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Xiaofan Li, Zeng-Zhen Hu, and Bohua Huang

Abstract

Evolutions of oceanic and atmospheric anomalies in the equatorial Pacific during four strong El Niños (1982/83, 1991/92, 1997/98, and 2015/16) since 1979 are compared. The contributions of the atmosphere–ocean coupling to El Niño–associated sea surface temperature anomalies (SSTA) are identified and their association with low-level winds as well as different time-scale variations is examined. Although overall SSTA in the central and eastern equatorial Pacific is strongest and comparable in the 1997/98 and 2015/16 El Niños, the associated subsurface ocean temperature as well as deep convection and surface wind stress anomalies in the central and eastern equatorial Pacific are weaker during 2015/16 than that during 1997/98. That may be associated with a variation of the wind–SST and wind–thermocline interactions. Both the wind–SST and wind–thermocline interactions play a less important role during 2015/16 than during 1997/98. Such differences are associated with the differences of the low-level westerly wind as well as the contribution of different time-scale variations in different events. Similar to the interannual time-scale variation, the intraseasonal–interseasonal time-scale component always has positive contributions to the intensity of all four strong El Niños. Interestingly, the role of the interdecadal-trend time-scale component varies with event. The contribution is negligible during the 1982/83 El Niño, negative during the 1997/98 El Niño, and positive during the 1991/92 and 2015/16 El Niños. Thus, in addition to the atmosphere–ocean coupling at intraseasonal to interannual time scales, interdecadal and longer time-scale variations may play an important and sometimes crucial role in determining the intensity of El Niño.

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Huiyan Xu, Guoqing Zhai, and Xiaofan Li

Abstract

In this study, the WRF Model is used to simulate the torrential rainfall of Typhoon Fitow (2013) over coastal areas of east China during its landfall. Data from the innermost model domain are used to trace trajectories of particles in three major 24-h accumulated rainfall centers using the Lagrangian flexible particle dispersion model (FLEXPART). Surface rainfall budgets and cloud microphysical budgets as well as precipitation efficiency are analyzed along the particles’ trajectories. The rainfall centers with high precipitation efficiency are associated with water vapor convergence, condensation, accretion of cloud water by raindrops, and raindrop loss/convergence. The raindrop loss/convergence over rainfall centers is supported by the raindrop gain/divergence over the areas adjacent to rainfall centers. Precipitation efficiency is mainly determined by hydrometeor loss/convergence. Hydrometeor loss/convergence corresponds to the hydrometeor flux convergence, which may be related to the increased vertical advection of hydrometeors in response to the upward motions and upward decrease of hydrometeors, whereas hydrometeor gain/divergence corresponds to the reduction in hydrometeor flux convergence, which may be associated with the decreased horizontal advection of hydrometeors in response to the zonal decrease in hydrometeors and easterly winds and the meridional increase in hydrometeors and southerly winds. The water vapor convergence and associated condensation do not show consistent relationships with orographic lifting all the time.

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Ting Liu, Jianping Li, Juan Feng, Xiaofan Wang, and Yang Li

Abstract

Recent work suggests that the boreal autumn Southern Hemisphere annular mode (SAM) favors a tripole pattern of winter precipitation anomalies in the Northern Hemisphere. This study focuses on the abilities of climate models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5) to reproduce the physical processes involved in this observed cross-seasonal connection. A systematic evaluation suggested that 16 out of 25 models were essentially capable of reproducing this cross-seasonal connection. Two categories of models were selected to explore the underlying reasons for these successful simulations. Models that successfully simulated the cross-seasonal relationship were placed in the type-I category, and these performed well in reproducing the related physical mechanism, known as the “coupled ocean–atmosphere bridge,” in terms of the SST variability associated with the SAM and response of the meridional circulation to these SST anomalies. In contrast, the type-II category of models showed poor performance in representing the related processes and associated feedbacks, and the model biases compromised the performance of the simulated cross-seasonal relationship. These results demonstrate that the capability of the CMIP5 models to reproduce SST variability associated with the boreal autumn SAM and related coupled ocean–atmosphere bridge process plays a decisive role in the successful simulation of the cross-seasonal relationship.

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Xiaofan Li, C-H. Sui, and K-M. Lau

Abstract

The phase relation between the perturbation kinetic energy (K′) associated with the tropical convection and the horizontal-mean moist available potential energy (P) associated with environmental conditions is investigated by an energetics analysis of a numerical experiment. This experiment is performed using a 2D cloud resolving model forced by the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) derived vertical velocity. The imposed upward motion leads to a decrease of P through the associated vertical advective cooling, and to an increase of K′ through cloud-related processes, feeding the convection. The maximum K′ and its maximum growth rate lags and leads, respectively, the maximum imposed large-scale upward motion by about 1–2 h, indicating that convection is phase locked with large-scale forcing. The dominant life cycle of the simulated convection is about 9 h, whereas the timescales of the imposed large-scale forcing are longer than the diurnal cycle.

In the convective events, the maximum growth of K′ leads the maximum decay of the perturbation moist available potential energy (P′) by about 3 h through vertical heat transport by perturbation circulation, and perturbation cloud heating. The maximum decay of P′ leads the maximum decay of P by about 1 h through the perturbation radiative processes, the horizontal-mean cloud heating, and the large-scale vertical advective cooling. Therefore, maximum gain of K′ occurs about 4–5 h before maximum decay of P.

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Xiaofan Li, C-H. Sui, and K-M. Lau

Abstract

Dominant cloud microphysical processes associated with a tropical oceanic convective system are investigated based on a 2D cloud resolving simulation. The model is forced by the zonal-mean vertical velocity, zonal wind, sea surface temperature, and horizontal temperature and moisture advections measured and derived from the TOGA COARE period. The analysis of cloud microphysics budgets shows that cloud water forms due to vapor condensation, but most of the conversion of cloud water to precipitation occurs primarily through two mechanisms, depending on the temperature when they occur: through riming of cloud water onto precipitation ice (snow or graupel) at colder than 0°C and collection of cloud water by rain at warmer temperatures. Processes involving the liquid phase are dominant during the early stages of convection development. The collection process produces rain, and the riming process enhances ice clouds. Ice processes are more dominant during the later stages. The melting of precipitation ice and vapor deposition become important in producing rain and ice clouds, respectively.

Based on the analysis of dominant cloud microphysical processes, a simplified set of cloud microphysics parameterization schemes are proposed. Simulations with the simplified and original sets show similar thermodynamic evolution and cloud properties.

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Jian-Jian Wang, Xiaofan Li, and Lawrence D. Carey

Abstract

A two-dimensional cloud-resolving simulation is combined with dual-Doppler and polarimetric radar analysis to study the evolution, dynamic structure, cloud microphysics, and rainfall processes of monsoon convection observed during the South China Sea (SCS) summer monsoon onset.

Overall, the model simulations show many similarities to the radar observations. The rainband associated with the convection remains at a very stable position throughout its life cycle in the northern SCS. The reflectivity pattern exhibits a straight upward structure with little tilt. The positions of the convective, transition, and stratiform regions produced by the model are consistent with the observations. The major difference from the observations is that the model tends to overestimate the magnitude of updraft. As a result, the maximum reflectivity generated by the model appears at an elevated altitude.

The surface rainfall processes and associated thermodynamic, dynamic, and cloud microphysical processes are examined by the model in terms of surface rainfall, temperature and moisture perturbations, circulations, and cloud microphysical budget. At the preformation and dissipating stages, although local vapor change and vapor convergence terms are the major contributors in determining rain rate, they cancel each other out and cause little rain. The vapor convergence/divergence is closely related to the lower-tropospheric updraft/subsidence during the early/late stages of the convection. During the formation and mature phases, vapor convergence term is in control of the rainfall processes. Meanwhile, water microphysical processes are dominant in these stages. The active vapor condensation process causes a large amount of raindrops through the collection of cloud water by raindrops. Ice microphysical processes including riming are negligible up to the mature phase but are dominant during the weakening stage. Cloud source/sink terms make some contributions to the rain rate at the formation and weakening stages, while the role of surface evaporation term is negligible throughout the life cycle of the convection.

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Chung-Hsiung Sui, Xiaofan Li, and Ming-Jen Yang

Abstract

A modified definition of precipitation efficiency (PE) is proposed based on either cloud microphysics precipitation efficiency (CMPE) or water cycling processes including water vapor and hydrometeor species [large-scale precipitation efficiency (LSPE)]. These PEs are examined based on a two-dimensional cloud-resolving simulation. The model is integrated for 21 days with the imposed large-scale vertical velocity, zonal wind, and horizontal advections obtained from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). It is found that the properly defined PEs include all moisture and hydrometeor sources associated with surface rainfall processes so that they range from 0% to 100%. Furthermore, the modified LSPE and CMPE are highly correlated. Their linear correlation coefficient and root-mean-squared difference are insensitive to the spatial scales of averaged data and are moderately sensitive to the time period of averaged data.

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Xiaofan Li, Zeng-Zhen Hu, Ping Liang, and Jieshun Zhu

Abstract

In this work, the roles of El Niño–Southern Oscillation (ENSO) in the variability and predictability of the Pacific–North American (PNA) pattern and precipitation in North America in winter are examined. It is noted that statistically about 29% of the variance of PNA is linearly linked to ENSO, while the remaining 71% of the variance of PNA might be explained by other processes, including atmospheric internal dynamics and sea surface temperature variations in the North Pacific. The ENSO impact is mainly meridional from the tropics to the mid–high latitudes, while a major fraction of the non-ENSO variability associated with PNA is confined in the zonal direction from the North Pacific to the North American continent. Such interferential connection on PNA as well as on North American climate variability may reflect a competition between local internal dynamical processes (unpredictable fraction) and remote forcing (predictable fraction). Model responses to observed sea surface temperature and model forecasts confirm that the remote forcing is mainly associated with ENSO and it is the major source of predictability of PNA and winter precipitation in North America.

Open access
Yaping Wang, Xiaopeng Cui, Xiaofan Li, Wenlong Zhang, and Yongjie Huang

Abstract

A set of kinetic energy (KE) budget equations associated with four horizontal flow components was derived to study the KE characteristics during the genesis of Tropical Cyclone (TC) Durian (2001) in the South China Sea using numerical simulation data. The genesis process was divided into three stages: the monsoon trough stage (stage 1), the midlevel mesoscale convective vortex (MCV) stage (stage 2), and the establishment stage of the TC vortex (stage 3). Analysis showed that the KE of the symmetric rotational flow (SRF) was the largest and kept increasing, especially in stages 2 and 3, representing the symmetrization process during TC genesis. The KE of the SRF was mainly converted from the KE of the symmetric divergent flow (SDF), largely transformed from the available potential energy (APE). It was found that vortical hot towers (VHTs) emerged abundantly, aggregated, and merged within the MCV region in stages 1 and 2. From the energy budget perspective, massive moist-convection-produced latent heat was concentrated and accumulated within the MCV region, especially in stage 2, and further warmed the atmosphere, benefiting the accumulation of APE and the transformation from APE to KE. As a result, the midlevel circulation (or MCV) grew strong rapidly. In stage 3, the intensity and number of VHTs both decreased. However, affected by increasing lower-level inward radial wind, latent heat released by the organized convection, instead of disorganized VHTs in the first two stages, continuously contributed to the strengthening of the surface TC circulation as well as the warm core.

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Xiaofan Li, C-H. Sui, K-M. Lau, and M-D. Chou

Abstract

The simulations of tropical convection and thermodynamic states in response to different imposed large-scale forcing are carried out by using a cloud-resolving model and are evaluated with the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment observation. The model is forced either with imposed large-scale vertical velocity and horizontal temperature and moisture advections (model 1) or with imposed total temperature and moisture advections (model 2). The comparison of simulations with observations shows that bias in temperature and moisture simulations by model 1 is smaller than that by model 2. This indicates that the adjustment of the mean thermodynamic stability distribution by vertical advection in model 1 is responsible for better simulations.

Model 1 is used to examine effects of different parameterized solar radiative and cloud microphysical processes. A revised parameterization scheme for cloud single scattering properties in solar radiation calculations is found to generate more solar heating in the upper troposphere and less heating in the middle and lower troposphere. The change in the vertical heating distribution is suggested to stabilize the environment and to cause less stratiform cloud that further induces stabilization through cloud–IR interaction. The revised scheme also causes a drier middle and lower troposphere by weakening vertical moisture flux convergence. Also tested is the effect of a revised parameterization scheme for cloud microphysical processes that tends to generate more ice clouds. The cloud-induced thermal effect in which less ice cloud leads to less infrared cooling at cloud top and more heating below cloud top is similar to the effect of no cloud–radiation interaction shown in a sensitivity experiment. However, the exclusion of cloud–radiation interaction causes drying by enhancing condensation, and the reduction of ice clouds by the microphysics scheme induces moistening by suppressing condensation.

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