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A numerical experiment with a 2-km resolution was conducted using the Weather Research and Forecasting (WRF) Model to investigate physical processes driving nocturnal precipitation over the Himalayas during the mature monsoon seasons between 2003 and 2010. The WRF Model simulations of increases in precipitation twice a day, one in the afternoon and another around midnight, over the Himalayan slopes, and of the single nocturnal peak over the Himalayan foothills were reasonably accurate. To understand the synoptic-scale moisture transport and its local-scale convergence generating the nocturnal precipitation, composite analyses were conducted using the reanalysis dataset and model outputs. In the synoptic scale, moisture transport associated with the westward propagation of low pressure systems was found when nocturnal precipitation dominated over the Himalayan slopes. In contrast, moisture was directly provided from the synoptic-scale monsoon westerlies for nocturnal precipitation over the foothills. The model outputs suggested that precipitation occurred on the mountain ridges in the Himalayas during the afternoon and expanded horizontally toward lower-elevation areas through the night. During the nighttime, the downslope wind was caused by radiative cooling at the surface and was intensified by evaporative cooling by hydrometeors in the near-surface layer. As a result, convergence between the downslope wind and the synoptic-scale flow promoted nocturnal precipitation over the Himalayas and to the south, as well as the moisture convergence by orography and/or synoptic-scale circulation patterns. The nocturnal precipitation over the Himalayas was not simulated well when we used the coarse topographic resolution and the smaller number of vertical layers.

The intensity of extreme precipitation has been projected to increase with increasing air temperature according to the thermodynamic Clausius–Clapeyron (C-C) relation. Over the last decade, observational studies have succeeded in demonstrating the scaling relationship between extreme precipitation and temperature to understand the projected changes. In mid-latitude coastal regions, intense precipitation is strongly influenced by synoptic patterns and a particular characteristic is the long-lasting heavy precipitation driven by abundant moisture transport. However, the effect of synoptic patterns on the scaling relationship remains unclear. Here we conduct an event-based analysis using long-term historical records in Japan, to distinguish extreme precipitation arising from different synoptic patterns. We find that event peak intensity increases more sharply in persistent precipitation events, which lasted more than 10 h, sustained by atmospheric river-like circulation patterns. The long duration-accumulated precipitation extremes also increase with temperature at a rate considerably above the C-C rate at higher temperatures. Our result suggests that long-lasting precipitation events respond more to warming compared with short-duration events. This greatly increases the risks of future floods and landslides in the mid-latitude coastal regions.

Polar mesocyclones (PMCs) occur frequently over the northern Sea of Japan. In the present study, topographic effects on PMC genesis in this region were investigated using long-term numerical simulations extending over 36 winter seasons. Sensitivity experiments showed that PMC genesis decreases in the part of the northern Sea of Japan when the mountain region at the eastern end of the Eurasian continent is removed. For instance, the generation of PMCs over offshore west of Hokkaido decreases significantly when the mountain range is removed, whereas the generation of PMCs over the Strait of Tartary remains unchanged. According to a composite analysis, this result can be attributed to the different responses of subregional oceanic surface wind to the removal of the mountains. In the experiment without mountains, cold air outbreaks from the continent blow directly over the Sea of Japan causing strong westerly winds over the offshore west of Hokkaido. Consequently, PMCs tend to make landfall earlier and before reaching maturity. The uniformly distributed westerly wind also has a negative impact on PMC genesis because of weakened horizontal wind shear and meridional temperature gradient. By contrast, the low-level wind over the Strait of Tartary before PMC genesis is unaffected by the mountains, and thus, topographic effects are not required for PMC genesis in this region. These results indicate that the responses of PMCs to topographic forcing have a regional variability.

Amplified Arctic warming and its relevance to mid-latitude cooling in winter have been intensively studied. Observational evidence has shown strong connections between decreasing sea ice and cooling over the Siberian/East Asian regions. However, the robustness of such connections remains a matter of discussion because modeling studies have shown divergent and controversial results. Here, we report a set of general circulation model experiments specifically designed to extract memory effects of land processes that can amplify sea ice–climate impacts. The results show that sea ice–induced cooling anomalies over the Eurasian continent are memorized in the snow amount and soil temperature fields, and they reemerge in the following winters to enhance negative Arctic Oscillation-like anomalies. The contribution from this memory effect is similar in magnitude to the direct effect of sea ice loss. The results emphasize the essential role of land processes in understanding and evaluating the Arctic–mid-latitude climate linkage., The connection between Arctic sea ice loss and mid-latitude cooling in Eurasia has been widely debated. Here, model experiments reveal that the persistence of sea ice loss-related snow and soil temperature anomalies in Eurasia may lead to further cooling in the following winters.

It is well observed that the monsoon climate experiences substantial climatic changes following explosive volcanism. Likewise, previous studies show that the monsoon climate regimes, especially, the African and South Asian tropical regions, are adversely affected by El Nino-Southern Oscillation (ENSO) events. Hence, studying the sensitivity of the monsoon regions to the effect of these forcing factors, that is, explosive volcanism and volcanic-induced ENSO forcing, is essential for better understanding the driving mechanism and climate variability in these regions. Using observations and a high resolution atmospheric model, effectively at 50- and 25-km grid spacing, this study shows that ENSO and tropical eruptions together weaken the upward branch of Northern Hemisphere (NH) Hadley cell, that is, Intertropical Convergence Zone. This results in a significant decrease of monsoonal precipitation, suggesting severe drought conditions over the NH tropical rain belt regions. The volcanic-induced direct radiative cooling and associated land-sea thermal contrast result in significant warming and drying due to the reduction of clouds over the monsoon regions in boreal summer. The posteruption ENSO circulation also results in warming and drying over NH tropical rain belt regions. This study confirms that the monsoon climate regime responds vigorously to posteruption direct radiative and indirect circulation impacts caused by volcanic-induced ENSO forcing. Hence, quantification of magnitude and spatial pattern of these postvolcanic direct and indirect climatic responses is important for better understanding of climate variability and changes in Asian and African monsoon regions.

This study investigated future changes in monthly extreme precipitation in Japan during summer (June–August). The uncertainties in estimating extreme monthly precipitation were analyzed using large-ensemble regional climate simulations for both present and 4-K warmer climates. The main diagnostics were based on the 100-yr return values of monthly total precipitation PT100 estimated from a best-fit probability distribution. Under the warmer climate, PT100 was projected to increase in approximately 87%, 88%, and 78% of the total number of stations for June, July, and August, respectively, suggesting that once-per-century monthly precipitation will increase as temperature increases over a wide area of Japan, although large regional variations will exist. The western part of Kyushu and the Hokkaido region showed significant and moderately robust increases in PT100 throughout the summer months. In contrast, a considerable and robust increase was projected only in June in the Nansei Islands. The percentage change in PT100 was small in western and eastern Japan, and thus the sign of the change was uncertain. Further analysis indicated that uncertainty in internal variability is more important than uncertainty in the SST scenario for future projections of monthly precipitation extremes.

This study investigated the impact of land surface heterogeneity on Mesoscale Convective System (MCS) initiations in East Asia, using geostationary satellite data during June-August from 1996 through 2018. The detected MCSs over land exhibited clear diurnal variation with the lowest existence frequency at 10:00 and highest initiation frequency during 12:00-17:00 local time. To quantify land surface heterogeneity, the spatial standard deviation of equivalent Black-Body Temperature (TBB) within a cloud-free 0.35 degrees x 0.35 degrees box (sigma LSTBB:Land Surface TBB) was computed for 10:00 each day. A comparison of the sigma LSTBB and MCS databases revealed that the probability of MCS initiations increased with increasing sigma LSTBB in East Mongolia while the probability was not sensitive to sigma LSTBB in East China. This indicated that MCSs tend to form over heterogeneous land surface conditions in the semiarid region. We found that the impact of land surface heterogeneity on MCS initiations was highest over flat terrain in East Mongolia, where the convection trigger due to topographically-induced circulation was absent. These results suggest that the impact of land surface heterogeneity on MCS initiations during the warm season varies with climate zones and terrain complexities in East Asia, with strongest impact in semiarid and flat regions.

Earlier studies suggest that the frequency and the strength of ENSO Modoki events increased considerably since last few decades (1970–2010) resulting in a need to revisit the climatic impacts of varying magnitude of ENSO Modoki. Hence, to better understand the impact of ENSO Modoki amplitude over the tropical and extra-tropical regions, especially in the Pacific, Asian and African regions, we conducted ENSO Modoki sensitivity experiments using ICTP-AGCM (SPEEDY). One of the main interests of this study is to see how ICTP-AGCM can reproduce teleconnections induced by ENSO Modoki events and how sensitive is the global and regional climate to ENSO Modoki strength. SPEEDY model qualitatively reproduces the impact of ENSO Modoki over the Pacific, Atlantic, North and South America and African regions very well. However, it underestimates ENSO Modoki-induced teleconnection patterns and associated changes in South Asia, particularly in the Indian region. This study suggests a nonlinear climatic response to increased magnitude of ENSO Modoki. Our results reveal that like conventional ENSO, ENSO Modoki also induces considerable impact over North Pacific (Atlantic) region and initiates strong PNA (NAO) like response. ENSO Modoki-induced negative/positive NAO-like response and associated changes in Southern Europe and North Africa region get significantly strong following increased intensity of El Nino/La Nina Modoki in the boreal winter. We further find that ENSO Modoki magnitude significantly impacts tropical and high latitude circulation cells. The positive phase of ENSO (El Nino) overall strengthens Hadley Cell and a reverse is true for La Nina phase. ENSO Modoki-induced strengthening and weakening of Hadley Cell induce significant impact over South Asian and African ITCZ convective regions through modification of ITCZ/monsoon circulation system.

Polar mesocyclones (PMCs) are mesoscale, maritime cyclones that occur around the high latitudes in the cold seasons. Over the northern Sea of Japan, PMC frequently occurs with cold air outbreaks from the east of the Eurasian Continent. In this study, effects of the mountains on the eastern end of the Eurasian Continent (Sikhote-Alin mountain range) on the PMCs genesis were examined by 36-years long-term numerical experiments. The sensitivity experiment, in which the Sikhote-Alin mountain range is removed, shows that the number of PMC genesis decreases and the duration between PMCs genesis and landfall becomes shorter compared with realistic experiment. These differences arise only in the southern part of the sea. This result suggests that the effect of the orographic forcing on PMC's behavior varies with the location of the PMCs genesis.

Precipitation features around the Meghalaya Plateau, northeast India, during summer are investigated using a 17-year (1998–2014) high-spatial-resolution TRMM precipitation radar dataset. Precipitation around the plateau fell into two distinct regimes based on the low-level wind direction that fluctuates on intraseasonal timescales over Bangladesh, windward of the plateau: a westerly regime (WR) and an easterly regime (ER). Under the WR, strong low-level onshore southwesterlies across Bangladesh encounter the plateau, and localized strong low-level southerlies running parallel to the Arakan Mountains (i.e., the barrier jet) also blow toward the plateau, concentrating convective unstable air onto its southern slopes. The low-level wind fields and large-scale upper-level divergent fields promote frequent and intense orographic rainfall along the southern slopes due to forced uplift, generating high precipitation. In contrast, under the ER, strong southeasterlies that blow along the Gangetic Plain without encountering the plateau and subsidence inhibit upward motion around the plateau, resulting in low precipitation. Diurnal variations in precipitation significantly affect the daily precipitation around the plateau under both regimes. High rainfall frequency persists over the southern slopes between 2100 and 1200 LT of the next day under the WR, whereas modest rainfall frequency occurs between 0000 and 0600 LT under the ER, with a daytime minimum and nocturnal maximum in both regimes. The atmospheric boundary layer processes over Bangladesh regulate the wind speed and vertical structure of the low-level wind toward the plateau, with deceleration during daytime and acceleration at night (i.e., nocturnal jet) that result in the nocturnal rainfall maximum.

Persistent abnormal hot weather can cause considerable damage to human society and natural environments. In northern Eurasia, the recent change in summer surface air temperature exhibits a heterogeneous pattern with accelerated warming around the Eastern European Plain and Central Siberia, forming a wave train-like structure. However, the key factors that determine the magnitude and spatial distribution of this summer temperature trend remain unclear. Here, a huge ensemble of general circulation model (GCM) simulations show that the recent summer temperature trend has been intensified by two factors: steady warming induced by external forcing and inhomogeneous warming induced by internal atmosphere–land interactions that amplify quasi-stationary waves. The latter is sensitive to both snow cover and soil moisture anomalies in the spring, suggesting the potential of land surface monitoring for better seasonal prediction of summer temperatures. Dramatic changes in the circumpolar environment, characterised by Eurasian snow variation and Arctic Ocean warming, collectively affect summertime climate via memory effects of the land surface.

The Pacific–Japan (PJ) teleconnection pattern leads to a meridional precipitation dipole between the subtropics and East Asia in the summer of El Niño decaying years. However, observational analysis and model experiments indicate that increased sea surface temperature (SST) in the Kuroshio–Oyashio Extension (KOE) region, where a strong northward-decreasing SST gradient occurs, induces a northward shift of baiu rainfall with the subtropical jet, forming a tripolar precipitation anomaly pattern over the northwest Pacific. In July, the leading empirical orthogonal function (EOF) mode for precipitation represents the meridional dipole between subtropical and baiu rainfalls, while EOF-2 features the northward-shifted baiu rainband. The PJ atmospheric forcing increases KOE SST, which enhances northward shifts in the subtropical jet and baiu rainband, resulting in oceanic forcing over the KOE. In experiments using a regional climate model the subtropical jet response to KOE SST forcing explains about 20% of the anomalies in the control experiment. The northward-shifted baiu condensational heating also enhances the northward shift of the subtropical jet with a northeastward extension. In August when the baiu rainband disappears, the northward shift of the subtropical jet is enhanced mainly by KOE SST variability. During the summer, as a result of increased KOE SST and the northward-shifted baiu heating, cyclonic anomalies intensify over the Sea of Okhotsk and anticyclonic anomalies intensify over the northwest Pacific, affecting the western Pacific subtropical high. The results suggest that air–sea interaction over the KOE must be considered to better understand the summer northwest Pacific climate.

The Middle East and North Africa (MENA), primarily the Arabian Peninsula (AP), is a region where the rate of mean surface temperature rise per decade is among the highest globally known during the recent past. Moreover, MENA regional climate is very sensitive to internal and external climate drivers. Therefore, it is of significant practical importance to analyze MENA sensitivity to climate trends as well as leading variability modes such as El Nino Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Indian summer monsoon (ISM). Using multiple regression technique on observations and the high-resolution atmospheric model output, this study investigates the role of climate trends and leading circulation modes such as NAO, ENSO, and ISM in inducing temperature and precipitation variability in MENA region for the period 1979-2008. Our results show substantial regional temperature and precipitation responses of ENSO, NAO, and ISM over MENA. Both the model and the observations indicate that positive phase of NAO and ENSO significantly cools central parts of MENA, in particular, the AP in winter. However, in boreal summer, the warm ENSO phase produces significant warming and drying over the tropical region. The strengthening (weakening) of ISM suggests cooling (warming) and wetting (drying) over MENA rain-belt region. Moreover, ISM induces a dipole precipitation structure over the tropics caused by Intertropical Convergence Zone shift and associated cloud distribution. High-resolution atmospheric model slightly underestimates NAO and ENSO winter cooling over the AP; however, overall patterns are well reproduced. The conducted analysis sheds light on the internal mechanisms of MENA climate variability.

A 30-year meteorological station and reanalysis datasets revealed that the frequency and intensity of hot extremes have increased dramatically around Mongolia since the late 1990s. Many recent heat waves (HWs) were triggered by atmospheric forcing with a mid-tropospheric ridge pattern which appears frequently around Mongolia in 2000s. An observational data analysis for each HW event indicates that the rate of temperature increase at the beginning stage of HW is sensitive to the soil moisture condition. Therefore, it is suggested that the recent drier soil conditions played a role in enhancing the frequency and intensity of HWs and localization around Mongolia.

This study used a 4-km resolution regional climate model to examine the sensitivity of surface air temperature on the Pacific coast of Japan to sea surface temperature (SST) south of the Pacific coast of Japan during summer. The authors performed a control simulation (CTL) driven by reanalysis and observational SST datasets. A series of sensitivity experiments using climatological values from the CTL SST datasets over a 31-yr period was conducted. The interannual variation in surface air temperature over the Pacific coast was well simulated in CTL. The interannual variation in SST over the Kuroshio region amplified the interannual variation in surface air temperature over the Pacific coast. It was found that 30% of the total variance of interannual variation in surface air temperature can be controlled by interannual variation in SST. The calculated surface air temperature on the Pacific coast increased by 0.4 K per 1-K SST warming in the Kuroshio region. Note that this sensitivity was considerably greater during nighttime than during daytime. Concurrent with the warming in surface air temperature, downward longwave radiation at the surface was also increased. In summer, the increase in latent heat flux was considerably larger than that in sensible heat flux over the ocean because of SST warming, according to the temperature dependence of the Bowen ratio. This implies that the primary factor for the increase in surface air temperature in summer is increased moisture in the lower troposphere, indicating that the regional warming was caused by an increase in H2O greenhouse gas.

The influence of historical urbanization on warming trends observed on Hokkaido Island (Japan) is discussed with an emphasis on seasonally and diurnally differential responses of air temperature to urban effects. Two numerical experiments using past and current land use scenarios successfully simulated the observed temperature trends, showing a greater rate of warming in winter than in summer and a greater increase in daily minimum than daily maximum temperatures. The results suggest that seasonal and diurnal variations in the thermal structure of the planetary boundary layer play a leading role in determining the warming rate of surface air. Under strongly stable stratification, anomalous heating within the urban canopy dissipates into a near-surface shallow layer, resulting in increased daily minimum temperatures during winter. In summer, however, anomalous urban heating due to increased Bowen ratios is attenuated by vertical mixing in the convective daytime boundary layer, suppressing the impact of urban heating on surface warming.

The summer western Pacific subtropical high (WPSH) has intensified during the past three decades. However, the underlying mechanism is not yet well understood. Here, it is shown that baiu rainband activity in midsummer, which is part of the East Asian summer monsoon, plays an important role in recent intensification in the WPSH along the baiu rainband. In contrast with the WPSH, the summer Okhotsk high, which is located to the north of the baiu rainband, has weakened during the past three decades. The north–south contrasting changes between the two highs reflect a response to northward-moved and enhanced baiu heating, which intensifies the upper-tropospheric ridge, resulting in the baroclinic intensification of the WPSH. Regional climate model experiments also support the observational analysis. Therefore, baiu convective activity in midsummer can act as a major driver for the WPSH intensification. The results here suggest that the mechanism intensifying the summer North Pacific subtropical high clearly differs between the western and eastern Pacific.

The experiments with three general circulation models (GCMs) by three regional atmospheric models (RAMs) for the dynamical downscaling (DDS) have been performed to evaluate the uncertainty in the global warming response during summertime in Hokkaido, Japan. The results of a 10-year RAM integration nested into GCM under present or future climate conditions were synthesized after applying bias correction. For the target decades during which the global-mean temperature increases by 2 K in each GCM, the DDS results indicate that surface air temperature and precipitation mostly depend on the GCM imposed as the lateral boundary condition.

This study investigated the spatiotemporal characteristics of high-temperature events in Hokkaido, Japan, using observational data of 26 years. Statistical analyses revealed that the annual mean frequency of these events was lower (higher) at stations on the western (eastern) side of Hokkaido. The frequency of these events showed clear seasonal variation with two distinct peaks occurring in January and May. In addition, the local time of the high-temperature onset was strongly dependent on the season; the onset occurred more frequently from 1600 to 0400 Japan Standard Time (JST) in January and from 0700 to 1300 JST in May. The seasonal dependence mechanism of the high-temperature onset was investigated in eastern Hokkaido, where the frequencies of both January and May high-temperature events were the highest. In January, an extratropical cyclone passage caused intensified warm advection and increased precipitable water vapor, leading to weakened radiative cooling during the night. In May, the high-temperature events were triggered by two different mechanisms related to solar insolation. The first mechanism is explained by dynamic foehn, which forms the subsidence of the high potential temperature layer on the lee of mountains. However, the nocturnal inversion layer prevented vertical mixing of the foehn-induced warm air aloft and cold air near the ground. The surface air temperature dramatically increased after sunrise when the nocturnal inversion layer disappeared. The second mechanism is explained by the combination of airflow diabatically heated by surface sensible heat flux and dynamic foehn. Therefore, solar insolation is the key factor that controlled the diurnal variation in high-temperature events in May.

[1] The role of sea surface temperature (SST) anomaly in modulating the terrestrial precipitation in winter around Japan was investigated using a regional atmospheric model. Large amount of snowfall occurs as the westerly winter monsoon carries abundant moisture from the Japan Sea. An experiment with realistic SST gives improved representation of terrestrial precipitation distribution compared to the reanalysis. The standard deviation of interannual variation of precipitation in the experiment was approximately 20% larger over the Kuroshio extension than that in the experiment with climatology SST, suggesting that the SST variability enhances the oceanic precipitation variability. For the role of Japan Sea, the terrestrial precipitation over the Japan Sea side (JSS) region in northern Japan was sensitive to the offshore SST anomaly through affecting moisture flux toward Japan. Since the offshore SST was clearly warmer in the 2000s relative to the 1980s, the effect of the long-term SST variation on the terrestrial precipitation trend was examined. The experiment with realistic SST simulated the observed trend in terrestrial precipitation in the JSS region. In contrast, the precipitation trend was significantly reduced in the experiment with climatology SST. Therefore, the long-term SST trend is an important factor for the precipitation trend in the region of Japan and the adjacent oceans where SST has significant trends. Precipitation in southern Japan facing the Pacific Ocean indicated increasing trend even without the SST trend. This suggests that the long-term variations in extratropical cyclones are also an important factor for precipitation trends around the Kuroshio extension.

Mesoscale processes that cause tremendous amounts of precipitation around the Meghalaya Plateau, northeast India, were investigated using a regional atmospheric model, with a particular focus on the possible role of an intraseasonal oscillation (ISO) and the diurnal cycle. A numerical experiment was conducted using the Weather Research and Forecasting Model (WRF) to simulate a prominent submonthly-scale ISO event observed during June–July 2004. A WRF experiment successfully simulated the timing and magnitude of precipitation during the first active period and subsequent inactive period of the ISO, despite large biases in the second active period. The WRF experiment revealed that the magnitude of prevailing southwesterly winds in the lower troposphere differs significantly between active and inactive periods; strong southwesterly flow transports abundant moisture to the southern slope of the Meghalaya Plateau during the active period, which triggers forced lifting and brings orographic precipitation. However, low-level wind during the inactive period is too weak to overcome the vertical stratification barrier; hence, it is unable to reach the condensation level. The WRF experiment suggested that the diurnal cycle of precipitation is enhanced during the active period, exhibiting an evening to early morning maximum, as observed by the Tropical Rainfall Measuring Mission (TRMM). The experiment also indicated a prominent nocturnal low-level jet (LLJ) appearing at 900 hPa. The direction of the LLJ varies clockwise with time, which accelerates the preexisting southwesterly flow during 1800–0600 LT. The diurnal variation of the LLJ is responsible for the evening to early morning maximum of precipitation, thus contributing to the precipitation variability in the ISO.

Spatial patterns of future atmospheric warming presented by climate model projections indicate that the temperatures of the lower troposphere will increase markedly at high latitudes in the Northern Hemisphere and also the upper troposphere at low-middle latitudes. In this study, potential impacts of the spatial patterns of future atmospheric warming on Asian dust emission were investigated by performing numerical experiments using a dynamical downscaling method. After three significant Asian dust outbreak events were reproduced as control runs, initial and boundary conditions of the control runs were updated by adding differences in atmospheric variables between the future climate (2091–2100) and the recent climate (1991–2000) to execute pseudo global warming runs. The results showed that the dust emission fluxes in the main Asian dust sources (MADSs), i.e., the Taklimakan and Gobi deserts, decrease markedly in the future climate condition. The future decreases in the dust emission fluxes are likely to be caused by a combination of the relatively large increases in sea level pressures (SLPs) in MADSs and the small SLP changes to the north of MADSs, which reduces the meridional SLP gradients between the two areas and consequently weakens cold air outbreaks. The large SLP increases in MADSs may be due to a northward shift of storm tracks and increased atmospheric stabilities caused by the large upper-tropospheric warming at low-middle latitudes. The small SLP changes to the north of MADSs corresponded to the large increases in surface air temperatures, which would be influenced by the large lower-tropospheric warming at high latitudes.

The downscaling framework, which is originally proposed by Emanuel et al. (2008), is modified to improve the interannual variation of the tropical cyclone features in Western North Pacific (WNP). The original framework, which utilizes axisymmetric model driven using monthly mean atmosphere and ocean variables, well reproduced interannual variation of tropical cyclone indices over North Atlantic while there is room for further improvement over WNP. The modified treatment of relative vorticity plays an essential role for improving interannual variation of genesis counts and power dissipation index in WNP. In this update, we introduced probability of cyclogenesis special to WNP which is a priori calculated using monthly mean 850-hPa level relative vorticity and best track. The result suggests that the monthly mean relative vorticity represents a potential of characteristic initiating disturbance in WNP such as monsoon trough and intraseasonal variation. Another experiment shows increased influence of vertical wind shear in middle troposphere improves the distribution of tropical cyclogenesis in WNP while it slightly deteriorates the interannual variation of genesis count.

Global weather forecasts do not have sufficient performance to predict the local severe weather events that are accompanied with cyclones and cold fronts due to their coarse horizontal resolution. This study investigated the performance of dynamical downscaling (DD) using mesoscale model to simulate the severe windstorm in eastern Mongolia which occurred on 26-27 May 2008.Our results revealed that the DD experiments were successful in capturing the general features of the windstorm in terms of wind and temperature patterns. The timing and amplitude of drastic changes in the simulated temperature and wind speed were very similar to that observed than that obtained from the global atmospheric data, suggesting that DD is capable of predicting extreme wind storm events in Mongolia. Analyses on the nested domains indicate that the DD has crucial impact on the performance for simulating severe storm even with a moderate resolution (27 km), and further nesting (9 and 3 km) plays a role to improve it. Furthermore, the maximum wind speed approaches the observed value more closely as the horizontal resolution increases, although it still underestimates the observed wind speed even in the 3 km mesh domain. On the other hand, the abrupt temperature change is captured well even in the low-resolution domain, suggesting a difference in necessary horizontal resolution for temperature change and maximum wind speed.

This study analyzes the diurnal cycle of precipitation simulated in a global cloud-resolving model (GCRM) named the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). A 30-day integration of NICAM successfully simulates the precipitation diurnal cycle associated with the land–sea breeze and the thermally induced topographic circulations as well as the horizontal propagation of diurnal cycle signals. The first harmonic of the diurnal cycle of precipitation in the 7-km run agrees well with that from satellite observations in its geographical distributions although its amplitude is slightly overestimated. The NICAM simulation revealed that the precipitation diurnal cycle over the Maritime Continent is strongly coupled with the land–sea breeze that controls the convergence/divergence pattern in the lower troposphere around the islands. The analysis also suggests that the cold pool often forms over the open ocean where the precipitation intensity is high, and the propagation of the cold pool events is related to the precipitation diurnal cycle as well as the land–sea breeze. Sensitivity experiments suggest a prominent horizontal resolution dependence of the simulated precipitation diurnal cycle. Over continental areas the 14-km run induces the diurnal peak about three hours later than the 7-km run. The 3.5-km run produces the peak time and amplitude that are very similar to those in Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) observations. Meanwhile, the resolution dependence in phase and amplitude is negligibly small over the open oceans. This contrast sensitivity to the horizontal resolution is attributed to the differences in structure and life cycle of convective systems over land and ocean. Diurnal peaks of precipitable water vapor, precipitation, and outgoing longwave radiation (OLR) are compared over land areas using the NICAM 7-km run. The daily precipitable water vapor maximum appears around 1500 local time (LT), which is followed by the precipitation peak around 1630 LT. The diurnal cycle of high clouds tends to peak around 1930 LT, three hours later than the precipitation peak. These results from NICAM simulations can explain the cause of the phase differences among precipitation products based on several satellite observations. The authors demonstrate that the GCRM is a promising tool for realistically simulating the precipitation diurnal cycle and could be quite useful for studying the role of the diurnal cycle in the climate systems in a global context.

We synthesize palaeoclimate records from the mid-latitude arid Asian region dominated today by the Westerlies ("arid central Asia" (ACA)) to evaluate spatial and temporal patterns of moisture changes during the Holocene. Sediment records from I I takes with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five-class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200-year time slices. The proxies used in these records include pollen and diatom assemblages, sediment lithology, lake levels, and geochemistry (mainly isotope) data. The results of our synthesis show that ACA as a whole experienced synchronous and coherent moisture changes during the Holocene, namely a dry early Holocene, a wetter (less dry) early to mid-Holocene, and a moderately wet late Holocene. During the early Holocene most of the lakes experienced very low water levels and even dried out before ca 8 ka (1 ka = 1000cal a BP). Hence the effective-moisture history in ACA is out-of-phase with that in monsoonal Asia as documented by numerous palaeoclimate records. In monsoonal Asia, a strong summer monsoon and humid climate characterized the early Holocene, and a weakened summer monsoon and drier climate prevailed during the late Holocene, which were mainly controlled by changes in low-latitude summer insolation. In contrast, we propose that the pattern of Holocene effective-moisture evolution in the westerly dominated ACA was mainly determined by North Atlantic sea-surface temperatures (SSTs) and high-latitude air temperatures that affect the availability, amount and transport of water vapor. Also, topography of the Tibetan Plateau and adjacent Asian highlands could have contributed to the intensification of dry climate in ACA during the early Holocene, as a result of strengthening the subsidence of dry air masses, associated with stronger uplift motion on the plateau by intense heating under a stronger summer insolation. Summer insolation might have played a key role in directly controlling moisture conditions in ACA but only after the northern hemisphere ice-sheets had disappeared in the mid- and late Holocene. (C) 2007 Elsevier Ltd. All rights reserved.

Sensitivity of precipitation diurnal cycle to the horizontal grid spacing was investigated using a mesoscale model without cumulus parameterization. Four numerical experiments with changing horizontal resolution are performed over one month with NCEP reanalysis boundary forcing. The studied range of grid spacing is from 3.5 km to 28 km which has been known as the intermediate scale to simulate mesoscale systems with/without cumulus parameterization. The target area is the Tibetan Plateau where pronounced diurnal cycle of convective systems is observed during the spring season.Lower resolution runs (14 and 28 km grid) show delayed formation and delayed mature stage of the cumulus convection in comparison to satellite observations. On the other hand, higher resolution runs (3.5 and 7 km grid) reproduce the proper development of the clouds after local noon which is consistent with observations. The total hydrometeor content and rainfall rate increase with grid size. Such systematic relationship of resolution dependencies are confirmed even in the monthly mean diurnal cycle, although most of previous studies examined only short periods. These results suggest that finer resolution at less than 7 kilometer is necessary to simulate realistic phase of the precipitation diurnal cycle over the Tibetan Plateau.The mechanism that is responsible for the resolution dependency is discussed. We suggest that the daytime convection which initially occurs due to unstable stratification over the Tibetan Plateau in spring tends to have a horizontal scale smaller than that is resolvable by the coarse resolution runs. The delayed cloud formation induces larger downward shortwave radiation, which increases surface fluxes and results in too strong rainfall in the coarser resolution runs.

The diurnal variations of water vapor in central Japan were investigated with GPS-derived precipitable water (PWV) and surface meteorological data as classified to three kinds of locations. Twenty-five clear days in central Japan in August 2000 were investigated to clarify the role of water vapor in the nocturnal maximum in the diurnal cycle of convective rainfall. The diurnal variations of PWV and some meteorological factors were composite during the selected days at 6 stations. The PWV shows a clear diurnal cycle with the amplitude of 3.4 mm to 8.8 mm and changes little during the period from the morning to noon. The daily amplitude of PWV is the largest in basin and smallest in plain although mean of PWV keeps high value in plain. A typical feature of the diurnal variation in central Japan is a maximum appearing in the evening. The time of maximum is from 1800 LST to 2000 LST, and minima appears at noon nearly in mountainous area and basin, while in early morning in plain. The diurnal maximum of PWV appears earlier in mountainous region than in plain.

The roles of the Tibetan Plateau (TP) upon the transition of precipitation in the south Asian summer monsoon are investigated using a simplified regional climate model. Before the onset of the south Asian monsoon, descending flow in the midtroposphere, which can be considered as a suppressor against precipitation, prevails over northern India as revealed by the NCEP–NCAR reanalysis data. The descending motion gradually weakens and retreats from this region before July, consistent with the northwestward migration of the monsoon rainfall. To examine a hypothesis that the dynamical and thermal effects of TP cause the midtropospheric subsidence and its seasonal variation, a series of numerical experiments are conducted using a simplified regional climate model. The mechanical effect of the TP generates robust descending flow over northern India during winter and spring when the zonal westerly flow is relatively strong, but the effect becomes weaker after April as the westerly flow tends to be weaker. The thermal effect of the TP, contrastingly, enhances the descending flow over north India in the premonsoonal season. The descending flow enhanced by the thermal effect of the TP has a seasonal cycle because the global-scale upper-level westerly changes the energy propagation of the thermal forcing response. The subsidence formed by the mechanical and thermal effects of the TP disappears over northern India after the subtropical westerly shifts north of the plateau, the seasonal change of which is in good agreement with that in the reanalysis data. The retreat of the descending flow can be regarded as the withdrawal of the premonsoon season and the commencement of the south Asian monsoon. After that, the deep convection, indicating the onset of the Indian summer monsoon, is able to develop over north India in relation to the ocean–atmosphere and land–atmosphere interaction processes. Northwest India is known to be the latest region of summer monsoon onset in south Asia. Thus, the thermal and mechanical forcing of the TP has great impact on the transition of the Indian monsoon rainfall by changing the midtropospheric circulation.

Summary Climate change due to global warming is of concern to the public and may cause significant changes in the hydrological regimes in arid/semi-arid areas including Mongolia, which locates at a boundary between arid and humid regions. However, general circulation models (GCMs) are not sufficient to evaluate climate change on a regional-scale. In this study, two kinds of dynamical downscaling (DDS), referred to as method-G and method-R, using a regional climate model (RCM) are applied to investigate the rainfall change over Mongolia in July due to the global warming. Method-G is a traditional DDS method in which an RCM is directly nested within a GCM, while method-R is newly suggested in this study and aims to improve the reproductivity of a regional climate. For current climate simulation, method-R uses reanalysis data as a boundary forcing of the RCM while a specially created boundary condition, in which projected changes of meteorological variables in a GCM simulation are added on reanalysis data, is used for global warming simulation. Compared with in situ observations, the rainfall amount for July is very well reproduced by the RCM, even in a smaller area of four subregions in Mongolia. Rainfall intensity by method-R is very close to actual observations; on the other hand, method-G fails to simulate heavy rainfall events stronger than 16 mm day −1 . The two DDS methods show similar results with respect to the changes of precipitation in July due to the global warming, which are that precipitation decreases over northern and increases over southern Mongolia. In method-R, a decrease of precipitation of middle to heavy rainfall intensity, stronger than 4 mm day −1 , contributes largely to the decreased July precipitation in northern Mongolia. Soil moisture over Mongolia also tends to decrease in July because of the combined effect caused by the decrease of precipitation and the increase of potential evaporation due to rising air temperature. This situation indicates that severe droughts may occur more frequently from the effects of global warming.

Convective rainfall often shows a clear diurnal cycle. The nighttime peak of convective activity prevails in various regions near the world's mountains. The influence of the water vapor and convective instability upon nocturnal precipitation is investigated using a numerical model and observed data. Recent developments in GPS meteorology allow the estimation of precipitable water vapor (PWV) with a high temporal resolution. A dense network has been established in Japan. The GPS analysis in August 2000 provides the following results: In the early evening, a high-GPS-PWV region forms over mountainous areas because of the convergence of low-level moisture, which gradually propagates toward the adjacent plain before midnight. A region of convection propagates simultaneously eastward into the plain. The precipitating frequency correlates fairly well with the GPS-PWV and attains a maximum value at night over the plain. The model also provides similar characteristics in the diurnal cycles of rainfall and high PWV. Abundant moisture accumulates over the mountainous areas in the afternoon and then advects continuously toward the plain by the ambient wind. The specific humidity greatly increases at about the 800-hPa level over the plain at night, and the PWV reaches its nocturnal maximum. The increase in the specific humidity causes an increase of equivalent potential temperature at about the 800-hPa level; as a result, the convective instability index becomes more unstable over the plain at night. These findings are consistent with the diurnal cycle of the observed precipitating frequency.

The role of mountain ranges upon the arid climate formation in northwestern China is investigated using regional climate model. Two kinds of numerical experiments are carried out with real topography and with modified topography which does not contain the TianShan Mountains. Simulated rainfall distribution with real topography shows maximum precipitation in upwind slope of the mountain and minimum precipitation in downwind side of it, which coincides with the features of rain-shadow in other regions of the world. However, numerical experiment without the TianShan Mountains also simulates the area of distinct minimum precipitation in the same location. The arid climate, therefore, is formed even though the rain shadow effect is absent. The difference in rainfall amount between these two experiments is very small around downwind arid region. On the other hand, more intense and frequent rainfall events tend to occur in upwind of the mountain range due to forced lifting of environmental flow, which just emphasizes the precipitation contrast between upwind and downwind side of the mountain. The precipitation accompanied with the synoptic scale disturbances is suppressed over the arid region whether the TianShan Mountains exist or not.

Simple numerical experiments using a two-dimensional model were conducted to investigate the diurnal variation of water vapor in the lee of a mountain range, which strongly affects nighttime precipitation. In the daytime, moisture increases around the convergence zone in the lee of the mountain were formed by thermally induced local circulations in moderate ambient wind. Then, a region of enhanced precipitable water vapor (PWV) was formed. The region of enhanced PWV was transported to the lee side of the mountain by the ambient wind and finally reached the plain during the night. The convective instability in the lower troposphere and the possibility of deep convection were controlled by the advection of water vapor in the lee of the mountain, especially by the position of the enhanced PWV region. As a result, the diurnal cycle of the convective instability showed a phase difference between the mountainous region and the plain. The profile of equivalent potential temperature tended to be unstable in the lee of the mountain at night and contributed to the production of nighttime precipitation. The propagation speed of the enhanced PWV region was roughly determined by the ambient wind speed. Some sensitivity experiments using bulk microphysics have suggested that the propagation speed of the enhanced PWV region is insensitive to the occurrence of a deep convection. The enhanced PWV region generated over a wider mountain range tends to travel farther, retaining its magnitude even until night. The water vapor transport process presented in this study agrees well with the actual diurnal variations of rainfall observed in many regions over land.

Large topographic features, like the Tibetan Plateau (TP) and the Rocky Mountains, have significant impacts on Earth's climate. Numerical experiments were carried out using a regional climate model in order to study the sensitivity of rainfall distribution to the TP's thermal/dynamic effects and meridional position of the subtropical westerly jet. The sensitivity experiments under the present winter conditions, characterized by weaker solar radiation and westerly jet located to the south of the TP, reveal that broad and climate characterizes Northeast Asia, covering North China and Mongolia. The results are consistent with previous studies, indicating that TP dynamic forcing is dominant in winter. In sensitivity experiments under present summer conditions, characterized by stronger solar radiation and the westerly jet located to the north of TP, a prominent subsidence occurred over North China, where the Taklimakan and Gobi deserts are located. In summer, dynamic forcing of TP is very weak, whereas the thermal forcing of TP mainly induces subsidence. Relatively smaller mountain ranges, such as the TianShan Mountains, also modify the atmospheric circulation even during the summer season. The active convection zone along the Baiu front, which produces an elongate rain belt that extends from East China to Korea and Japan, is near the boundary of two moisture sources, transported by the westerly wind over the Eurasian continent and by the Asian summer monsoon flow. The millennial-scale variation in the location of the boundary reflects corresponding changes in East Asian climate, including monsoon intensity, storm activity, and the subtropical jet. (C) 2008 Elsevier Ltd and INQUA. All rights reserved.

[1] Origin of water vapor, which falls as precipitation over arid/semiarid area in northeast Asia, is investigated by modeling and observational approaches. The regional climate model evaluates the spatiotemporal variations of precipitation and water vapor budget, which are used to drive the single-layer isotope circulation model. Intraseasonal variations of δ18O in precipitation are well simulated during June-July-August of 2003. The δ18O in water vapor experiences rapid decreases according to the passage of synoptic-scale disturbances. The rapid decreases of δ18O are attributed to two processes. (1) The δ18O in air mass locally decreases over the western mountains associated with the mountain precipitation; the light vapor is advected to eastern Mongolia by the prevailing westerly wind. (2) Convective systems pass by in the vicinity of the observation site, which persistently keep the lower δ18O in the air mass. Origin of the water vapor during June-July-August in 2003 is evaluated by the colored moisture analysis in which the tracers are assigned depending on the region where the water vapor finally evaporated. Seasonal mean result indicates that the regions contributing to precipitation in Mongolia are not low-latitude regions but central Asia and western Siberia located to the northwest of Mongolia. Observed multilevel isotopic composition supports the model estimation. The moisture transport along the southwesterly wind of the Asian summer monsoon has difficulty reaching Mongolia as a monthly/seasonal mean perspective. However, eastern Mongolia and northeast China are situated on the border area between westerly wind moisture transport (by midlatitude synoptic cyclones) and southerly wind moisture transport (by Asian summer monsoon).

[1] The formation mechanism of the arid climate over northeast Asia is investigated by a series of regional climate model experiments. Precipitation there is suppressed by a prominent regional-scale subsidence during summertime. An experiment without diabatic heating shows very gentle vertical motion there, whereas the subsidence is prominent when diabatic heating by solar radiation is included in the model. Additionally, the other experiments indicate that an artificial heating over the Tibetan Plateau enhances the subsidence over the arid area which is similar to the observed distribution. These results suggest that the thermal effects of the plateau induce the prominent subsidence. The mechanical effects of the topography, such as stationary mountain waves, are not a principal reason for the subsidence nor the arid climate. The stationary Rossby wave driven by the diabatic heating over the plateau tends to propagate northward during boreal summer, since subtropical westerly jet locates north of the plateau.

[1] By the numerical experiments, we clarify the regional wind systems that generate dust storms in the Tarim basin, Northwest China. In the basin, dust storms are generated by a mesoscale cold wind system induced by a synoptic-scale cold air mass behind a cold front. The intruding course of the mesoscale cold wind into the basin can be classified into three patterns. The first pattern is characterized by the easterly mesoscale cold wind, which is separated from the synoptic-scale cold westerly and turns westward after going around the eastern side of the TianShan Mountains. The mesoscale cold wind categorized in the second pattern directly crosses over the TianShan Mountains, which often gives a very strong wind in the basin with the largest dust storms in the three patterns. The third pattern indicates that the cold wind crosses over Pamir plateau, although the magnitude of the dust storm is usually small.