In April 2002, a severe dust storm occurred in the Taklimakan Desert. A large amount of the dust was lifted up by the dust storm and gradually removed in the following few days. The whole event of the dust storm was observed by the Mie-scattering depolarization lidar at Aksu, Xinjiang, China (40.62°N, 80.83°E, 1028 m above mean sea level). This paper describes the dust event and the removal process that was observed by the lidar. During the dust storm (April 13-16), a dense dust layer developed from the ground up to 5.5 km. The backscattering ratio was 20 or more, and the depolarization ratio was 15-25%. Due to the absorption of the laser beam by the heavy dust, a normal lidar observation was impossible for several hours. In this study, we estimated the backscattering ratio at the lowest height during the dust storm by solving the lidar equation directly. After the dust storm (April 17-20), a clear diurnal variation of the top of the dust layer was found by the lidar. An investigation of the lidar signals at different heights shows that there were two types of the removal process of the dust. The lidar signals at lower heights (less than the 2 km) gradually decreased during the post-dust storm period. This result indicates that the gravitational settling of the relatively large sized dust (coarse particles with a diameter of 10 μm or more) occurs near the ground. On the other hand, lidar signals at 2-4 km had a clear diurnal variation with spike-like peaks from evening to midnight. These peaks suggest that the advection of the relatively small sized dust picked up in other location is due to the local circulation that occurs in the Tianshan Mountains and Tarim Basin.
This paper presents an observational and numerical study of the southwesterly flow and heavy rainfall associated with Typhoon Mindulle (2004). When Mindulle made landfall on the east coast of Taiwan on 1 July 2004, a secondary low formed over the Taiwan Strait. The low then dissipated after Mindulle moved out over the ocean north of Taiwan. Subsequently, the accompanying strong southwesterly flow brought extremely heavy rainfall over southern and central Taiwan. The modeling study of the 15-km and 5-km grids shows that the southwesterly flow, which resulted from a downgradient acceleration toward the low system over the Taiwan Strait, transported the convectively unstable air northeastward over the northern South China Sea. When low-level air convergence provided enough lifting, strong mesoscale convective systems (MCS) were triggered in this region. This contributed to the formation of a series of mesolows and mesohighs which prevented air from accelerating farther northeastward. Only after these MCSs dissipated and the Pacific high extended westward was the moist unstable air able to accelerate into the southern Taiwan Strait, where it converged with the westerly flow of the typhoon circulation, producing convective rainbands southwest of Taiwan. When moving over land, these MCSs were enhanced, resulting in heavy rainfall. It is concluded that although the typhoon circulation was critical on the rainband development, the importance of the southwesterly flow in bringing the moist unstable air to converge with the typhoon circulation can not be left out. The southwesterly flow was first induced by typhoon's low pressure system at early stage, and was enhanced as a result of the westward extension of the Pacific high at later stage. The moisture budget analysis further shows that the development of the MCSs was mainly contributed by the horizontal moisture transport in the southwesterly flow. Evaporation locally from the ocean played only a minor role.
An eastward propagating Kelvin wave of period near 7 days is observed in the radiosonde winds and temperature in the upper troposphere and lower stratosphere (UTLS) region and a wave of similar periodicity is observed simultaneously in the Mesosphere and Lower Thermosphere (MLT) winds acquired by MF radar at Pameungpeuk (7.5°S, 107.5°E) during the first Coupling Processes in the Equatorial Atmosphere (CPEA-1) Campaign (April 10-May 9, 2004). The horizontal and vertical characteristics of these waves are investigated using winds and temperature data acquired by TIDI (TIMED Doppler Interferometer) and Sounding of Atmosphere using Broadband Emission Radiometry (SABER) instruments respectively on TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) satellite and European Centre for Medium-Range Weather Forecasts (ECMWF) winds and temperature. The wave observed in the MLT region has a dominant period of approximately 6.5 days and is found to propagate westward with zonal wave number 1. The horizontal structures of the wave in winds and the temperature indicate that the wave is of gravest symmetric wave number 1 Rossby wave. The downward phase propagation from the MLT region to tropopause indicates that the wave may have a source in the troposphere. The Kelvin wave observed in the UTLS region has zonal wave number 3. However, this wave becomes damped above 23 km, where the above mentioned westward propagating wave begins to amplify. During the observation period, the OLR distribution in the tropics shows similar periodicities, propagating eastward with zonal wave number 3 and westward with zonal wave number 1. These observations suggest that tropical convective heating may be a common source for these waves.
Annual values of sunshine duration (SS) measured in Japan between 1890 and 2002 were used as a proxy for global irradiance (Eg↓) to study trends and changes in solar forcing at the Earth's surface. Proxy relationships established for the two SS recorders used in the JMA network both yielded estimates of mean annual values of Eg↓ with RMS &1t; 6%. A first order integrated moving average model (ARIMA) adequately described the time course of SS and Eg↓, which indicated a small, irregular but significant annual increase in solar forcing during the 20th century averaging 0.08 W m-2 or 2.3 hours of Jordan SS recorder sunshine, equivalent to 0.5% per decade. The rate of increase was four times the average in the first four and last three decades of the century reaching a maximum after 1980. The negative effect of the five major volcanic eruptions on Eg↓ was shown to yield a significant linear negative forcing of -41 W m-2 per unit AOD stratospheric aerosol optical depth). The degree of negative solar forcing was related to latitude: between 25° and 44°N each degree shift to the North was associated with an annual increase in Eg↓averaging 0.02%. The time course of changes in solar radiation in Japan during the 20th century resembled that measured in air temperature; correlations between annual values of Eg↓and those in the air temperature of the Northern Hemisphere were very highly significant (P < 0.001) both for the concurrent and preceding year.
In climate change projections, inter-model differences in cloud feedback have been identified as the largest source of uncertainty. The source terms of the cloud condensate tendency equation (CCTD) are expected to be useful diagnostics to better understand the different cloud responses to a CO2 increase in GCMs. To demonstrate the idea, analysis of the CCTD response to CO2 doubling is presented using two versions of a climate model with different climate sensitivities of 6.2°C ('HS'version) and 4.1°C ('LS'version). The model's response to CO2 doubling is characterized with a marked difference in the cloud feedback between the two versions, which is consistent with the cloud response in the southern middle latitudes: cloud decreases in the HS version and increases in the LS version. Analysis of the source terms reveals that the difference in cloud response is attributable to the ice sedimentation process. The results also suggest the importance of the vertical cloud ice profile which controls the ice sedimentation response to a CO2 increase, indicating the potential for providing constraints on the aspect of cloud feedback.
The relationships among structures, development processes, and diabatic heating profiles of a maritime mesoscale convective system (MCS) and a continental MCS in the inactive phase of a large-scale disturbance associated with Madden-Julian Oscillation (MJO) were studied mainly using dual-Doppler radar data around Darwin, Australia during the southern summer in 1998-1999. The maritime MCS (15 January 1999) formed in a west to southwesterly flow of moist air at the lowmid levels during the monsoon active regime, and had several convective lines oriented parallel to the mid-level shear in 1-2 hours cycle within an extensive stratiform region. Radar analyses revealed that a new convective line formed on a convergence between the outflow of a weak downdraft within the stratiform precipitation and the moist westerly in the low-level, and developed more deeply within a mid-level moist rear inflow. A cyclonic circulation (mesovortex) in the mid-level was subsequently found in the stratiform region. The analysis of heating profiles is indicative of a convective heating maximum at 6 km and a stratiform region with large heating and relatively small cooling above and below the melting level, respectively. The developments of the convective line and mesovortex are attributed to the small low-level cooling and convergence of moist air with the large mid-level heating, respectively, and both are considered to play a significant role in the maintenance of the MCS. The continental MCS (21 January 1999) formed in a strong mid-level east-southeasterly flow of dry air that was influenced by tropical depressions during the break regime. Radar analyses showed that the MCS formed a number of convective lines oriented parallel to low- and mid-level shears with relatively strong negative vortices and convergence in the low-mid level. Positive vorticity were found with a descending rear inflow in the rear of the lines and the stratiform region of the MCS below the mid-level. The heating profiles represented a convective heating maximum at 5.5 km, and a moderate stratiform cooling below the melting level. It is suggested that the stratiform cooling due to the mid-level dryness made a contribution to the advection of positive vorticity below about 5 km, resulting in the development of convective lines with a intensive vortex couplet. A comparison of the heating profiles in the stratiform parts of two MCSs indicated a larger low-level cooling in the continental MCS and a larger heating at the mid-upper level in the maritime MCS. It was suggested that the different stratiform heating of two MCSs in the inactive phase of the MJO played significant roles in their structures and development processes.
This paper discusses the effect of cumulus suppression (CS), applied in the Arakawa-Schubert (AS) cumulus parameterization, on the Baiu front simulated with an atmospheric general circulation model (AGCM; T106L56: horizontal resolution ~1.1°, 56 vertical layers). As an additional condition of the AS cumulus parameterization, CS permitted cumulus convection only when the environmental relative humidity averaged in the modeled cumulus cloud of the AS scheme exceeded 80%. To evaluate the effect of CS, the results of simulations with CS (CS run) are compared with those without CS (NOCS run). Detailed analysis was carried out for June, because the Baiu front in the real atmosphere is usually sustained quasi-steadily during June. Whereas the distribution and amount of 5-year June mean Baiu precipitation in the CS run was generally similar to that in the NOCS run, the standard deviation of daily precipitation of 5-year June in the CS run was significantly larger than that in the NOCS runs. The CS condition increased heavy rainfall and temporal variation of Baiu precipitation. The CS run also suppressed precipitation in the southern area of the Baiu front, as seen in observations. Examination of the Baiu front during intense-rainfall periods in the model showed that characteristics of the Baiu front and frontal disturbances were reasonably simulated, in some years, by both the CS and NOCS runs, to a certain extent. However, the CS condition improved the occurrence of intense rainfall events at the Baiu front, and the reproduction of the Baiu frontal disturbances. Both of these features became closer to actual observed features.
I examined features of the South Indian Ocean convergence zone (SICZ) and the North American convergence zone (NACZ) simulated using an atmospheric general circulation model (AGCM; T106L56: a spectral primitive-equation model with 56 σ levels and triangular spectral truncation at wave-number 106). The 24-year model integration from 1979 to 2002 was constrained by observed sea-surface temperature and sea-ice distribution. I selected a typical case for each zone (SICZ and NACZ) from the 1985-1996 simulation. The AGCM properly simulates African and Indian Ocean monsoon circulation and precipitation. The precipitation zone of the SICZ extends southeastward from the southeastern part of Africa to the southwestern rim of the Mascarene high during Southern Hemisphere summer. North American summer monsoon circulation and precipitation were also correctly reproduced. The precipitation zone of the NACZ extends northeastward along the southeastern coast of North America to the northwestern rim of the Bermuda high during the North American summer monsoon season. I compared the features of the simulated SICZ and NACZ with features of the South Atlantic convergence zone (SACZ) and the Baiu frontal zone (BFZ) simulated using the same AGCM. The SACZ, SICZ, NACZ, and BFZ were characterized as subtropical convergence zones (STCZs) and are commonly sustained along their respective subtropical anticyclones that form over the ocean east of continents. However, their geographical environments differ significantly. Whereas the respective cool oceans at the poleward sides of the SACZ and SICZ provide significant baroclinicity for the SACZ and SICZ, the respective hot continents to the poleward sides of the BFZ and NACZ create weak baroclinicity for the BFZ and NACZ.
Double ridges in the Western North Pacific Subtropical High (WPSH) in 1998 are studied as a particular case using NCEP/NCAR reanalysis data and the Black Body Temperature (TBB) data. In contrast to typical single ridge in WPSH, double ridges are found during July in 1998 and associated with the second Meiyu activity in the Yangtze River valley. The second ridge of WPSH is built up in the south of the first one and contributes to the stagnancy of WPSH in the south in middle of July. It is different from the previous view that the stagnancy of WPSH in the south resulted from the sudden southward withdrawal of the original ridge. The characteristics of double ridges including the structure, evolution and possible mechanisms are investigated with the case of 1998. The results show that there are a lot of differences in the large-scale circulation, temperature and humidity between the single ridge and double ridges of the WPSH. The north branch of the double ridges has coherent characteristics with the traditional single ridge, while the south branch is characterized by the tropical systems. There are evidences to support that the formation of the double ridges is associated with the northward movement of equatorial buffer zone and the persistence is related to the evolution of tropical convections. The rainfall pattern in eastern China associated with the double ridges of WPSH is examined using the instrumental records of precipitation in China. Two rain belts shaped as an italic "L"-like pattern in eastern China are found to be related to the double ridges. This study provides new evidence to understand the variability of WPSH and associated East Asian summer monsoon activity on synoptic scales.
On 9 October 2004, Typhoon Ma-on hit the southern Kanto district in eastern Japan. Strong winds were observed in the left-rear quadrant of Ma-on near the typhoon center during its passage over the southern Kanto district in spite of the rapid translation speed of about 70 km h-1. Numerical simulations of Ma-on were performed using a nonhydrostatic model with a horizontal grid spacing of 2 km. The simulation results showed that the strong winds on the left-rear quadrant of Ma-on were low-level phenomena after landfall. The low-level jet (LLJ) associated with the observed strong winds formed over the Sagami bay just after the typhoon center had passed. The typhoon moved over the pre-existing low-level cold air in the Kanto region. When the typhoon center reached the Sagami bay, the northerly flow of the low-level cold air formed a narrowed channel between the typhoon center and the Kanto Mountains in the west of the Kanto plain. The LLJ corresponded to the outflow response from the narrowed channel to the Sagami bay. Trajectory analysis illustrated that the parcels which passed through the LLJ traveled southward parallel to the Kanto Mountains. When they passed near the Tanzawa Mountains at the southern tip of the Kanto Mountains, significant subsidence, spreading and acceleration occurred toward the Sagami bay. The horizontal momentum budget analysis and diagnostic evaluation of pressure gradient force (PGF) revealed that the LLJ was mainly supported by the large-scale southward PGF due to Ma-on. However, locally generated mesoscale forcing due to the decreasing depth of cold layer worked around the exit of the narrowed channel of the cold air. Thus, we conclude that the dynamics and structure of the LLJ were close to those of"gap wind". The sensitivity experiments showed that the low-level cold air as well as the relative position between the high mountainous topography and the typhoon was essential for the LLJ formation.
In this preliminary study, we have developed a method to retrieve rain rate on a scale of 20 km from the brightness temperatures measured by the TRMM microwave imaging radiometer (TMI) over the tropical oceans, using the estimates of rain rate RPR made by the TRMM Precipitation Radar (PR) as a benchmark. The purpose of this study is to demonstrate with a limited amount of PR and TMI data the feasibility of improving the TRMM operational rain retrieval method V6 over the tropical oceans. This study utilizes the TMI-measured brightness temperatures T19H, T37H, and T85H of horizontally polarized microwave radiances at 19, 37 and 85 GHz, respectively, to deduce a salient non-linear parameter ξ that is highly correlated with RPR over the oceans. Two additional parameters generated from TMI data, ω and γ, add significant amounts of rain information to our retrieval method. The parameter &omega is based on T19V and T21V, the brightness temperatures measured by TMI for vertically polarized microwave radiances at 19 and 21 GHz respectively. This parameter takes advantage of the independent information contained in T21V. The parameter Γ depends on the average horizontal gradient of the TMI-measured T85V (vertically-polarized 85 GHz radiance) in a 20 km footprint. Initially our TMI rain retrieval algorithm is tuned with the help of RPR for seven cases of 2° × 3° area over tropical oceans. Then it is applied to 13 other independent tropical ocean cases. For these independent cases, the rain rate R* estimated from our method correlates better with RPR than the rain rate RV6 retrieved from the present TMI V6 operational retrieval method. On a 20 km scale, the correlation between RPR and R* is better by about 6% compared to that between RPR and RV6. The slope of the regression line between the rain rates RPR and RV6 is about 0.5. With respect to RPR, the rain rate RV6 retrieved from operational V6 method tends to underestimate high rain rates and overestimate low rain rates. The slope of the regression line between RPR and the rain rate R* retrieved with our method is about 0.8, another indication of the improvement of R* over RV6. In addition, the area average rain rate on a scale of 2° × 3° deduced from our method agrees better with that of PR by about 7%.
A prominent mode of low-frequency variability in the northern extratropical winter known as the Pacific/North American (PNA) teleconnection pattern prevails not only on seasonal but also on intraseasonal timescales. In this study, processes governing the intraseasonal PNA are investigated using daily fields during 1957-2002. The results of the vorticity budget analysis illustrate that the positive phase of the PNA tends to grow by linear processes such as the barotropic energy conversion from the zonally asymmetric climatological flow. For the negative phase of the PNA, nonlinear low-frequency vorticity advection is as important as the linear processes. Composite life cycle of the PNA shows that at 9 days before the peak a pronounced wave train was observed along the Asian jet stream and it eventually developed to the PNA near the jet exit region. This wave train is found to be excited by divergent winds primarily associated with anomalous convection of the Madden-Julian Oscillation (MJO). Probability density functions of the MJO calculated separately following the polarity of the PNA reveal a phase locking between the PNA and the MJO. When the active (inactive) convection associated with the MJO reaches the Bay of Bengal to the western Pacific, occurrence frequency of the negative (positive) phase of the PNA is the highest. This MJO triggering explains roughly 30% of the total PNA events, suggesting that, even though the PNA may be inherent to the extratropical atmosphere, a specific tropical forcing is of importance to realize the PNA as dominant mode.
The canonical ensemble correlation (CEC) prediction method is applied to build a statistical prediction model of East Asian summer monsoon (EASM) rainfall. The predictors are regional sea surface temperature (SST) and sea ice concentration (SIC) fields in selected optimal periods of time. Results show this CEC model is much more skillful than the commonly used canonical correlation analysis (CCA) model which uses only winter tropical Pacific SST as predictor. The skillful regions of the new model cover almost half of the area of East Asian land, and from 1980 to 2005, there are 20 years bearing significant spatial pattern correlation. The careful selection of predictors is the reason of the high skill achieved. By dividing the global ocean into five ocean basins and selecting five regions with large variation of SIC, the CEC prediction based on these regional SSTs or SICs can recognize more regional and weaker forcing from the sea. Moreover, for each SST and SIC region, the optimal period of time leading to the most skillful CCA forecast of EASM rainfall is a special month or combination of two months from winter and spring respectively instead of seasons. The selection of optimal periods of time is the key for the good performance of the CEC prediction by improving the forecast skill of its ensemble members.
This study shows that high resolution rainfall data indicate an increase in number of extreme rainfall events over India in the last few decades. Unravelling the mechanism behind this increasing number of extreme rainfall events in central India is important as these intense rainfall events cause floods and related damage to the life and property of more than 400 million people living in this region. The natural disaster risk hotspots analysis projected the central Indian region as the most vulnerable region for floods in the Indian subcontinent. Here, we present first evidence that the extreme rainfall events in central India in recent decades are strongly modulated by cool sea surface temperature anomalies in the south-eastern equatorial Indian Ocean. The ongoing warming trend of the Indian Ocean coupled with the increase in the number of Indian Ocean Dipole years in recent decades suggests more frequent extreme rainfall events and related hazards in central India in forthcoming years.