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Jian-Jian Wang

Abstract

The evolution and structure of mesoscale convection in the South China Sea (SCS) region are documented for the first time mainly using the dual-Doppler radar dataset collected during the South China Sea Monsoon Experiment (SCSMEX) in 1998. In particular, this study focuses on the convection associated with a subtropical frontal passage during the early onset of the southeast Asian monsoon (SEAM).

For the case of 15 May 1998, interaction between the tropical monsoon flow and frontal circulation played an important role in the evolution and structure of mesoscale convection. In the prefrontal region, the southwesterly monsoon flow converged with the southwesterly frontal flow to generate northeast-to-southwest-oriented convection. In the postfrontal region, the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east-to-west orientation. In addition, the convergence between the southerly monsoon flow and the northerly postfrontal flow generated deeper and stronger low-level convergence. The postfrontal convection was more intense and deeper than the prefrontal convection.

The precipitation and kinematic structure of mesoscale convection were studied with special attention to significant departures from archetypal tropical oceanic convection. On 15 May, prefrontal convection showed a straight upward rainfall and updraft pattern with little tilt as a result of moderate vertical wind shear. The maximum low-level convergence and updraft were 20–30 km behind instead of within 1–2 km of the leading edge. Although the convection was intense with maximum reflectivity over 50 dBZ, both pre- and postfrontal convection had a very limited stratiform region as a result of a dry environmental upper layer. The observed mesoscale convection had a tendency to form stratiform rain ahead of the convective rain, and two different modes of leading stratiform structure were found separately in pre- and postfrontal convection.

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Jian Zhang and Shunxin Wang

Abstract

An automated 2D multipass velocity dealiasing scheme has been developed to correct velocity fields when wind velocities are very large compared to the Nyquist velocity of the weather Doppler radars. The new velocity dealiasing algorithm is based on the horizontal continuity of velocity fields. The algorithm first determines a set of reference radials and gates by finding the weakest wind region. Then from these reference radials and gates, the scheme checks continuities among adjacent gates and corrects for the velocity values with large differences that are close to 2 × (Nyquist velocity). Multiple passes of unfolding are performed and velocities identified as “folded” with low confidence in an earlier pass are not unfolded until a discontinuity is detected with high confidence at a subsequent pass. The new velocity dealiasing scheme does not need external reference velocity data as do many existing algorithms, thus making it more easily applicable. Over 1000 radar volume scans that include tornadoes, hurricanes, and typhoons are selected to test and to evaluate the new algorithm. The results show that the new algorithm is very robust and very computationally efficient. In cases with many data voids, the new algorithm shows improvements over the current WSR-88D operational velocity dealiasing scheme. The new dealiasing algorithm is a simple and stand-alone program that can be a very useful tool to various Doppler radar data users.

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Yi-Leng Chen and Jian-Jian Wang

Abstract

The diurnal variation in the surface thermodynamic fields over the island of Hawaii is presented for the first time. The diurnal ranges in temperature and dewpoint vary considerably over the island. In addition to the diurnal heating cycle, the thermodynamic fields over the island are also related to orography, airflow, and distributions of clouds and rains.

On the windward side, the onset of upslope (downslope) flow in the morning (evening) is closely related to the thermal contrasts between the slope surface and the upstream air. After sunrise around 0600 HST, the onset of upslope flow occurs at approximately 0730 HST on the upper slopes where the virtual temperature first becomes warmer than the environment. In the early evening, the onset of downslope flow occurs just before sunset about 1900 HST in the windward lowland where the virtual temperature first drops below the environmental value. These results suggest that the diurnal evolution of the surface flow on the windward side is closely controlled by the evolution of the surface thermodynamic fields.

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Jian-Jian Wang and Yi-Leng Chen

Abstract

The near-surface winds and thermal profiles on the windward side of the island of Hawaii are studied using the dataset from the Hawaiian Rainband Project (HaRP), with special attention to the early morning and late afternoon transition periods that separate the daytime upslope and nighttime downslope flow regimes. The effects of rain showers on the development of daytime upslope and nighttime downslope flow are also discussed.

Before sunrise (∼0600 HST), the temperature profiles are usually characterized by a nocturnal inversion about 50–150 m above the ground, with a strength of 1–4 K. The stable downslope flow layer usually extends above the nocturnal inversion with a nocturnal jet beneath the inversion. For rain cases, the nocturnal inversion and the nocturnal jet are weaker with a deeper stable downslope flow than clear cases because of rain evaporation aloft, reduced infrared radiation heat loss at the lowest levels, and vertical mixing associated with precipitation. After sunrise the nocturnal inversion weakens and eventually disappears as a result of surface heating. Three types of upslope flow onset are observed: 1) the upslope flow appears simultaneously within the downslope flow layer after this layer becomes well mixed and warmer than the environment; 2) slope flow develops near the surface because of surface heating, progressing upward; and 3) upslope flow onset occurs above the nocturnal inversion, progressing downward because of turbulence mixing after sunrise.

In the early afternoon, a superadiabatic layer is observed in the lowest levels and disappears before sunset (∼1900 HST). Normally, the surface temperature of the slope surface becomes colder than the environment about 1–2 h before sunset with the nocturnal inversion forming near the surface. About 30–45 min later, the downslope flow starts at the surface and progresses upward. It strengthens gradually during the night. Several cases during HaRP show a much earlier downslope flow onset and a deep downslope flow layer on the windward lowland because of precipitation. The early downslope flow onset on the lower slopes is caused by the surface virtual temperature on the lower slopes becoming colder than the environment at an earlier time because of the evaporative cooling of failing raindrops. The development of a deep downslope layer is caused by the evaporative cooling aloft. Occasionally, the downdraft outflow from continuous afternoon rain showers on the upper slopes can also contribute to the early downslope flow onset on the lower slopes. This type of downslope flow is localized in nature.

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Yi-Leng Chen and Jian-Jian Wang

Abstract

The effects of precipitation on the surface temperature and airflow over the island of Hawaii, which are not considered in previous studies, are presented. It is found that clouds and rains can modify the surface thermal fields and result in changes in the intensity of diurnal circulations and the timing of wind shifts from downslope (upslope) to upslope (downslope) flow at the surface in the early morning (late afternoon). The onset of upslope (downslope) flow at the surface is closely related to the virtual temperature anomalies on the slope surface from the adjacent environment for both the rain and dry cases.

Prior to sunrise, the rain cases feature higher surface temperatures in contrast to the dry cases because of a more extensive cloud cover, which reduces the longwave radiation, and the precipitating downdrafts, which bring the warmer air above the nocturnal inversion to the surface. Hence, at the surface a weaker (stronger) downslope flow is observed for the rain (dry) cases, which is consistent with warmer (colder) temperatures on the slope surface. After sunrise, because of reduced insulation by clouds, evaporative cooling of raindrops, and slower heating of the wet surface, the rain cases have a slower surface temperature increase than the dry cases. For the rain cases, the latest turning from downslope to upslope flow at the surface occurs in the Hilo area where the total rainfall is the largest. For the dry cases, the latest upslope flow onset is at the eastern tip of the island where the surface temperature remains colder than the environment after sunrise.

In the afternoon, the extensive cloud cover, the evaporative cooling of rain showers, and moist soil conditions contribute to a lower surface temperature and result in the weaker upslope flow at the surface for the rain cases than for the dry cases. During the evening hours, the surface temperature decrease is slower for the rain cases than for the dry cases because of a reduction of longwave radiation heat loss due to a more extensive cloud cover. For the rain cases, the evaporative cooling and precipitation downdrafts cause a low surface temperature and the early downslope flow onset at the surface in the Hilo area, whereas on the upper slope, the orographic clouds reduce the outgoing longwave radiation and delay the turning from the upslope to downslope flow.

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Jian-Jian Wang and Yi-Leng Chen

Abstract

A case study of trade-wind rainbands observed on 22 August 1990 during the Hawaiian Rainband Project is presented. It shows that the interaction between the morning rainbands and the island-induced airflow is important for the evolution of the rainbands. In the early morning of 22 August, there are two convective periods, 0400–0600 and 0700–0900 HST (Hawaii standard time), on the windward side of the island of Hawaii. For both periods, preexisting rain cells are observed in the trade-wind flow at least 40 km upstream of the island and move westward toward the island.

At night and in the early morning, the offshore flow opposes the trade winds resulting in a convergent region over the area immediately upstream of the island. As the first group of rain cells (0400–0600 HST) moves toward the island, the low-level convergent airflow provides a favorable kinematic background for the enhancement of the coming rain cells. These rain cells merge in the convergent zone and become a well-defined rainband. However, after the first rainband meets the offshore flow, the cool air feeds into the lowest levels of the rainband. This is an unfavorable thermal condition for the rainband and is thus partly responsible for the decay of the first rainband over the windward lowlands. After the arrival of the first rainband, the depth of the offshore flow at Hilo increases from about 250 m to over 500 m. Its horizontal extent also extends from approximately 10 km to more than 20 km offshore.

The second group of rain cells (0700–0900 HST) also becomes a well-defined rainband as it moves over the convergent zone. Interacting with a deep and extensive offshore flow resulting from precipitation effects from the first rainband, the rain cells associated with the second rainband are much deeper and stronger than the first rainband. The second rainband moves toward the island during the morning transition, during which the offshore flow retreats and onshore flow begins. After the onset of the onshore flow, the low-level airflow in the Hilo Bay region diverges and splits around the island. This provides an unfavorable dynamic condition for the maintenance of the rainband. Therefore, the second rainband weakens. It dissipates and only reaches the eastern tip of the island.

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Jian-Jian Wang and Lawrence D. Carey

Abstract

A primary goal of the South China Sea Monsoon Experiment (SCSMEX), a major field campaign of the Tropical Rainfall Measuring Mission (TRMM), is to define the initiation, structure, evolution, and dynamics of precipitation processes associated with the onset of the South China Sea (SCS) summer monsoon. In this study, dual-Doppler and dual-polarimetric radar analysis techniques are used to investigate the development and structure of a squall-line system observed on 24 May 1998. The focus is the linkage between the airflow and the microphysical fields through the system.

The squall-line system, including three distinct lines, persisted from 1200 UTC 24 May to the following day. A detailed study was performed on the structure of the second and most intense line, lasting for over 10 h. Compared to tropical squall lines observed in other regions, this narrow squall-line system had some interesting features including 1) maximum reflectivity as high as 55 dBZ; 2) relatively little stratiform rainfall that preceded instead of trailed the convective line; and 3) a broad vertical velocity maximum in the rear part of the system, rather than a narrow ribbon of vertical velocity maximum near the leading edge.

Polarimetric radar–inferred microphysical (e.g., hydrometeor type, amount, and size) and rainfall properties are placed in the context of the mesoscale morphology and dual-Doppler-derived kinematics for this squall-line system. A comparison is made between results from this study for SCSMEX and the previous studies for the TRMM Large-Scale Biosphere–Atmosphere experiment (LBA). It was found that precipitation over the SCS monsoon region during the summer monsoon onset was similar to the precipitation over the Amazon monsoon region during the westerly regime of the TRMM–LBA, which has previously been found to be closer to typical conditions over tropical oceans. Both of these cases showed lower rain rates and rainwater contents, smaller raindrops, and significantly lower ice water contents between 5 and 8 km than the precipitation over the Amazon during the easterly regime of the TRMM–LBA with more tropical continental characteristics.

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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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Bin Wang, Xiao Luo, and Jian Liu

Abstract

Instrumental observations (1901–2017) are used to uncover the seasonality, regionality, spatial–temporal coherency, and secular change of the relationship between El Niño–Southern Oscillation (ENSO) and Asian precipitation (AP). We find an abrupt seasonal reversal of the AP–ENSO relationship occurring from October to November in a large area of Asia north of 20°N due to a rapid northward shift of the ENSO-induced subsidence from Indonesia to the Philippines. We identified six subregions that have significant correlations with ENSO over the past 116 years with |r| > 0.5 (p < 0.001). Regardless of the prominent subregional differences, the total amount of AP during a monsoon year (from May to the next April) shows a robust response to ENSO with r = −0.86 (1901–2017), implying a 4.5% decrease in the total Asian precipitation for 1° of SST increase in the equatorial central Pacific. Rainfall in tropical Asia (Maritime Continent, Southeast Asia, and India) shows a stable relationship with ENSO with significant 31-yr running correlation coefficients (CCs). However, precipitation in North China, the East Asian winter monsoon front zone, and arid central Asia exhibit unstable relationships with ENSO. Since the 1950s, the AP–ENSO relationships have been enhanced in all subregions except over India. A major factor that determines the increasing trends of the AP–ENSO relationship is the increasing ENSO amplitude. Notably, the AP response is asymmetric with respect to El Niño and La Niña and markedly different between the major and minor ENSO events. The results provide guidance for seasonal prediction and a metric for assessment of climate models’ capability to reproduce the Asian hydroclimate response to ENSO and projected future change.

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Chunhan Jin, Bin Wang, and Jian Liu

Abstract

An accurate prediction of land monsoon precipitation (LMP) is critical for the sustainable future of the planet as it provides water resources for more than two-thirds of the global population. Here, we show that the ensemble mean of 24 CMIP6 (phase 6 of the Coupled Model Intercomparison Project) models projects that, under the Shared Socioeconomic Pathway 2–4.5 (SSP2–4.5) scenario, summer LMP will very likely increase in South Asia (~4.1% °C−1), likely increase in East Asia (~4.6% °C−1) and northern Africa (~2.9% °C−1), and likely decrease in North America (~−2.3% °C−1). The annual mean LMP in three Southern Hemisphere monsoon regions will likely remain unchanged due to significantly decreased winter precipitation. Regional mean LMP changes are dominated by the change in upward moisture transport with moderate contribution from evaporation and can be approximated by the changes of the product of the midtropospheric ascent and 850-hPa specific humidity. Greenhouse gas (GHG)-induced thermodynamic effects increase moisture content and stabilize the atmosphere, tending to offset each other. The spatially uniform increase of humidity cannot explain markedly different regional LMP changes. Intermodel spread analysis demonstrates that the GHG-induced circulation changes (dynamic effects) are primarily responsible for the regional differences. The GHGs induce a warm land–cool ocean pattern that strengthens the Asian monsoon, and a warm North Atlantic and Sahara that enhances the northern African monsoon, as well as an equatorial central Pacific warming that weakens the North American monsoon. CMIP6 models generally capture realistic monsoon rainfall climatology, but commonly overproduce summer rainfall variability. The models’ biases in projected regional SST and land–sea thermal contrast likely contribute to the models’ uncertainties in the projected monsoon rainfall changes.

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