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Jun Li, Yi-Leng Chen, and Wen-Chau Lee

Abstract

A heavy rainfall event during the Taiwan Area Mesoscale Experiment intensive observing period 13 has been studied using upper-air, surface mesonet, and dual-Doppler radar data. The heavy rainfall (≥231 mm day−1) occurred over northwestern Taiwan with the maximum rainfall along the northwestern coast and was caused by a long-lived, convective rainband in the prefrontal atmosphere. It occurred in an upper-level divergence region and along the axis of the maximum equivalent potential temperature at the 850-hPa level.

As a Mei-Yu front advanced southeastward, the postfrontal cold air in the lowest levels was retarded by the hilly terrain along the southeastern China coast. As a result, a low-level wind-shift line associated with a pressure trough at the 850-hPa level moved over the Taiwan Strait before the arrival of the surface front. The westerly flow behind the trough interacted with a barrier jet along the northwestern coast of Taiwan. The barrier jet is caused by the interaction between the prefrontal southwest monsoon flow and the island obstacle. A low-level convergence zone (∼3 km deep) was observed along the wind-shift line between the westerly flow coming off the southeastern China coast and the barrier jet. A long-lived rainband developed within the low-level convergence zone and moved southeastward toward the northwestern Taiwan coast with the wind-shift line.

There were several long-lived (>2 h) reflectivity maxima embedded in the rainband. They often had several individual cells with a much shorter lifetime. The reflectivity maxima formed on the southwestern tip of the rainband and along the low-level wind-shift line. They intensified during their movement from the southwest to the northeast along the rainband. The continuous generation of the reflectivity maxima along the wind-shift line and the intensification of them over the low-level convergence zone maintained the long lifetime of the rainband and produced persistent heavy rainfall along the northwestern coast as these reflectivity maxima moved toward the coast. During the early stage of their lifetime, the reflectivity maxima were observed along the wind-shift line with upward motion in the lower troposphere. As they moved toward the northeastern part of the rainband and matured, the reflectivity maxima were observed southeast of the convergence zone with sinking motion in the lower troposphere. The upward motion was rooted along the wind-shift line and tilted southeastward with height. The reflectivity maxima dissipated as they moved inland. During the early stage of the rainband, the reflectivity maxima on the northeastern part of the rainband also merged with the convective line associated with the land-breeze front offshore of the northwestern coast.

The Mei-Yu front was shallow (<1 km) and moved slowly southward along the western coast. Convection associated with the front was weak with echo tops (∼10 dBZ) below 6 km.

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Xu Zhang, Jian-Wen Bao, Baode Chen, and Evelyn D. Grell

Abstract

A new three-dimensional (3D) turbulent kinetic energy (TKE) subgrid mixing scheme is developed using the Advanced Research version of the Weather Research and Forecasting (WRF) Model (WRF-ARW) to address the gray-zone problem in the parameterization of subgrid turbulent mixing. The new scheme combines the horizontal and vertical subgrid turbulent mixing into a single energetically consistent framework, in contrast to the conventionally separate treatment of the vertical and horizontal mixing. The new scheme is self-adaptive to the grid-size change between the large-eddy simulation (LES) and mesoscale limits. A series of dry convective boundary layer (CBL) idealized simulations are carried out to compare the performance of the new scheme and the conventional treatment of subgrid mixing to the WRF-ARW LES dataset. The importance of including the nonlocal component in the vertical buoyancy specification in the newly developed general TKE-based scheme is illustrated in the comparison. The improvements of the new scheme with the conventional treatment of subgrid mixing across the gray-zone model resolutions are demonstrated through the partitioning of the total vertical flux profiles. Results from real-case simulations show the feasibility of using the new scheme in the WRF Model in lieu of the conventional treatment of subgrid mixing.

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Jinling Piao, Wen Chen, Qiong Zhang, and Peng Hu

Abstract

The moisture supplies over Siberia and Northeast Asia are investigated by comparing their similarities and differences, enlightened by the seesaw pattern in their summer precipitation. Based on the rotated empirical orthogonal functions in the 3-month standardized precipitation evapotranspiration index (SPEI_03), Siberia and Northeast Asia are defined as the regions within 55°–70°N, 80°–115°E and 40°–55°N, 90°–115°E, respectively. Our results show that over both regions, evaporation contributes the most to the precipitation amount at the annual time scale, and moisture convergence contributes the most on the interannual time scale. For moisture convergence, both the stationary and transient terms are subject to impacts of the midlatitude westerlies. For the annual cycle, the net moisture supply over both Siberia and Northeast Asia is closely associated with both stationary and transient moisture transport. However, on the interannual time scale, the net moisture convergence is closely related to the stationary term only. The examination of the boundary moisture transport shows that in addition to the zonal component, the meridional stationary moisture transport plays a key role in the net moisture convergence. The transient moisture transport mainly depends on moisture transport through the western and southern boundaries, with a comparable magnitude to that of the stationary one, further confirming the importance of the stationary and transient terms on the moisture supply for the annual cycle. In addition, the circulations responsible for moisture transport anomalies indicate that the stationary moisture circulation is the key factor for the moisture supply anomalies over both Siberia and Northeast Asia, with limited impacts from the transient moisture circulation.

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Xiuzhen Li, Wen Zhou, Deliang Chen, Chongyin Li, and Jie Song

Abstract

The water vapor transport and moisture budget over eastern China remotely forced by the cold-tongue (CT) and warm-pool (WP) El Niño show striking differences throughout their lifetime. The water vapor transport response is weak in the developing summer but strong in the remaining phases of CT El Niño, whereas the opposite occurs during WP El Niño. WP El Niño causes a moisture deficit over the Yangtze River valley (YZ) in the developing summer and over southeastern China (SE) in the developing fall, whereas CT El Niño induces a moisture surplus first over SE during the developing fall with the influential area expanding in the decaying spring and shifting northward in the decaying summer. It is the divergence of meridional water vapor transport that dominates the total water vapor divergence anomaly, with the divergence of zonal transport showing an opposite pattern with smaller magnitude.

Investigation of the vertical profile of moisture budget shows a great baroclinicity, with the strongest abnormal moisture budget occurring in different levels. The moisture transport via the southern boundary plays a crucial role in the regional moisture budget anomalies and is located near the surface over SE, in the lower troposphere over the YZ, and at the lower-middle troposphere over the eastern part of northern China. The enhanced moisture surplus near the surface forced by WP El Niño over SE in the mature winter and decaying spring is offset by a moisture deficit within the lower-middle troposphere due to a diverse response circulation at different vertical levels.

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Wen Wang, Wei Cui, Xiaoju Wang, and Xi Chen

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The Global Land Data Assimilation System (GLDAS) is an important data source for global water cycle research. Using ground-based measurements over continental China, the monthly scale forcing data (precipitation and air temperature) during 1979–2010 and model outputs (runoff, water storage, and evapotranspiration) during 2002–10 of GLDAS models [focusing on GLDAS, version 1 (GLDAS-1)/Noah and GLDAS, version 2 (GLDAS-2)/Noah] are evaluated. Results show that GLDAS-1 has serious discontinuity issues in its forcing data, with large precipitation errors in 1996 and large temperature errors during 2000–05. While the bias correction of the GLDAS-2 precipitation data greatly improves temporal continuity and reduces the biases, it makes GLDAS-2 precipitation less correlated with observed precipitation and makes it have larger mean absolute errors than GLDAS-1 precipitation for most months over the year. GLDAS-2 temperature data are superior to GLDAS-1 temperature data temporally and spatially. The results also show that the change rates of terrestrial water storage (TWS) data by GLDAS and the Gravity Recovery and Climate Experiment (GRACE) do not match well in most areas of China, and both GLDAS-1 and GLDAS-2 are not very capable of capturing the seasonal variation in monthly TWS change observed by GRACE. Runoff is underestimated in the exorheic basins over China, and runoff simulations of GLDAS-2 are much more accurate than those of GLDAS-1 for two of the three major river basins of China investigated in this study. Evapotranspiration is overestimated in the exorheic basins in China by both GLDAS-1 and GLDAS-2, whereas the overestimation of evapotranspiration by GLDAS-2 is less than that by GLDAS-1.

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Xiao Feng, Renguang Wu, Jiepeng Chen, and Zhiping Wen

Abstract

The present study investigates the year-to-year variations of September–October rainfall in Hainan, China, for the period 1965–2010. The dominant circulation anomalies feature a cyclone (an anticyclone) over the Indochina Peninsula and northern South China Sea, an anticyclone (a cyclone) over subtropical western North Pacific and lower-level convergence (divergence) over the Maritime Continent in the wet (dry) years. These circulation anomalies are responses to an east–west sea surface temperature (SST) anomaly pattern with negative (positive) SST anomalies in the equatorial central Pacific and positive (negative) SST anomalies around the Maritime Continent in the wet (dry) years. Although the SST anomaly pattern is similar (but with opposite anomaly), the SST signal in the equatorial central Pacific is more significant in the dry years than in the wet years. This difference indicates a larger case-to-case variability in the wet years than in the dry years. The large variability in the wet years is attributed to contributions of tropical cyclones (TCs) and intraseasonal oscillations (ISOs). There are more TCs impinging on Hainan and the TC tracks are closer to the island in the wet years than in the dry years. The rainfall shows large intraseasonal variations with periods of 10–20 and 30–60 days during September–October in the wet years. The 10–20-day ISO originates from the Maritime Continent, whereas the 30–60-day ISO develops over tropical Indian Ocean and propagates northeastward to northern South China Sea. In contrast, the ISO signal is much weaker in the dry years.

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Jingliang Huangfu, Wen Chen, Ronghui Huang, and Juan Feng

Abstract

This paper investigates how La Niña Modoki modulates the impacts of the warm Indian Ocean basin mode (IOBM) on the boreal summer climate and the genesis of tropical cyclones (TCs) over the northwest Pacific (NWP). The results showed that the influence of the Indian Ocean sea surface temperature (SST) on TC genesis is the primary mechanism during the boreal summer, while La Niña Modoki exerts a secondary influence. However, although the summertime index of the IOBM shows a high negative correlation with the number of TCs generated over the NWP, warm IOBM events without La Niña Modoki have only limited influences on the boreal summertime circulations and TC genesis. The present study showed that when warm IOBM events and La Niña Modoki coexisted, the average location of TC genesis shifted westward, and the annual number of generated TCs substantially decreased. La Niña Modoki–related cold sea surface temperature anomalies over the central Pacific further suppressed convective activities over the eastern NWP compared with warm IOBM events without La Niña Modoki. Upper-level convergence and enlarged tropospheric vertical wind shears both contributed to the weakening of the low-level relative vorticity in the coupled cases, leading to a suppressed NWP monsoon trough. Additionally, together with the weaker moisture supply, the impacts of warm IOBM cases were significantly enhanced under the modulation of La Niña Modoki, leading to poorer TC genesis conditions over the eastern NWP. In addition, the energy conversion processes in the aforementioned modulation showed that joint cases will provide fewer initial disturbance seedlings for TC genesis. These results are useful for further understanding the role of warm IOBM cases in TC genesis over the NWP.

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Mengyan Feng, Weihua Ai, Guanyu Chen, Wen Lu, and Shuo Ma

Abstract

One-dimensional synthetic aperture microwave radiometer (1D-SAMR) can provide remote sensing images at a higher spatial resolution than those from traditional real aperture microwave radiometers. As 1D-SAMR operates at multiple incidence angles, we proposed a multiple linear regression method to retrieve sea surface temperature at an incidence angle between 0° and 65°. Assuming that a 1D-SAMR operates at various frequencies (i.e., 6.9, 10.65, 18.7, 23.8 and 36.5 GHz), a radiation transmission forward model was developed to simulate the brightness temperature measured by the 1D-SAMR. The sensitivity of the five frequencies to sea surface temperature was examined, and we evaluated the reliability of the regression method proposed in this study. Furthermore, 11 schemes with various frequency combinations were applied to retrieve sea surface temperature. The results showed that the five-frequency combination scheme performed better than the other schemes. This study also found that the accuracy of retrieved sea surface temperature is dependent on incidence angles. Finally, we suggested that the incidence angle range of the 1D-SAMR is necessary to be 30°–60° based on the relationship between the accuracy of retrieved sea surface temperature and the incidence angles.

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Sai Wang, Debashis Nath, Wen Chen, and Lin Wang

Abstract

In the last three decades, rapid surface warming is observed in the land areas of northern high latitudes during boreal summer months. Although the warming trend is thought to be driven by early snowmelt, the exact causes, especially its relationship with atmospheric circulation changes, remain a subject of debate. By analyzing ERA-Interim data, this study examines the possible factors for rapid subarctic warming. It is found that more than half of the warming trend over the entire subarctic and 80% over northern Canada and eastern Siberia (regions with maximum amplification) can be explained by enhanced downward infrared radiation (IR). Downward IR is largely driven by horizontal atmospheric moisture flux convergence and warm-air advection. The positive trend in geopotential height over the Greenland region is key for moisture flux convergence over northern Canada and eastern Siberia through changes in the storm tracks. An enhanced summertime blocking activity in the Greenland region seems responsible for the positive trend in geopotential heights.

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Lin Wang, Ronghui Huang, Lei Gu, Wen Chen, and Lihua Kang

Abstract

Interdecadal variations of the East Asian winter monsoon (EAWM) and their association with the quasi-stationary planetary wave activity are analyzed by using the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis dataset and the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset. It is found that the EAWM experienced a significant weakening around the late 1980s; that is, the EAWM was strong during 1976–87 and became weak after 1988. This leads to an obvious increase in the wintertime surface air temperature as well as a decrease in the frequency of occurrence of cold waves over East Asia. The dynamical process through which the EAWM is weakened is investigated from the perspective of quasi-stationary planetary waves. It is found that both the propagation and amplitude of quasi-stationary planetary waves have experienced obvious interdecadal variations, which are well related to those of the EAWM. Compared to the period 1976–87, the horizontal propagation of quasi-stationary planetary waves after 1988 is enhanced along the low-latitude waveguide in the troposphere, and the upward propagation of waves into the stratosphere is reduced along the polar waveguide. This results in a weakened subtropical jet around 40°N due to the convergence of the Eliassen–Palm flux. The East Asian jet stream is then weakened, leading to the weakening of the EAWM since 1988. In addition, the amplitude of quasi-stationary planetary waves is significantly weakened around 45°N, which is related to the reduced upward propagation of waves from the lower boundary after 1988. This reduced amplitude may weaken both the Siberian high and the Aleutian low, reduce the pressure gradient in between, and then weaken the EAWM. Further analyses indicate that zonal wavenumber 2 plays the dominant role in this process.

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