Weihua Yuan

Why Nocturnal Long-Duration Rainfall Presents an Eastward-Delayed Diurnal Phase of Rainfall down the Yangtze River Valley

Hourly observational records and 6-hourly reanalysis data were used to investigate the influences of large-scale forcings on the diurnal variation of summer rainfall along the Yangtze River (YR). The results show that longduration (more than six hours) rainfall events dominate the summer rainfall along the YR. These events tend to start during the night and to peak after several hours of development. The eastward-delayed initiation of the nocturnal long-duration rainfall events is thought to be due to the diurnal clockwise rotation of the lowtropospheric circulation, especially the accelerated nocturnal southwesterlies. In the early evening, the anomalous easterly flow toward the Tibetan Plateau (TP) causes low-level convergence over the Plateau’s eastern slope that induces the formation of rainfall in the upper YR valley. The anomalous wind sequentially rotates clockwise to a southerly flow at midnight and accelerates the meridional wind in the middle valley, resulting in the initiation of rainfall between 2300 and 0300 LST. In the early morning, the accelerated southwesterlies in southern China, when combined with decelerated winds in the north of the YR, causes a strong convergence along the YR and contributes to the early morning rainfall in the lower valley. Furthermore, the development of the convection systems is suppressed in the afternoon by the mid- and low-level warm advection downstream from the TP. This helps explain why long-duration events do not typically start in the afternoon in the upper YR valley.

Subseasonal characteristics of diurnal variation in summer monsoon rainfall over central eastern China.

Subseasonal characteristics of the diurnal variation of the summer monsoon rainfall over central eastern China(258‐408N,1108‐1208E) areanalyzedusinghourlystationraingaugedata.Resultsshowthattherainfall in the monsoon rain belt is dominated by the long-duration rainfall events (

Regimes of Diurnal Variation of Summer Rainfall over Subtropical East Asia

7 h) with early-morning peaks. The long-duration rainfall events and early-morning diurnal peaks experience subseasonal movement that is similar to that of the monsoon rain belt. When the monsoon rainfall is separated into the active and break periods, the long-duration early-morning precipitation dominates the active period, which is in sharp contrast to the short-duration (#6 h) rainfall with leading late-afternoon diurnal peaks during the break period. The combination of different diurnal features of monsoon rainfall in the active and break monsoon periods also explainsthelesscoherentdiurnalphasesofsummermeanrainfallovercentraleasternChina.Thecauseofthe early-morning peak of rainfall during the active monsoon period is discussed.

Precipitation over East Asia simulated by NCAR CAM5 at different horizontal resolutions

Using hourly rain gauge records and Tropical Rainfall Measuring Mission 3B42 from 1998 to 2006, the authors present an analysis of the diurnal characteristics of summer rainfall over subtropical East Asia. The study shows that there are four different regimes of distinct diurnal variation of rainfall in both the rain gauge and the satellite data. They are located over the Tibetan Plateau with late-afternoon and midnight peaks, in thewesternChinaplainwithmidnighttoearly-morningpeaks,intheeasternChinaplainwithdoublepeaksin lateafternoonandearlymorning,andovertheEastChinaSeawithanearly-morningpeak.Nopropagationof diurnal phases is found from the land to the ocean across the coastlines. The different diurnal regimes are highly correlated with the inhomogeneous underlying surface, such as the plateau, plain, and ocean, with physical mechanisms consistent with the large-scale ‘‘mountain‐valley’’ and ‘‘land‐sea’’ breezes and convective instability. These diurnal characteristics over subtropical East Asia can be used as diagnostic metrics to evaluate the physical parameterization and hydrological cycle of climate models over East Asia.

Diurnal Cycle of Summer Precipitation over Subtropical East Asia in CAM5

Long-term simulations using version 5.1 of the National Center for Atmospheric Research’ Community Atmosphere Model at low (T42), medium (T106), and high (T266) resolutions were carried out to study the impact of horizontal resolution on the models performance in reproducing the climatological features of precipitation over East Asia. The simulated spatial pattern of annual mean precipitation amount improves significantly with increased resolution. The low-resolution model is inadequate to reproduce the precipitation closely associated with fine-scale orographic forcing, such as the narrow large-rainfall belt along the southern edge of the Tibetan Plateau. The distribution of rainfall over and around the elevation of the Tibetan Plateau and high-altitude mountains becomes more realistic at higher resolutions. The proportion of the large-bias (small-bias) area continuously reduces (increases) when moving from T42 to T266 resolution. Simulations at all three resolutions can capture the key features of the major seasonal variation of rainfall arising from the onset and advancement of the Asian monsoon. A novel method is used to evaluate the sensitivity of the simulated intensity-frequency structure to the horizontal resolution. The proportion of light rain, which demonstrates large positive bias in climate models, decreases dramatically at higher resolution. The intensity-frequency structures averaged over steep-terrain regions and plain areas become more distinctive and realistic as the resolution increases.

Changes in Characteristics of Late-Summer Precipitation over Eastern China in the Past 40 Years Revealed by Hourly Precipitation Data

The simulations of summertime diurnal cycle of precipitation and low-level winds by the Community Atmosphere Model, version 5, are evaluated over subtropical East Asia. The evaluation reveals the physical cause of the observed diurnal rainfall variation in East Asia and points to the source of model strengths and weaknesses. Two model versions with horizontal resolutions of 2.88 and 0.58 are used. The models can reproduce the diurnal phase of large-scale winds over East Asia, with an enhanced lowlevel southwesterly in early morning. Correspondingly, models successfully simulated the diurnal variation of stratiform rainfall with a maximum in early morning. However, the simulated convective rainfall occurs at local noontime, earlier than observations and with larger amplitude (normalized by the daily mean). As a result, models simulated a weaker diurnal cycle in total rainfall over the western plain of China due to an out-of-phase cancellation between convective and stratiform rainfalls and a noontime maximum of total rainfall over the eastern plain of China. Over the East China Sea, models simulated the early-morning maximum of convective precipitation and, together with the correct phase of the stratiform rainfall, they captured the diurnal cycle of total precipitation. The superposition of the stratiform and convective rainfalls also explains the observed diurnal cycle in total rainfall in East Asia. Relative to the coarse-resolution model, the high-resolution model simulated slight improvement in diurnal rainfall amplitudes, due to the larger amplitude of stratiform rainfall. The two models, however, suffer from the same major biases in rainfall diurnal cycles due to the convection parameterization.

Changes in Duration-Related Characteristics of Late-Summer Precipitation over Eastern China in the Past 40 Years

Abstract Using hourly station rain gauge data during 1966–2005, the authors studied changes in the characteristics of the late-summer (July–August) rainfall, which has exhibited a so-called southern flooding and northern drought (SFND) pattern over eastern China in recent decades. Although the rainfall amount and frequency have significantly increased (decreased) in the mid–lower reaches of the Yangtze River valley (North China) during this period, the rainfall intensity has decreased (increased). This finding differs from previous results based on daily data, which showed that the rainfall intensity has increased in the mid–lower reaches of the Yangtze River valley. In this region, the mean rainfall hours on rainy days have increased because of the prolonged rainfall duration, which has led to an increased daily rainfall amount and to a decreased hourly rainfall intensity. Results also show that the SFND pattern is mostly attributed to changes in precipitation with moderate and low intensity (≤10 mm h−1)...

A possible cause for different diurnal variations of warm season rainfall as shown in station observations and TRMM 3B42 data over the southeastern Tibetan plateau

AbstractDuration is a key feature of rainfall events that is closely related to rainfall mechanisms and influences. This study analyzes the decadal change in the duration-related characteristics of late-summer (July–August) precipitation over eastern China during 1966–2005. Accompanying the southern-flooding and northern-drought (SFND) pattern of rainfall amount over the eastern China in recent decades, the duration-related rainfall structure also experienced significant changes. In North China, the frequency of short duration rainfall events decreased and their intensity increased. The decadal decreases of rainfall amount over North China are largely contributed by long duration rainfall events, especially those occurring between midnight and morning. In the mid-to-lower reaches of the Yangtze River valley, both the frequency and amount of long duration precipitation have significantly increased. The mean and maximum duration time of late-summer precipitation has increased 0.85 and 7.61 h, respectively. ...

Progress in Studies of the Precipitation Diurnal Variation over Contiguous China

In this study, records from a 3-yr intensified observational experiment at eight stations along the hillside of Seqilashan over the southeastern Tibetan Plateau were analyzed and combined with records at 28 routine observation stations in the Chinese National Meteorological Station Network to investigate the influences of station location on the different diurnal rainfall variations between station records and Tropical Rainfall Measuring Mission (TRMM) data products. The results indicate that the diurnal variation of warm season rainfall is closely related to location of stations. The prevailing nocturnal rainfall peak in observations at routine stations can be largely attributed to the relatively lower location of the stations, which are mostly situated in valleys. The records at Seqilashan stations on hillsides revealed an evident diurnal afternoon peak of warm season rainfall, similar to that indicated by TRMM data. The different diurnal phases between valley and hillside stations are closely related to the orographically induced regional circulations caused by the complex topography over the Tibetan Plateau. The results of this study indicate that the prevailing nocturnal rainfall associated with the relatively lower location of routine observation stations can partially explain the diurnal rainfall variations between observation station records and TRMM data.

Dynamic and Thermodynamic Relations of Distinctive Stratus Clouds on the Lee Side of the Tibetan Plateau in the Cold Season

This paper summarizes the recent progress in studies of the diurnal variation of precipitation over contiguous China. The main results are as follows. (1) The rainfall diurnal variation over contiguous China presents distinct regional features. In summer, precipitation peaks in the late afternoon over the southern inland China and northeastern China, while it peaks around midnight over southwestern China. In the upper and middle reaches of Yangtze River valley, precipitation occurs mostly in the early morning. Summer precipitation over the central eastern China (most regions of the Tibetan Plateau) has two diurnal peaks, i.e., one in the early morning (midnight) and the other in the late afternoon. (2) The rainfall diurnal variation experiences obvious seasonal and sub-seasonal evolutions. In cold seasons, the regional contrast of rainfall diurnal peaks decreases, with an early morning maximum over most of the southern China. Over the central eastern China, diurnal monsoon rainfall shows sub-seasonal variations with the movement of summer monsoon systems. The rainfall peak mainly occurs in the early morning (late afternoon) during the active (break) monsoon period. (3) Cloud properties and occurrence time of rainfall diurnal peaks are different for long- and short-duration rainfall events. Long-duration rainfall events are dominated by stratiform precipitation, with the maximum surface rain rate and the highest profile occurring in the late night to early morning, while short-duration rainfall events are more related to convective precipitation, with the maximum surface rain rate and the highest profile occurring between the late afternoon and early night. (4) The rainfall diurnal variation is influenced by multi-scale mountain-valley and land-sea breezes as well as large-scale atmospheric circulation, and involves complicated formation and evolution of cloud and rainfall systems. The diurnal cycle of winds in the lower troposphere also contributes to the regional differences in the rainfall diurnal variation. (5) Evaluation of the model performance shows that the present numerical models are weak in simulating the rainfall diurnal variation over contiguous China. The simulations are not significantly improved by increasing the model horizontal resolution alone. The key is to reduce the uncertainty in physical parameterizations related to the rainfall processes.