Abstract

A new index measuring the East Asian winter monsoon is defined using the mean wind shears of upper-tropospheric zonal wind based on the belief that the physical processes of both higher and lower latitudes, and at both lower and upper troposphere, should be considered to depict the variability of monsoon. When the index is high (low), the westerly jet is strong (weak), the East Asian trough is deep (shallow), the Siberian high is strong (weak), and anomalous low-level northerlies (southerlies) prevail over East Asia. As a result, the surface and lower-tropospheric temperature over East Asia decreases (increases) and the cold surges over Southeast Asia and tropical western Pacific are more (less) active. The index, which exhibits distinct interannual variations, is also strongly correlated with the Arctic Oscillation and Niño-3.4 sea surface temperature (SST) index. Compared to previous indexes, this index takes into account more influencing factors and better elucidates the physical processes associated with monsoon, enhancing interpretations of the variability of monsoon and its effects on regional weather and climate. Furthermore, the monsoon index is significantly linked to antecedent tropical Pacific SST and is highly predictable in the NCEP Climate Forecast System, indicating the advantage of the index for operational predictions of monsoon.

Abstract

The Moderate Resolution Imaging Spectroradiometer (MODIS) dual spectral rain algorithm (MODRA) is developed for rain retrievals over the northern midlatitudes. The reflectance of the MODIS water vapor absorption channel at 1.38 μm (R _{1.38 μm}) has a potential to represent the cloud-top height displayed by the brightness temperature (TB) of the MODIS channel at 11 μm, because of an excellent negative relationship (correlation coefficient ≤−0.9) between R _{1.38 μm} and TB_{11 μm} for optically thick clouds with reflectance (R _{0.65 μm}) greater than 0.75. With a training rainfall dataset from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) aboard the same Aqua satellite platform, two MODIS channels (R _{1.38 μm} and R _{0.65 μm}) are applied to form multiregression curves to estimate daytime rainfall. Results demonstrate that the instantaneous rain rates from MODRA, independent AMSR-E rainfall products, and surface rain gauge measurements are consistent. This study explores a new way to estimate rainfall from MODIS water vapor and cloud channels. The resulting technique could be applied to other similar satellite instruments for rain retrievals.

Abstract

This study investigates the variations and feedback attributions of changes in surface temperature between strong and weak East Asian winter monsoons. The variations of winter-mean surface air temperature are dominated by two distinct principal modes that account for 70.9% of the total variance. The first mode features high correlation with the high-latitude atmospheric circulation, including a correlation coefficient of −0.53 with the Arctic Oscillation in January, and the second mode is significantly linked to El Niño–Southern Oscillation, with a correlation coefficient of −0.37. The surface temperature anomalies of each mode are decomposed into partial temperature anomalies resulting from radiative and nonradiative feedback processes by applying a coupled climate feedback–response analysis method to quantify contributions from thermodynamic and dynamic processes. Results indicate that the surface cooling associated with both modes is mainly attributed to the nonradiative feedback processes of atmospheric dynamics and surface sensible heating and to the radiative feedback processes of water vapor and clouds. The first mode exhibits a deep barotropic anomalous high that weakens the high-latitude westerly jet stream but strengthens the midlatitude westerly jet stream. This circulation feature traps cold and dry air over northern East Asia. For the second mode, the ocean and land heat storage processes induce a large thermal gradient over eastern China and the northwestern Pacific, resulting in a large pressure gradient. Northerly anomalies further reinforce the pressure gradient, which favors cold air intruding southward into the tropics.

Abstract

Based on observational data, a linear baroclinic model, and an atmospheric general circulation model (AGCM), the major modes of spring precipitation over the tropical Asian and Pacific regions are identified. and the influence of latent heating over the tropical western Pacific (TWP) on global climate is investigated. Results show that the first mode of empirical orthogonal function analysis explains 20% of the total variance in March, the largest in spring, with the maximum center located over the TWP. The precipitation is highly positively correlated with local sea surface temperature (SST) in March, which suggests that the warming SST is the trigger for the precipitation over the TWP. Further analysis suggests that an increase in latent heating over the TWP, especially in March, can produce Rossby waves along the westerly jet, which causes an increase in surface temperature over North America. The propagation intensity decreases from March to May. The changes in location and strength of the westerly jet stream in the Northern Hemisphere are responsible for this decrease. Experiments with both a linear baroclinic model and an AGCM verify the above hypothesis. The study highlights that the spatial distributions of latent heating and westerly jet stream are the two key factors for the formation of teleconnection patterns from eastern Asia to North America.

Abstract

Using multiple datasets and a partial correlation method, the authors analyze the different impacts of eastern Pacific (EP) and central Pacific (CP) El Niño on East Asian climate, focusing on the features from El Niño developing summer to El Niño decaying summer. Unlike the positive–negative–positive (+/−/+) anomalous precipitation pattern over East Asia and the equatorial Pacific during EP El Niño, an anomalous −/+/− rainfall pattern appears during CP El Niño. The anomalous dry conditions over southeastern China and the northwestern Pacific during CP El Niño seem to result from the anomalous low-level anticyclone over southern China and the South China Sea, which is located more westward than the Philippine Sea anticyclone during EP El Niño. The continuous anomalous sinking motion over southeastern China, as part of the anomalous Walker circulation associated with CP El Niño, also contributes to these dry conditions.

During the developing summer, the impact of CP El Niño on East Asian climate is more significant than the influence of EP El Niño. During the decaying summer, however, EP El Niño exerts a stronger influence on East Asia, probably due to the long-lasting anomalous warming over the tropical Indian Ocean accompanying EP El Niño.

Temperatures over portions of East Asia and the northwestern Pacific tend to be above normal during EP El Niño but below normal from the developing autumn to the next spring during CP El Niño. A possible reason is the weakened (enhanced) East Asian winter monsoon related to EP (CP) El Niño.

Abstract

The eastern Pacific (EP) El Niño–Southern Oscillation (ENSO) and the central Pacific (CP) ENSO exert different influences on climate. In this study, the authors analyze the hindcasts of the NCEP Climate Forecast System, version 2 (CFSv2), and assess the skills of predicting the two types of ENSO and their impacts on East Asian climate. The possible causes of different prediction skills for different types of ENSO are also discussed.

CFSv2 captures the spatial patterns of sea surface temperature (SST) related to the two types of ENSO and their different climate impacts several months in advance. The dynamical prediction of the two types of ENSO by the model, whose skill is season dependent, is better than the prediction based on the persistency of observed ENSO-related SST, especially for summer and fall. CFSv2 performs well in predicting EP ENSO and its impacts on the East Asian winter monsoon and on the Southeast Asian monsoon during its decaying summer. However, for both EP ENSO and CP ENSO, the model overestimates the extent of the anomalous anticyclone over the western North Pacific Ocean from the developing autumn to the next spring but underestimates the magnitude of climate anomalies in general. It fails to simulate the SST pattern and climate impact of CP ENSO during its developing summer. The model’s deficiency in predicting CP ENSO may be linked to a warm bias in the eastern Pacific. However, errors in simulating the climate impacts of the two types of ENSO should not be solely ascribed to the bias in SST simulation.

Abstract

The early season rainfall (ESR) over southern China, usually occurring from April to June, is a prominent meteorological phenomenon of the East Asian monsoon system. In this paper, output from the 45-day hindcast by the NCEP Climate Forecast System, version 2 (CFSv2), and various observational datasets are analyzed to assess the predictability of the ESR and associated atmospheric circulation. Results show that CFSv2 can successfully predict the ESR and associated circulation patterns over southern China. The lower-tropospheric convergence and upper-tropospheric divergence as well as the local upward motion over southern China lead to the formation of ESR. Analysis of bias shows small differences and close relationships between the predicted and observed ESR values when the forecast lead time is less than 2 weeks. The skill in the ESR predictions by CFSv2 decreases significantly when the lead time is longer than 2 weeks. Overall, CFSv2 has a higher level of skill when predicting the southern China ESR compared to the rainfall over other Asian regions during the same period of time.

Abstract

This study addresses the retrieval of tropical open-ocean latent heating using Special Sensor Microwave Imager (SSM/I) satellite measurements. The analysis is carried out for the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observation period in the western Pacific, much of it focused on the study area of the third WCRP–GPCP Algorithm Intercomparison Project (AIP-3) situated over the TOGA COARE Inner Flux Array (IFA). The retrieval algorithm is a profile-type physical inversion scheme based on the use of multispectral passive microwave (PMW) measurements. It estimates vertically distributed rain rate and latent heating by first retrieving mixing ratio profiles of liquid and frozen hydrometeors and then calculating rain fallout rates and vertical derivatives of the liquid–ice mass fluxes. Various modifications to the existing algorithm are discussed, including a combined visible–infrared–PMW–radar screening scheme for distinguishing among “clear,” “cloud without rain,” and “cloud with rain pixels” to better delineate vertical heating structure. Validation of retrieved rain rates over the AIP-3 study area indicates acceptable accuracy/precision uncertainty levels in terms of intensity, distribution, and time variation.

A procedure is developed for improving the initially retrieved heating profiles based on calibration to shipboard radar measurements. The modified algorithm and calibration scheme were applied to the IFA for estimating vertical profiles of latent heating. An optimum high-quality sounding period (1–17 February 1993) was selected for large-scale diagnostic calculations of apparent heating (Q ₁) and moistening (Q ₂) to analyze heat-moisture budgets of convective and stratiform cloud systems. Comparison and sensitivity tests indicate that the retrieved latent heating and Q ₁/Q ₂ calculations are representative. Moisture budget analyses over the IFA were carried out to study the detailed heating structures of clouds, particularly the cumulus scale heating process. This was accomplished by using residuals between the SSM/I-retrieved latent heating and the large scale Q ₂ diagnostics. Results show that estimates of daily eddy vertical moisture flux divergence contain sizable uncertainties, however, by averaging over extended periods and vertically integrating to obtain surface latent heat flux transfer, close agreement to independently derived surface evaporation rates is found. This suggests that by combining the SSM/I retrievals with large-scale sounding data, it is possible to shed light on the role of cumulus convection on diabatic heating.

Abstract

Using precipitation data from rain gauge stations over China, the authors examine the long-term variation of the durations of persistent rainfall over eastern China for the past 40 years. The variation in the regional rainfall was related to a change in the global-mean surface temperature from the relatively cold period of the 1960s–70s to the relatively warm period of the 1980s–90s. Compared to the cold period, the persistent rainfall in the warm period began earlier and ended later over southern China, lengthening the rainy season by 23 days, but it began later and ended earlier over northern China, shortening the rainy season by 14 days. This change in the durations of persistent rainfall contributed to the pattern of the long-term change in rainfall: southern floods and northern droughts. The earlier beginning of the rainy season over southern China was associated with a more westward subtropical high over the western North Pacific and a stronger low-level low near the eastern Tibetan Plateau during spring. On the other hand, the later ending of the rainy season over southern China and the shorter rainy season over northern China were related to a more westward subtropical high over the western Pacific and a weaker trough near the eastern Tibetan Plateau during summer.

The snow cover over the Tibetan Plateau exhibited a positive trend in winter and spring, which increased the local soil moisture content and cooled the overlying atmosphere during spring and summer. The sea surface temperature over the tropical Indian Ocean and the western North Pacific also displayed a positive trend. The cooling over land and the warming over oceans reduced the thermal contrast between East Asia and the adjacent oceans. Moreover, the low-level low pressure system over East Asia weakened during summer. Under such circumstances, the East Asian summer monsoon circulation weakened, with anomalous northerly winds over eastern China. Correspondingly, the mei-yu front stagnated over the Yangtze River valley, and the associated pattern of vertical motions increased the rainfall over the valley and decreased the rainfall over northern China.

Abstract

This study investigates the variability of convective and stratiform rainfall from 8 yr (1998–2005) of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and TRMM Microwave Imager (TMI) measurements, focusing on seasonal diurnal variability. The main scientific goals are 1) to understand the climatological variability of these two dominant forms of precipitation across the four cardinal seasons and over continents and oceans separately and 2) to understand how differences in convective and stratiform rainfall variations ultimately determine how the diurnal variability of the total rainfall is modulated into multiple modes.

There are distinct day–night differences for both convective and stratiform rainfall. Oceanic (continental) convective rainfall is up to 25% (50%) greater during nighttime (daytime) than daytime (nighttime). The seasonal variability of convective rainfall’s day–night difference is relatively small, while stratiform rainfall exhibits very apparent day–night variations with seasonal variability. There are consistent late evening diurnal peaks without obvious seasonal variations over ocean for convective, stratiform, and total rainfall. Over continents, convective and total rainfall exhibit consistent dominant afternoon peaks with little seasonal variations—but with late evening secondary peaks exhibiting seasonal variations. Stratiform rainfall over continents exhibits a consistent strong late evening peak and a weak afternoon peak, with the afternoon mode undergoing seasonal variability. Thus, the diurnal characteristics of stratiform rainfall appear to control the afternoon secondary maximum of oceanic rainfall and the late evening secondary peak of continental rainfall. Even at the seasonal–regional scale spatially or the interannual global scale temporally, the secondary mode can become very pronounced, but only on an intermittent basis. Overall, the results presented here demonstrate the importance of partitioning the total rainfall into convective and stratiform components and suggest that diurnal modes largely arise from distinct diurnal stratiform variations modulating convective variations.