Impact of MJO on the diurnal cycle of rainfall over the western Maritime Continent in the austral summer

This paper investigates the impact of the Madden-Julian Oscillation (MJO) on the diurnal cycle of rainfall over the western Maritime Continent during the austral summer. For this purpose, cyclostationary empirical orthogonal function analysis is applied to the tropical rainfall measuring mission rain rate and the Japanese Reanalysis-25 data for the period 1998–2008. The real-time multivariate MJO index by Wheeler and Hendon (Mon Wea Rev 132:1917–1932, 2004) is adopted to define the intensity and the phase of MJO. It is demonstrated that the hourly maximum rain rate over the domain tends to increase when convectively active phase of MJO approaches the Maritime Continent. In contrast, the hourly maximum rain rate tends to decrease when convectively suppressed phase of MJO resides over the region. The changes in the rain rate due to MJO differ over the ocean and the land. This difference is the greatest when the MJO is in the mature stage. Throughout the day during this stage, terrestrial rain rates show minimum values while diurnally varying oceanic rain rates record maximum values. Thus, precipitation becomes more intense in the morning over the Java Sea and is weakened in the evening over Borneo and Sumatra during the mature stage of MJO. During the decaying stage of MJO over the Maritime Continent, the diurnal cycle of precipitation weakens significantly over the ocean but only weakly over land. Analyses suggest that the anomalous lower level winds accompanied by MJO interact with the monsoonal flow over the Maritime Continent. Westerlies induced by MJO convection in the mature stage are superimposed on the monsoonal westerlies over the equator and increase wind speed mainly over the Java Sea due to the blocking effect of orography. Mountainous islands induce flow bifurcation, causing near-surface winds to converge mainly over the oceanic channels between two islands. As a result, heat flux release from the ocean to the atmosphere is enhanced by the increased surface wind resulting in instability as described in the wind-induced surface heat exchange mechanism. This may contribute to heavy rainfall over the Java Sea in the morning during the mature stage. On the other hand, convergence and vertical velocity over the islands, which play important roles in inducing nighttime rainfall, tend to be weak in the evening during the mature stage of MJO. Strong westerlies arising from MJO and the seasonal flow during the mature stage tend to interrupt convergence over islands. This interruption of convergence by MJO gives rise to decreased rain rates over the land regions.     

Seasonal Prediction of Boreal Winter Rainfall over the Western Maritime Continent during ENSO

Since the beginning of the Association of Southeast Asian Nations Climate Outlook Forum(ASEANCOF)in 2013,the most difficult challenge has been the rainfall forecast in boreal winter.This is the Maritime Continent monsoon season during which rainfall reaches maximum in the annual cycle.This forecast difficulty arises in spite of the general notion that seasonal predictability of the Maritime Continent rainfall may be higher than most places because of the strong and robust influences of ENSO.The lower predictability is consistent with the lower correlation between ENSO and western Maritime Continent rainfall that reaches minimum during the boreal winter monsoon.Various theories have been proposed to explain this low correlation.In this paper,we review the research on ENSO–Maritime Continent rainfall relationship and show that the main cause of the forecast difficulty is the wind–terrain interaction involving the Sumatran and Malay Peninsula mountains,rather than the effect of sea surface temperature(SST).The wind–terrain interaction due to the low-level regional scale anomalous horizontal circulation offsets the anomalous Walker circulation during both El Ni?o and La Ni?a.The net result of these two opposing responses to ENSO is a lower local predictability.We propose to call this low-predictability region the WIMP(Western Indonesia–Malay Peninsula)region both for its geographical location and its special characteristic of causing difficulties for forecasters to make a confident forecast for the boreal winter.Our result suggests that climate models lack skills in forecasting rainfall in this region because their predictability depends strongly on SST.     

Climate Variability over the Maritime Continent and Its Role in Global Climate Variation: A Review

The Maritime Continent(MC) consists of multiple islands with varying sizes and topography, and surrounding seas. It is characterized by rainfall(convection) variability on multiple spatial and temporal scales. Various largescale atmospheric, oceanic, and coupled climate systems, such as the El Ni?o–Southern Oscillation(ENSO), Indian Ocean Dipole(IOD), Madden–Julian Oscillation(MJO), and cold surge, exert significant influences on the spatiotemporal complexity of the MC climate and climate variability. As a major tropical heat source located within the warmest oceanic area(the western Pacific warm pool), the MC has been identified as a region of great importance for climate variation on the global scale. However, prediction of climate variability over the MC and its surrounding areas and the relationships to large-scale atmospheric circulation patterns are big challenges, even for state-of-the-art climate models. In this paper, we provide a thorough review on current understanding of the spatiotemporal complexity and prediction of climate variability over this important region, and its influence on global climate variation.     

The sensitivity of the tropical circulation and Maritime Continent precipitation to climate model resolution

The dependence of the annual mean tropical precipitation on horizontal resolution is investigated in the atmospheric version of the Hadley Centre General Environment Model. Reducing the grid spacing from about 350 km to about 110 km improves the precipitation distribution in most of the tropics. In particular, characteristic dry biases over South and Southeast Asia including the Maritime Continent as well as wet biases over the western tropical oceans are reduced. The annual-mean precipitation bias is reduced by about one third over the Maritime Continent and the neighbouring ocean basins associated with it via the Walker circulation. Sensitivity experiments show that much of the improvement with resolution in the Maritime Continent region is due to the specification of better resolved surface boundary conditions (land fraction, soil and vegetation parameters) at the higher resolution. It is shown that in particular the formulation of the coastal tiling scheme may cause resolution sensitivity of the mean simulated climate. The improvement in the tropical mean precipitation in this region is not primarily associated with the better representation of orography at the higher resolution, nor with changes in the eddy transport of moisture. Sizeable sensitivity to changes in the surface fields may be one of the reasons for the large variation of the mean tropical precipitation distribution seen across climate models.     

Interannual variability of summertime outgoing longwave radiation over the Maritime Continent in relation to East Asian summer monsoon anomalies

The Maritime Continent (MC) is under influences of both the tropical Pacific and the Indian Ocean. Anomalous convective activities over the MC have significant impacts on the East Asian summer monsoon (EASM) and climate in China. In the present study, the variation in convective activity over the MC in boreal summer and its relationship to EASM anomalies are investigated based on regression analysis of NCEP–NCAR reanalysis and CMAP [Climate Prediction Center (CPC) Merged Analysis of Precipitation] data, with a focus on the impacts of ENSO and the Indian Ocean Dipole (IOD). The most significant interannual variability of convective activity is found over 10°S–10°N, 95°–145°E, which can be roughly defined as the key area of the MC (hereafter, KMC). Outgoing longwave radiation anomaly (OLRA) exhibits 3- to 7-yr periodicities over the KMC, and around 70% of the OLRA variance can be explained by the ENSO signal. However, distinct convection and precipitation anomalies still exist over this region after the ENSO and IOD signals are removed. Abnormally low precipitation always corresponds to positive OLRA over the KMC when negative diabatic heating anomalies and anomalous cooling of the atmospheric column lead to abnormal descending motion over this region. Correspondingly, abnormal divergence occurs in the lower troposphere while convergence occurs in the upper troposphere, triggering an East Asia–Pacific/Pacific–Japan (EAP/PJ)-like anomalous wave train that propagates northeastward and leads to a significant positive precipitation anomaly from the Yangtze River valley in China to the islands of Japan. This EAP/PJ-like wave pattern becomes even clearer after the removal of the ENSO signal and the combined effects of ENSO and IOD, suggesting that convective anomalies over the KMC have an important impact on EASM anomalies. The above results provide important clues for the prediction of EASM anomalies and associated summer precipitation anomalies in China.     

Study of a high SO2 event observed over an urban site in western India

Continuous measurements of SO₂, NO_x and O₃ along with sampling based measurements of CO, CH₄, NMHCs and CO₂ were carried out during May, 2010 at Ahmedabad. The diurnal variations of SO₂ in ambient air exhibited elevated values during the night and lower levels during the sunlit hours. The mean concentration of SO₂ during the study period was 0.95 ± 0.88 ppbv. However, the ambient SO₂ exceeded 17 ppbv in the night of 20 May, 2010. On the same day, tropospheric columnar SO₂ from OMI showed almost 350% increase corroborating the surface observations over an extended height regime. This was also the highest columnar value of SO₂ during the summer of 2010. Columnar loadings were also found to be high for formaldehyde, precipitable water vapor and aerosol optical depth on 20 May. Elevated concentrations were also recorded for other trace gases like NO₂ and O₃. Analysis of related data of trace gases indicated characteristics of fresh emissions with dominant contributions from mobile sources during the study period. However, SO₂/NO₂ ratio of 0.36 during the event period on 20th May connotes non-local influences. Analyses of meteorological parameters suggest combined impacts of transport and inversion causing higher levels of SO₂ and other pollutants during 20?C21 May. Episodes of such enhancements may perturb chemical and radiative balance of the atmosphere.     

Tropical synoptic-scale wave disturbances over the western Pacific simulated by a global cloud-system resolving model

Lower-tropospheric tropical synoptic-scale disturbances (TSDs) are associated with severe weather systems in the Asian Monsoon region. Therefore, exact prediction of the development and behavior of TSDs using atmospheric general circulation models is expected to improve weather forecasting for this region. Recent state-of-the art global cloud-system resolving modeling approaches using a Nonhydrostatic Icosahedral Atmospheric Model (NICAM) may improve representation of TSDs. This study evaluates TSDs over the western Pacific in output from an Atmospheric Model Intercomparison Project (AMIP)-like control experiment using NICAM. Data analysis compared the simulated and observed fields. NICAM successfully simulates the average activity, three-dimensional structures, and characteristics of the TSDs during the Northern summer. The variance statistics and spectral analysis showed that the average activity of the simulated TSDs over the western Pacific during Northern summer broadly captures that of observations. The composite analysis revealed that the structures of the simulated TSDs resemble the observed TSDs to a large degree. The simulated TSDs exhibited a typical southeast- to northwest-oriented wave-train pattern that propagates northwestward from near the equator around 150 ^° E toward the southern coast of China. However, the location of the simulated wave train and wave activity center was displaced northward by approximately a few degrees of latitude from that in the observation. This displacement can be attributed to the structure and strength of the background basic flow in the simulated fields. Better representation of the background basic states is required for more successful simulation of TSDs.     

Simulation of snowstorm over the Yellow Sea using a mesoscale coupled model

This study aims to examine the favorable conditions for an ocean effect snowstorm across the Yellow Sea over the southwestern coast of Korea on 21 December 2005, using a coupled model with a Coupled Ocean/Atmosphere Mesoscale Prediction System as the atmospheric component and the Regional Ocean Modeling System as the oceanic component. Simulation of heavy snowfall event, which was 44.3 cm of snow accumulated in 24-hour, was performed to investigate the mesoscale structure, dynamics and development mechanisms in the snowstorm. As a result from 48-hour integration, the results of simulation showed that barotropic instability and turbulent heat fluxes played important roles in the formation of snowstorm. The enhanced surface diabatic heating was dominant in the latent heat flux, and eventually induced convective instability. An additional factor was the favorable condition of synoptic environment, accessing the cold air transport by the approach of the upper-level cold vortex over the warm ocean. Besides these factors, conditional symmetric instability (CSI) is a mechanism which can result in a heavy snowfall with sufficient moisture and upward vertical motion. A slantwise convection from the release of CSI could support a complex snowfall event with heavier than expected amounts. The result comparison between a coupled model and an uncoupled model supports that airsea coupling has an impact of decreasing of about 10% in a snowfall amount on the snowstorm.     

Regional climate simulations over Vietnam using the WRF model

Time-irreversible symmetry is a fundamental property of nonlinear time series. Time-irreversible behaviors of mean temperature measured on 182 stations over China from 1960 to 2012 are analyzed by directed horizontal visibility graph (DHVG for short), and significance of results has been estimated by Monte Carlo simulations. It is found that dominated time irreversibility emerges in nearly all daily temperature anomaly variations over China. Further studies indicate that these time-irreversible behaviors result from asymmetric distributions of persistent daily temperature increments and decrements, and this kind of symmetry can be quantified by distributions of consecutive daily mean temperature increasing or decreasing steps. At the same time, the findings above have been confirmed by artificially generated time series with given value of multiscale asymmetry.     

漯河市秋季强降水预报

利用1996～1998年9～11月份欧洲中心数值预报产品资料和日本传真图资料，根据不同天气类型，建立了漯河市秋季强降水预报方法。     

Very high-resolution climate simulation over Fiji using a global variable-resolution model

A high-resolution climate model simulation has been performed for the first time for Fiji’s climatology. The simulation involved a numerical experiment for a 10-year period (1975–1984), and was conducted at a horizontal resolution of 8 km in a stretched-grid configuration, which is currently the highest resolution at which a global climate model has been applied for regional climatological simulations. Analysis of model-generated data demonstrates a fairly good skill of the CSIRO Conformal-Cubic Atmospheric Model (C-CAM) in the simulation of the annual cycles of maximum and minimum temperatures and rainfall at selected locations in Fiji. The model has also successfully reproduced the pattern of maximum and minimum surface air temperatures between the western and central divisions of Fiji. Model simulation of spatial and temporal distribution of monthly total rainfall (10-year mean) over the main island of Viti Levu in Fiji shows that it reproduces the observed intraseasonal and interannual variability; the influence of the El Niño phenomena has also been captured well in the model-simulated rainfall.     

Extreme precipitation over the Maritime Alps and associated weather regimes simulated by a regional climate model: Present-day and future climate scenarios

Summary We use the regional climate model RegCM nested within time-slice atmospheric general circulation model experiments to investigate the possible changes of intense and extreme precipitation over the French Maritime Alps in response to global climate change. This is a region with complex orography where heavy and/or extended precipitation episodes induced catastrophic floods during the last decades. Output from a 30-year simulation of present-day climate (1961–1990) is first analysed and compared with NCEP reanalysed 700 hPa geopotential heights (Z700) and daily precipitation observations from the Alpine Precipitation Climatology (1966–1999). Two simulations under forcing from the A2 and B2 IPCC emission scenarios for the period 2071–2100 are used to investigate projected changes in extreme precipitation for our region of interest. In general, the model overestimates the annual cycle of precipitation. The climate change projections show some increase of precipitation, mostly outside the warm period for the B2 scenario, and some increase in the variability of the annual precipitation totals for the A2 scenario. The model reproduces the main observed patterns of the spatial leading EOFs in the Z700 field over the Atlantic-European domain. The simulated large scale circulation (LSC) variability does not differ significantly from that of the reanalysis data provided the EOFs are computed on the same domain. Two similar clusters of LSC corresponding to heavy precipitation days were identified for both simulated and observed data and their patterns do not change significantly in the climate change scenarios. The analysis of frequency histograms of extreme indices shows that the control simulation systematically underestimates the observed heavy precipitation expressed as the 90^th percentile of rainday amounts in all seasons except summer and better reproduces the greatest 5-day precipitation accumulation. The main hydrological changes projected for the Maritime Alps consist of an increase of most intense wet spell precipitation during winters for both scenarios and during autumn for the B2 scenario. Case studies of heavy precipitation events show that the RegCM is capable to reproduce the physical mechanisms responsible for heavy precipitation over our region of interest.     

Variability of tropical cyclones over the southwest Pacific Ocean using a high-resolution climate model

Summary Knowledge of the variability in tropical cyclone (TC) frequency and distribution is essential in determining the possible impact of natural or human-induced climate change. This variability can be investigated using the available TC data bases and by carrying out long-term climate model simulations for both past and future climates. A coupled ocean-atmosphere climate model (referred to here as the OU-CGCM) is described and applied with a higher resolution (50 km) nested domain in the southwest Pacific region. Six-member ensembles of simulations with the OU-CGCM have been run for 80 years, from 1970 to 2050. During the period 1970–2000, the OU-CGCM runs were compared with the observed TC data base. For the period 2000–2050, two ensembles of simulations were performed, one with constant greenhouse gas concentrations and the second with increasing greenhouse gases. The OU-CGCM simulated well the observed TC frequency and distribution in the southwest Pacific during the period 1970–2000. It also produced clear interannual and interdecadal TC variability in both the fixed and enhanced greenhouse gas simulations during the period 2000–2050. The variability in TC frequencies was associated with the typical atmospheric and SST anomaly patterns that occur in periods of quiet and active TC frequencies. The main findings from the enhanced greenhouse gas scenario for the period 2000–2050 are: no change in the mean decadal number of TCs relative to the control run, but a marked increase of about 15% in the mean decadal number of TCs in the most severe WMO categories 4 and 5; the likelihood of TCs during the next 50-year period that are more intense than ever previously experienced in the Australian region; a poleward extension of TC tracks; and a poleward shift of over 2 degrees of latitude in the TC genesis region.     

Seasonal prediction of tropical cyclone genesis frequency over the western North Pacific using teleconnection patterns

This study has developed a multiple linear regression model for the seasonal prediction of the summer tropical cyclone genesis frequency (TCGF) in the western North Pacific using the three teleconnection patterns. These patterns are representative of the Siberian High Oscillation (SHO) in the East Asian continent, the North Pacific Oscillation in the North Pacific, and Antarctic Oscillation (AAO) near the Australia during the boreal spring (April–May). This statistical model is verified through the two analyses: (a) statistical method of cross validation and (b) differences between the high TCGF years and low TCGF years that is hindcasted by the statistical model. The high TCGF years are characterized by the following anomalous features: Three anomalous teleconnection patterns such as anticyclonic circulation (positive SHO phase) in the East Asian continent, pressure pattern like “north-high and south-low” in the North Pacific, and cyclonic circulation (negative AAO phase) near the Australia were strengthened during the period from boreal spring to boreal summer. Thus, anomalous trade winds in the tropical western Pacific (TWP) were weakened by anomalous cyclonic circulations that located in the subtropical western Pacific (SWP) in both hemispheres. In consequence, this spatial distribution of anomalous pressure pattern suppressed convection in the TWP but strengthened convection in the SWP.     

An Analysis of Regional and Intra-annual Precipitation Variability over Iran using Multivariate Statistical Methods

Summary The temporal and spatial precipitation regime of Iran was analysed using multivariate analyses of monthly mean precipitation records for 71 stations. A Principal Component Analysis was applied to the correlation matrix in order to describe the intra-annual variations of precipitation. The Principal Component scores were mapped to visualize the spatial structure of the three derived precipitation regimes. By applying an agglomerative clustering (WARD) of the three Principal Component scores, five homogeneous spatial clusters, representing five precipitation regions, were developed. The intra-annual types of precipitation distribution, shown by the five clusters, are described and discussed. Received November 24, 1997 Revised July 17, 1998     

Formation of horizontal convective rolls in urban areas

The formation of horizontal convective rolls (HCRs) in urban areas is investigated in this paper using observations and fine-scale numerical simulations. Cloud streets organized parallel to the mean boundary-layer wind (a manifestation of HCRs) are seen in the Fengyun-2C satellite imagery around local noon in Beijing. Observed vertical velocity and horizontal wind fields from an urban wind profiler suggest that the time scale for alternating updraft and downdraft in the boundary layer is about 30 min, and the length of the updraft/downdraft is about 9 km. Numerical simulations show that most HCRs occur in the urban areas with − z_i / L < 25 (z_i: the boundary-layer depth, L: the Monin–Obukhov length). Sensitivity tests reveal that HCRs are common in urban boundary layers, while rural areas are more conducive to forming cellular convection; the aspect ratio of HCRs in urban areas is smaller than the typical value over natural landscapes.     

与澳高相关的海洋性大陆区域海温异常对中国夏季气候的可能影响

利用NCEP/NCAR全球再分析资料和中国夏季站点资料,采用相关分析、合成分析以及线性回归方法,找到了与澳大利亚高压(下称澳高)显著相关的海温关键区,并分析了此关键区海温异常的年际变化与中国夏季气候异常的联系。结果表明:海洋性大陆区域附近海区是与澳高变化相关的海温异常关键区,其与澳高指数的时间序列相关系数高达-0.64。在不考虑ENSO的影响时,关键区海温的年际变化与中国东部地区夏季气候存在密切联系,即当澳高指数偏强(偏弱)时,海洋性大陆区域附近的海温偏低(偏高),辽宁、吉林大部分以及江淮东部地区夏季降水较常年偏少(偏多),江南地区降水偏多(偏少)。关键区海温对中国夏季气候的可能影响途径是:印度尼西亚附近海温负异常时,低层风场异常辐散,辐散场作为涡度源在南海西太平洋地区激发类似EAP或P—J型波列,在30°N以南形成异常气旋环流,30°N-50°N形成异常反气旋环流,50°N以北形成异常气旋环流,这样的异常分布有利于"南涝北旱"降水格局的形成。另外,降水异常时气温亦存在显著异常,我国东部气温为异常的"南低北高"分布。当海洋性大陆区域附近的海温为正异常时,情况相反。