experiments, i.e., the mesoscale analysis without precipitation data and the JMA global analysis, with its quantitative precipitation forecast. This shows that the assimilation of precipitation data can have a positive impact on subsequent model forecasts and that it would be effective even for areas where successive rainfall observations are sparse.
Interannual variability of the Northern Hemisphere (NH) wave energy is diagnosed using 30-year reanalysis data. Particular attention is paid to a relationship between the stationary and transient wave energy in the extratropics. In the NH, the stationary wave energy is negatively correlated with the transient wave energy on interannual timescales. The correlation coefficient is significantly high during mid-winter and spring between the subtropics and midlatitudes. Throughout the year, negative correlation is also found between NH-mean generation rates of the stationary and transient wave energy, which results from the upward Eliassen-Palm flux. This suggests an essential role of the poleward eddy heat flux in the negative correlation of the wave energy. Standard deviation analysis shows that, in the NH, the interannual variability of the total wave energy is primarily explained by the stationary component. These facts provide a useful implication for understanding future projections of the NH extratropical wave activity.
The summer Arctic sea ice extent (SIE) during the period 1996-2008 had an accelerating trend of retreat. The difference of SIE between May and September has a significant correlation with the summer Northern Hemisphere Annular Mode (summer NAM). The summer NAM had a significant positive trend during the period 1958-1996, and a weak negative trend during the period 1996-2008. The atmospheric circulation during the recent period had a trend of anticyclonic circulation over the Arctic with easterly wind over the marginal seas. In addition, tropospheric warming was enhanced especially over the Arctic and near the ground surface.
To investigate the resolution dependence of singular vector (SV) for Typhoon SINLAKU in the recurvature stage, SVs were computed using the Japan Meteorological Agency (JMA) tangent linear and adjoint (hereafter TL/AD) model at three different resolutions of TL63L601, TL95L60 and TL159L60. The results show that depending on the horizontal resolution, SVs have their own peak energy height and spectral shifts from the initial to final time due to the difference of detectable influences from mid-latitude troughs and tropical cyclone (TC) surrounding flows. On the other hand, common features of wind fields are found among the three SV structures. As for the SV using the TL/AD model at a TL159L60 resolution, the 48 hour optimization time interval (OTI) provides a larger influence from mid-latitude troughs with higher peak energy height than in the case with 24 hour OTI. Those results suggest that in the present case of a recurving TC using dry SV with a dry total energy (TE) norm, a certain degree of resolution in the TL/AD model is required to effectively detect sensitive areas around the TC because the resolution can change the energy balance among the mechanisms contributing to the SV structure.
1 We use T as an abbreviation for triangular truncation with Gaussian grid, TL for triangular truncation with linear Gaussian grid and L for vertical layers. Therefore, TL63L60 denotes spectral triangular truncation at 63 wave numbers with linear Gaussian grid and 60 vertical layers.
The tropical-extratropical interaction associated with the Madden-Julian Oscillation (MJO) and El Niño-Southern Oscillation (ENSO) in boreal winter is examined. When MJO convection is activated over the Indian Ocean, an anomalous extratropical high appears over the north Pacific. In this study, a zonal shift of the high is found to depend on ENSO phases, and the high shifts westward in the El Niño developing (EV) phases. Northeasterly trade surges that originate from the high intrude the tropical western north Pacific only during EV phases and moisturize the area through convergence. Although convective activities are centered south of the equator in boreal winter, the moisturization by the surges results in the activation of MJO convection not only south of the equator but also north of the equator, facilitating the formation of the twin cyclonic disturbances straddling the equator. During the other ENSO phases, on the other hand, extratropical fluctuations and MJO activity do not interact each other. It has been shown in previous studies that these twin cyclonic disturbances produce westerly wind bursts, which can trigger El Niño. These results suggest that tropical-extratropical interaction, between the MJO and the north Pacific high accompanied by the trade surges, occurs preferentially during EV phases, and feeds back to the development of El Niño.
Severe convective clouds developed in the Tokyo Metropolitan Area in the afternoon of 12 July 2008, and the storm produced F0 downbursts. Two X-band multi-parameter radars operated by the National Research Institute for Earth Science and Disaster Prevention (NIED) captured the structure of the wind and hydrometeors within the cloud, as well as their time evolutions. The storm had a multi-cellular appearance, and the radars captured the development of four separate cells. The existence of wet hail aloft within the updrafts is suggested from the high rainfall rate estimated from reflectivity compared with that from the specific differential phase.
Two types of rainfall conditions for shallow landslide initiation are objectively identified in Japan. We examined rainfall intensity-duration conditions of 1,174 shallow landslides that occurred during 2006-2008 using the Normalized Soil Water Index (NSWI), which represents conceptual soil water contents. We classified rainfall in Japan into two types using principal component analysis: short-duration - high-intensity (SH) type, and long-duration - low-intensity (LL) type. Based on this classification, we proposed a new method herein for predicting shallow landslides resulting from the rapid increase of NSWI with short duration in the SH type, and from the gentle rise of NSWI followed by heavy rainfall with long duration in the LL type. We verified this method by analyzing the rainfall-induced shallow-landslide and debris disasters in 2009, for which we succeeded in predicting those of the SH and LL types, respectively, at an earlier stage of rainfall events. These results are important for shallow landslide disaster warnings in Japan.
Long-term trends of gridded land surface temperature data over Japan, provided by the Climatic Research Unit (CRU) at the University of East Anglia, NASA Goddard Institute for Space Studies (GISS), and the Japan Meteorological Agency (JMA) were compared with those at surface observation stations that are used for the JMA's climate monitoring. It was found that the CRU data (CRUTEM3v) for 1901 to 2008 have an anomalous trend of about 0.04°C/decade relatively to the surface stations, except over the Nansei Islands. The trend of the GISS temperature data (the Tsurf250 file) also has some positive departure until 1960. These facts indicate the possibility of urban bias in these gridded data over the Mainland Japan area.
A new parameterization scheme of air-sea momentum flux, heat flux and moisture flux is introduced into the Japan Meteorological Agency Non-Hydrostatic Model (JMA-NHM, Saito et al. 2006). The new formulation describes the saturation properties of bulk transfer coefficients of momentum, heat and moisture under high winds regime (> 30 m s-1). It also considers the effect of sea-wave drag in the roughness length. The proposed parameterization is applied to simulate an intense tropical cyclone Hagupit (0814). The impact of the new scheme on the tropical cyclone prediction is found in the increase of maximum surface wind speed and decrease of central pressure. Improvements in the forecasts of distribution of high wind areas and precipitation are obtained. This may help to improve the model predicted wind-pressure relationship for intense tropical cyclones.
Lidar observations of the stratospheric aerosol layer have been carried out at Tsukuba and Naha, Japan, and at Lauder, New Zealand. Evolution of the volcanic aerosols originating from the 1991 Mt. Pinatubo volcanic eruptions, and the subsequent trend and seasonal variations of the background aerosols, are evaluated. Stratospheric aerosol drastically increased to about 30 to 100 times of the background level after the eruption, and returned to calm condition after about five years. The e-folding time of the integrated backscattering coefficient of Pinatubo aerosols at the three stations shows similar values between 1.14 and 1.37 years. The column amount of background stratospheric aerosol over Lauder is about 3 times as much as that over Naha, and that over Tsukuba is between the other two sites. Seasonal variation of stratospheric aerosol over Tsukuba shows the largest value and is nearly 10 times as much as that over Naha. The background aerosols over Lauder increased after around 2000. The increasing rate observed over Lauder is about 3.8%/year from 2000 to 2009.
The effect of model resolution on projected climatological features of tropical cyclones (TCs) was investigated via 25-year present-day and future global warming projections using the Japan Meteorological Agency/Meteorological Research Institute Atmospheric General Circulation Model with four resolutions ranging from TL95 (180-km mesh) to TL959 (20-km mesh). The finest resolution (TL959) showed the highest skills in terms of TC intensity and interannual and seasonal variations in TC genesis number. Resolutions of TL319 (60-km mesh) and finer showed a significant future increase in the frequency of intense TCs, whereas resolutions coarser than TL319 showed no such change, indicating that TL319 is the critical resolution in projecting future change in the frequency of intense TCs. Overall, high model resolution is preferable for realistic and reliable climate projections. Resolutions of TL159 (120-km mesh) and finer showed similar skills, biases, and future changes in the spatial pattern of TC genesis frequency (TGF) and TC genesis number, indicating the potential use of lower model resolutions for minimizing uncertainties in future changes in the mean state of TGF and TC genesis number.
Structures of 266 tropical cyclones (TCs) that completed extratropical transition (ET) in the western North Pacific during 1979-2004 are examined using the JRA-25 dataset, in relation to the removal of TC wind profile retrievals (TCR) from each TC after its recurvature. The TCR is a kind of “TC bogusing” techniques, and it was expected that its removal may lead to an artificial change in the TC structure at the ET stage. Nearly half of the examined TCs have cold-core structures before the removal of the TCR. On average, the structural change associated with ET starts more than one day before the removal of the TCR. Direct impact of the warm-core structure of the TCR is limited within a radius of about 200 km from the TC center. Although discontinuity in the TC structure between the periods with and without TCR may be statistically significant, the differences in the indices are practically negligible.
Impacts of three kinds of GPS-derived water vapor data, i.e., precipitable water vapor (PWV), slant water vapor (SWV) and radio occultation (RO) data, were investigated using the Meso 4D-Var system of Japan Meteorological Agency (JMA) for a heavy rainfall case on 16 July 2004. When PWV or SWV data were assimilated individually, water vapor in the rainfall region was increased and on the northern sides was decreased, and then the shape of the rainfall region became similar to the observed one. However, the reproduced rainfall amount remained smaller than the observed one. Compared with PWV, SWV made the horizontal contrast of water vapor larger. When RO data were assimilated, the low-level water vapor was increased so that the rainfall amount was largely increased. However, the rainfall region became wider than the observed one. When SWV and RO data were assimilated simultaneously, low-level water vapor in the rainfall region and on its southern side was increased, and then both shapes of rainfall region and of rainfall amount became similar to the observed ones.
The transformed Eulerian mean (TEM) formulation for equations of motion, which was originally defined for waves on a zonally averaged flow, is extended three-dimensionally to a non-hydrostatic Boussinesq fluid in the general coordinate system. The three-dimensional antisymmetric diffusion tensor that plays a central role in defining the residual circulation and the Eliassen-Palm flux tensor is derived systematically by assuming that eddies consist of neutral waves. A modification of the formulation for application to practical data analyses is made by introducing a symmetric eddy diffusion tensor.
This paper discusses the availability of the vertically integrated liquid (VIL) water content product from 3-cm wavelength polarimetric radar in precipitation detection and estimation. Based on a case of localized severe rainfall around Zoshigaya, Tokyo, Japan on 5 August 2008, VIL values are compared with radar derived rain intensity and surface rain-gauge data. VIL is calculated by three methods: (1) using only the horizontal reflectivity (Z), (2) using only specific differential phase (KDP), and (3) using both Z and KDP. Two integral height scales are tested: a height near the melting layer (5 km) and a height over the echo top (15 km). The composite method of Z and KDP proved to be the best approach for calculating VIL. VIL values were consistent with gauge measurements: the maximum normalized cross-correlation coefficients at all three observation sites (Edogawa Elementary School, Daiichi Junior High School, and Toshima Branch) exceed 0.9, and detection by VIL has a 5-10 min lead time than gauge measurements.
Smoke plumes originating from a forest fire in northern Mongolia were observed with a two-wavelength (1064 nm, 532 nm) polarization (532 nm) lidar in Nagasaki at altitudes of 12 to 14 km and 3 to 10 km on June 10, 2007. Smoke from the same region was also observed in Tsukuba with a 532 nm high-spectral-resolution lidar (HSRL) at altitudes of 15 to 15.5 km on June 12, 2007. A two-wavelength data analysis method was applied to the Nagasaki data, and the extinction-to-backscatter ratio (the lidar ratio) at 532 nm was estimated to be 65 ± 5 sr (50 ± 5 sr) for the smoke at 12 to 14 km (3.5 to 4.5 km) altitudes. The particle depolarization ratio (PDR) was 0.14 ± 0.03 (0.12 ± 0.03), and the backscatter-related Angstrom exponent (BAE) between 532 nm and 1064 nm was 1.1 ± 0.2 (0.9 ± 0.1) for the high (low) altitude smoke. The optical thickness of the high (low) altitude plume was approximately 1.0 (0.03). The lidar ratio of the smoke in Tsukuba measured with the HSRL at 15 to 15.5 km was 75 ± 5 sr, and the PDR was 0.15 ± 0.04. The optical thickness was 0.03. The lidar ratio was comparable to those reported previously for forest-fire smoke in the lower troposphere. However, the PDR in the present case was two times higher, and the BAE was slightly lower. A possible explanation of the results involves mixing with solid particles such as those of ash and/or mineral dust in the strong convection found with pyrocumulonimbus. A discussion on lidar methods for characterizing smoke aerosols is also provided.
Cloud-top height (CTH) variability in the tropical tropopause layer (TTL) in association with equatorial Kelvin waves is investigated using a new CTH dataset based on MTSAT-1R geostationary satellite measurements with a statistical look-up table constructed based on CloudSat measurements. We focus on a case in the tropical Indian Ocean during October-December 2006, when shipboard radiosonde, TTL water vapor, and 95-GHz cloud radar measurements were taken during the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO) field campaign. At 10-15 km, the satellite-based CTH data agree well with the radar echo top heights from shipboard radar reflectivity data. During the MISMO campaign, cloud frequency was suppressed in the warm phase of equatorial Kelvin waves propagating in the TTL. The suppressed-cloud region moves eastward to the western Pacific together with Kelvin waves. We found that changes in CTH occurrence frequency over the vessel in association with Kelvin waves are much greater than those associated with the diurnal cycle. It is expected that the phase of equatorial Kelvin waves is important for the intraseasonal variabilities of both the radiative budget of the tropical atmosphere and water vapor transport in the TTL.
The contribution of tropical cyclones (TCs) to the seasonal change patterns of precipitation in the western North Pacific was studied using Japanese long-term reanalysis/JMA Climate Data Assimilation System (JRA-25/JCDAS) data from 1979 to 2006. The seasonal change patterns are based on the three types of Hattori et al. (2005) classified by a northward shift and increase in precipitation from June to August. We show that the contribution ratio of TC precipitation in the western North Pacific, on average and maximum being 19.8% and 46.7%, becomes larger as monthly mean total precipitation increases. Among the three types of seasonal change pattern, TC precipitation varies corresponding to the features of total precipitation and contributes 0.5-1.8 mm day-1 variation of the regional mean precipitation. Between the two types which show a northward shift and an increase in precipitation, TC causes 31%-41% of the total precipitation difference. TC precipitation highly contributes to the type that shows the largest northward shift and precipitation increase; its maximum ratio reaches 70% east of Taiwan and TCs contribute 3-5 degrees of the northward shift of the precipitation area in the western North Pacific.
The convergent and divergent wind over the Japanese islands is estimated using meteorological observation data to examine the general characteristics of land and sea breeze driven by differential heating of land and sea. The results show an annual cycle wind called the “Japanese Monsoon” blows, as well as a diurnal cycle flow. These winds are caused by the temperature difference between the Japanese islands and the surrounding sea. The divergent wind field over the Japanese islands can be expressed simply as a superposition of synoptic-scale phenomena (wind fields with seasonal and daily cycles). An annual variation in the amplitude of the diurnal cycle wind is closely related to the magnitude of solar radiation.
The predictability of large-scale atmospheric motions during the onset period of a prominent atmospheric blocking event occurring over the Euro-Atlantic sector during 12-21 December 2007 is examined using Japanese 25-year Reanalysis (JRA25)/JMA Climate Data Assimilation System (JCDAS) dataset and one-week ensemble forecast dataset provided by the Japan Meteorological Agency (JMA). First, it is found that the predictability in the blocking region is temporarily reduced during the onset period of the blocking event. Second, a simple sensitivity analysis introduced by Enomoto et al. (2007) indicates that low-frequency variations associated with a quasi-stationary Rossby wave train significantly affect the prediction of the blocking onset in comparison with high-frequency variations. This is also confirmed by the time evolution of the spread among ensemble members based on 300-hPa height field. Finally, the predicted vorticity flux divergence by the low-frequency variations, rather than the high-frequency variations, is also found to significantly correlate with the predicted blocking strength, which stresses the importance of the low-frequency variations for the prediction of the onset of this blocking event.
The present study provides a preliminary evaluation of the WRF model at 4-km horizontal resolution in central Japan with a 5-year control simulation. The results show that the spatial distribution of the annual mean temperature is well reproduced with a bias of +0.6°C. However, the diurnal range is underestimated by 1.8°C, which is mainly caused by an overestimation of the daily minimum temperature from October to April. Additional analyses indicate that there is a statistically significant correlation between the monthly bias and the number of clear sky days in a particular month, which implies the incomplete reproduction of the surface inversion layer at night. The model overestimates the annual precipitation amount averaged over the analysis domain by 7%, compared to the observations. Such a positive bias results from the overestimation of weak precipitation. On the other hand, the model underestimates the frequency of the heavy rainfall. The performance of the WRF model shown in this study is similar to that of the NHRCM (by Sasaki et al. 2008). The results indicate that the WRF model can be used for high resolution, regional climate simulation/projection research for the studied region.
The climatological characteristics of daily precipitation over Japan in the Kakushin regional climate change experiments using a non-hydrostatic model with a horizontal resolution of 5 km (NHM5km) are investigated from June to October between 1990 and 1999. Comparisons with the results of a global 20-km-mesh atmospheric climate model (AGCM20km), which provides the boundary conditions for NHM5km, show that NHM5km can improve the reproducibility of precipitation characteristics, including the frequencies of intense precipitation and wet days, from those in AGCM20km. Compared with observations, AGCM20km shows -30% and +33% biases in terms of the 10-year mean values of regional maximum precipitation and wet days, respectively, in the region 31°N-38°N and 129.8°E-142.0°E over Japan from June to October; these biases are reduced to +9% and +12% in the case of NHM5km. The largest biases in AGCM20km, overestimation of wet days in July and August, are successfully reduced in NHM5km. Probability density distributions of daily precipitation amount are superior in NHM5km than in AGCM20km for all analysis months, being in strong agreement with a raingauge-based daily precipitation dataset for June-August. These features indicate that NHM5km also provides improved seasonal variations in precipitation characteristics, which are crucial for reliable climate change experiments.
A ground-based lidar observation was carried out in the northwest of China to validate the space-borne lidar CALIOP on 23 March 2009. Combining backscatter profiles of the ground-based lidar and CALIOP, lidar ratio (extinction to backscattering ratio) was retrieved for 532 nm and 1064 nm wavelengths by using performance function that minimizing the difference between the ground-based lidar and CALIOP for backscattering coefficient. The correlation coefficients between them were 0.98 for 532 nm and 0.95 1064 nm, respectively. Using the retrieved lidar ratio, the color ratio and aerosol optical depth (AOD) were calculated. The observed aerosols and clouds were classified into three groups (boundary layer dust, free tropospheric aerosol and cirrus cloud) according to a relationship between color ratio and 532 nm-backscattering coefficient.
Dust emission estimated with a 4D-Var data assimilation system using ground-based lidar network data was compared with vegetation growth data based on visual observations in the Gobi desert in Mongolia in the spring of 2007. The dust emission flux estimated with the data assimilation system was less than that estimated without data assimilation in the dust event of May 21-30 and was the opposite in the event of March 25-April 3. The threshold surface friction velocity estimated from the results of the data assimilation was less than 0.3 m s-1 in the dust event of March 25-April 3 and was ∼0.36 m s-1 in the event of May 21-30. The difference between the two events was qualitatively explained by the vegetation growth data. The accumulated precipitation during the period was ∼2 mm. The results show that vegetation growth with slight precipitation in the Gobi desert may significantly reduce dust emission.
The analysis of satellite-based ocean color data shows that low concentrations of surface chlorophyll-a (chl-a) found in the equatorial region of the Pacific Ocean varies in phase with the eastern edge of the warm pool. As is true for high sea surface temperatures, the existence and maintenance of these low concentrations are linked to the upper ocean stratification due to salinity. The present study also establishes the quasi permanence of a frontal zone in chlorophyll-a separating the regimes of the western region and the eastern-central cold tongue and, through the identification of this front in satellite-based ocean color data, it provides, for the first time, a reliable method for locating the eastern edge of the warm pool from surface observations only. Finally, the recognition of this front offers the opportunity to define a simple and robust index of the horizontal extension of the equatorial Pacific warm pool within the context of the ENSO variability.
Observations show that optical depth over desert increase during daytime when a convective mixed layer develops under a light general wind condition. This implies that dust suspension by horizontal winds associated with convective motions occur even in the absence of general winds. In the present paper, a large eddy simulation is performed to study how much dust is suspended in a convective mixed layer without a general wind. The results show that dust particle concentration in the convective mixed layer can reach on the order of 10 μg m-3, which is in reasonable agreement with observations. Tiny dust particles that have small terminal velocities are easily brought up by convective winds during daytime and remain in the atmosphere throughout night. If a similar weather continues for several days, dust particles concentration on the evening of the second day can reach 1.8 times as large as that on the evening of the first day, accordingly.
The impact of stratospheric sudden warming event in September 2007 on the tropics was investigated based on satellite data (CALIOP, MLS and TRMM PR). Equatorial temperature and water vapor at 100 hPa decreased by about 1 K and 1 ppmv within 10 days, respectively. Changes in tropical clouds are observed together with the occurrence of the SSW as i) frequent formation of higher-level cirrus clouds over the Maritime Continent, to where water vapor was transported from Asian Monsoon and where the lowest temperature occurred, ii) intensification of deep convective activity in the TTL over African continent, and iii) southward shift of the convective clouds over South American continent.
To study regional climate change over Japan, we conducted continuous 26-year dynamical downscaling of JRA-25 (Japanese Re-Analysis 25 years) data using a 20 km-mesh Regional Climate Model (RCM20). The accuracy of the results of downscaling were examined using the observed precipitation data. We found that the downscaled precipitation data reproduce well at both monthly and daily time scales, as well as for extreme events. The data also correlate well with observed inter-annual variability of daily precipitation frequencies for heavy rain events over Japan. These results demonstrate that RCM20's ability for dynamical downscaling is quite strong and that the data are useful for investigating regional climate change.
The number (0.26 m) in the caption of Fig. 1 was incorrect. The correct sentence is given here: A 3rd sentence in the Fig. 1 caption. “The curves indicate Yanai wave dispersion relation for equivalent depths of 0.8 m and 0.23 m.”
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