2010jcli3719.1
Diurnal Variations of Precipitation,Deep Convection,and Lightning over and East of the Eastern Tibetan Plateau
W EIXIN X U AND E DWARD J.Z IPSER
Department of Atmospheric Sciences,University of Utah,Salt Lake City,Utah
(Manuscript received10March2010,in?nal form9September2010)
ABSTRACT
Diurnal cycles of total rainfall,precipitation features,mesoscale convective systems(MCSs),deep con-vection,precipitation vertical structure,and lightning over the eastern Tibetan Plateau(TP)and eastward through China are investigated using11yr of Tropical Rainfall Measuring Mission(TRMM)measurements. Diurnal cycles of rainfall and precipitation features present apparent phase propagation eastward from the eastern TP for about1000km.The phase propagation is most evident during the pre-mei-yu and mei-yu seasons.However,it weakens with the northward progress of the East Asian monsoon and ceases in mid-summer.During the pre-mei-yu season,diurnal cycles of storm population,total rainfall,deep convection, and lightning over the central and eastern TP foothills are in phase,peaking during the early morning.An-other striking feature of the pre-mei-yu season is that nocturnal rainfall and MCSs prevail over the south-eastern TP foothills following the deep convection and lightning maxima.These nocturnal peaks of deep convection and lightning over the foothills shift to afternoon after the onset of the monsoon,but nocturnal precipitation still dominates.Over the less mountainous region of eastern China,deep convection usually has an afternoon peak during the mei-yu whereas the rainfall maximum is at night.In midsummer,most parts of eastern China have strong afternoon peaks of deep convection,precipitation,and lightning,except in north-eastern China where deep convection has an afternoon peak followed by a nocturnal precipitation peak.
1.Introduction
The diurnal cycle of precipitation and convection is one of the most fundamental characteristics of a regional weather regime.It provides an important test for the validation of model physics in both weather and climate models(Lin et al.2000;Trenberth et al.2003;Dai et al. 1999;Dai and Trenberth2004).Because of the complex terrain and evident seasonal change of weather regimes (Tao and Chen1987;Chen2004;Ding and Chan2005; Xu et al.2009),East Asia has been used frequently for investigating diurnal variations of rainfall and convec-tive activities and their underlying physics(summarized in,e.g.,Domros and Peng1988;Zhao et al.2005). Over East Asia,in addition to the widespread after-noon rainfall peak,nocturnal rainfall peaks are often found in the valleys,foothills of high terrain,and over lakes and coastlines due to the low-level convergence by mountain–valley breezes and land–sea breezes(Ohsawa et al.2001;Fujinami et al.2005;Hirose and Nakamura 2005;Chen et al.2005;Li et al.2008).In addition,east-ward phase propagation of precipitation and cloudiness is found downstream of the eastern TP(Asai et al.1998; Wang et al.2004,2005;Yu et al.2007b;Zhou et al.2008; Chen et al.2009).This diurnal phase propagation is similar to that observed leeward of the Rocky Mountains (Wallace1975;Carbone et al.2002;Carbone and Tuttle 2008).It is further found that phase propagation of di-urnal cycles weakens along with the progress of the East Asian monsoon and almost ceases in midsummer(Asai et al.1998;Wang et al.2005;Chen et al.2009).Wavelike propagation of afternoon convection,mountain–plain circulation,and nocturnal low-level jets are thought to be possible mechanisms responsible for the propagation and nocturnal rainfall(He and Zhang2010;Huang et al. 2010).
Most of the above mentioned papers are focused on diurnal variations of precipitation and cloudiness by using just rain gauge or passive remote sensing obser-vations(e.g.,Asai et al.1998;Wang et al.2005;Yu et al. 2007b;Chen et al.2009).Three-dimensional information
Corresponding author address:Weixin Xu,Dept.of Atmo-spheric Sciences,University of Utah,135S1460E,Rm.819,Salt Lake City,UT84112-0110.
E-mail:weixin.xu@https://www.360docs.net/doc/9b16261330.html,
DOI:10.1175/2010JCLI3719.1
ó2011American Meteorological Society
of storms is never provided.It is hard to recognize what kind of storms(vertical structures)primarily contribute to the nocturnal rainfall in those studies.However,di-urnal cycles of convection,properties of storms,lightning, and precipitation vertical structures are necessary for a better understanding of the underlying physical mech-anisms,for evaluating rainfall retrievals,and for physical comparisons with model simulations(Nesbitt and Zipser 2003;Hirose and Nakamura2005;Nesbitt et al.2008;Liu and Zipser2008).
The Tropical Rainfall Measuring Mission(TRMM) Precipitation Feature(PF)database combines compre-hensive information from the Precipitation Radar,Mi-crowave Imager,and Lightning Sensor together on the storm scale(Nesbitt et al.2000;Liu et al.2008a).This database is collocated well enough to investigate the phase difference between diurnal cycles of precipitation,deep convection,shallow convection,mesoscale convective sys-tems(MCSs),and lightning(Nesbitt and Zipser2003;Liu and Zipser2008).It is now possible to measure how the vertical structures of precipitating systems vary diur-nally by this database(Liu and Zipser2008;Liu et al. 2008b).Furthermore,this feature-based database can pro-vide unique insights into the phase propagation of the di-urnal cycle from the perspective of precipitating storms. Starting from that point,this study seeks to answer the following motivating questions:1)Is there any phase propagation of diurnal variations of precipitating storms downstream from the eastern TP and how does it vary seasonally?2)What are the phase differences among diurnal cycles of precipitation,deep convection,and lightning,and how do these change seasonally?3)How does the precipitation vertical structure vary diurnally?
4)What are the possible mechanisms responsible for the phase propagation and nocturnal rain or convection? During the warm season,the most evident change in the large-scale circulation and rainfall patterns happens with the onset of the mei-yu pattern(Tao and Chen1987; Ding1992).After the onset of mei-yu,deep southwest-erlies from the tropics prevail over southern China and rainbands associated with the mei-yu front occur fre-quently(Chen1994;Ding and Chan2005;Chen2004). The convection intensity and storm properties of mei-yu systems also vary from those of before onset(Xu et al.2009).Therefore,this study focuses on the region of the eastern TP and continent downstream in different sea-sons:before the onset of mei-yu,the mei-yu season,and midsummer.
In this paper,details of the data and methods are described in section2.Section3shows the11-yr clima-tology of the large-scale?ow,rainfall,and storm activity. Results of phase propagation of diurnal cycles,phase differences among different parameters,and their sea-sonal variations are presented in https://www.360docs.net/doc/9b16261330.html,parisons between the afternoon and midnight peaks of rainfall and storm activity are given in section5.Section6 presents a summary showing how the changes in zonal wind pro?les may be related to the phase propagation, and how the diurnal wind changes in the low levels may affect the diurnal cycles of rainfall.A brief summary of our principal conclusions is given in section7.
2.Data and methods
a.ECMWF reanalysis data
The European Centre for Medium-Range Weather Forecasts(ECMWF)Re-Analysis Interim reanalysis data(ERA-Interim;Berrisford et al.2009;Dee and Uppala2009)between1998and2008are used to ex-amine seasonal transitions and diurnal variations.The ERA-Interim dataset has a horizontal spatial resolution of1.5831.58and6-hourly temporal resolution.This dataset has37vertical levels ranging from1000to1hPa. Most of the levels are at25-hPa intervals.
b.TRMM PF and3B42datasets
This study uses11yr(1998–2008)of the TRMM PF database(Nesbitt et al.2000;Liu et al.2008a).PFs are identi?ed as contiguous near-surface raining areas derived from the TRMM Precipitation Radar(PR).Measure-ments from different instruments are collocated before grouping them into PR pixels with the adjacent-pixel method(Liu et al.2008a).Standard products of1B11, 1B01,2A23,Lightning Imaging Sensor(LIS)orbital data, 2A25(Iguchi et al.2000),and2A12(Kummerow et al. 1998)are put into the collocation process.Parameters
T ABLE1.Parameters selected for the study of diurnal variations and their de?nitions.
Parameter Name De?nition
PF Num Population of precipitation features de?ned by grouping PR pixels(Nesbitt et al.2000;Liu et al.2008a) MCS Num Population of MCSs de?ned by PFs with area.2000km2having at least one convective pixel(Houze1993) Total rain Volumetric near-surface rainfall retrieved from PR(Iguchi et al.2000)
Deep Convec Area of deep convection with20-dB Z PR at12km(similar to Liu and Zipser2008)
Flash Counts Total?ash counts within all the selected PFs detected by LIS(Cecil et al.2005)
such as radar re?ectivity,lightning,and microwave or IR brightness temperature can be derived directly from each pixel of the PFs at the resolution of the PR,that is, 4.2km before and5.1km after the boost of the satellite in2001.
The TRMM Multisatellite Precipitation Analysis (TMPA)3B42rain product(Huffman et al.2007)is used to study the spatial distribution of seasonal rain-fall and diurnal–nocturnal rainfall.The3B42product has3-hourly temporal resolution and0.25830.258 spatial resolution,covering the globe from508S to508N, and is available from1998to2008.This algorithm uses both passive-microwave measurements from low-earth orbit satellites and infrared radiance measurements from geostationary satellites.Multiple passive microwave rain estimates are?rst calibrated to TRMM PR and TRMM Microwave Imager(TMI)estimates before their combi-nation.IR estimates are generated using the calibrated microwave estimates and are used to?ll the passive mi-crowave coverage gaps.The3B42rain estimates con-verge to TMI and PR estimates when they are available. Studies showed that3B42has good correspondence with rain gauge data for the rainfall amount and rainfall spatial pattern in China(Zhou et al.2008;Shen et al.2010). c.De?nition of total rain,MCS,and deep
convection
Most of the studies on diurnal cycles focus on pre-cipitation,but it is quite important to know the storm type, convection,and three-dimensional structure of precipita-tion systems contributing to the diurnal peaks.This study provides diurnal information on the structures of storms, and lightning,in addition to precipitation.Five parame-ters are selected and de?ned as follows(Table1).
1)P OPULATION OF PF
Precipitation features are de?ned by grouping con-tiguous near-surface PR pixels(Nesbitt et al.2000;Liu et al.2008a).In this study,a PF has at least100km2.The re?ectivity of the boundary of the PF is18–20dB Z,since the minimum detectable re?ectivity for PR is about ;(18–20)dB Z.
2)P OPULATION OF MCS
An MCS is de?ned as a PF with area.2000km2 having at least one convective pixel(Awaka et al.1997). This de?nition guarantees an MCS having a mesoscale horizontal precipitating area with at least one convec-tive cell embedded,as in the de?nition of Houze(1993). For this dataset,the population of MCSs is about10% of the total PFs(Table2).
3)T OTAL RAIN
Total rain is de?ned by the total volumetric near-surface rainfall of a PF retrieved from the PR2A25al-gorithm(Iguchi et al.2000).The total rain volume of a PF is the product of the area and mean rain rate of the feature.
4)D EEP CONVECTION
Deep convection is de?ned by the area of the20-dB Z radar echo at12km MSL(similar to Liu and Zipser 2008).By this de?nition,storms may or may not over-shoot the tropopause(about14;15km)but
have F IG.1.Map of the eastern TP and downstream area.Elevation is given by the color bar.Dashed black boxes are selected strips(from north to south):strips1,2,and3.Solid black boxes marked by capital letters are different regions for diurnal study.The Sichuan basin,Yangtze River,and TP regions are discussed in the text.
T ABLE2.Samples of precipitation features in the three latitude strips:strip1(328–368N),strip2(288–328N),and strip3(238–288N);
speci?cally,the eight boxes in Fig.1.
Strip1(328–368N)Strip2(288–328N)Strip3(238–288N)
A B C D E F G H Premonsoon480847473781669445074155932564576 Mei-yu117616724403597975266404853525526 Midsummer17127105099074158627160540469495390
precipitation-size ice particles lofted into the upper troposphere.
5)F LASH COUNTS
The total ?ash counts detected by LIS include both intracloud and cloud-to-ground lightning ?ashes within all the selected PFs (Nesbitt et al.2000;Cecil et al.2005).Although the number of total ?ashes may be caused by a single cell of a small thunderstorm or multiple cells of a large MCS,the presence of lightning is a well-known indicator of fairly intense convection (Cecil et al.2005;Zipser et al.2006).
d.Analysis methods
Generally,the East Asian summer monsoon has its early phase (mei-yu)?rst onset over South China in the middle of May (Chen 1983;Ding 1992).After the onset of mei-yu,the large-scale circulation and precipitation pattern change signi?cantly from before (Ding and Chan 2005;Chen 1994;Xu et al.2009).However,the westerly steering winds (300–500hPa)at 208–308N are diminishing with the progress of the monsoon and van-ish by mid-July (Murakami 1958;Murakami and Ding 1982;Chen 1993;Wang et al.2005).As one of the
major
F I
G https://www.360docs.net/doc/9b16261330.html,posite wind ?elds at (left)850and (right)500hPa over TP and downstream for (a),(b)pre-mei-yu,(c),(d)mei-yu,and (e),(f)midsummer during 1998–2008.Scale of wind vector is marked in the dashed box.Continental borders and elevations of 1500and 3000m are contoured.
purposes of the study is to investigate the seasonal var-iability of the diurnal cycles and the phase propagation phenomenon,it is essential to consider the seasonal change of the tropospheric ?ow.Therefore,it is rea-sonable to de?ne the ?rst time period as ‘‘pre-mei-yu’’during 1April–11May before the onset of the monsoon,the second as ‘‘mei-yu’’during 15May–25June in the monsoon phase with signi?cant steering winds,and the third as ‘‘midsummer’’during 1July–10August when steering winds disappear.
The spatial distribution of the PF population is cal-culated in each 28328box,with the latitude-dependent sampling bias removed.The sampling bias is considered by calculating the difference of the total PR pixels over each box between 1998and 2008.The bias is corrected by the bias factor de?ned as the fraction of the PR pixel number over each box to the mean PR pixel samples over the whole study region.The region of eastern TP and the downstream region (238–368N,908–1208E)is ?rst divided into three strips (dashed boxes in Fig.1):strip 1(328–368N),strip 2(288–328N),and strip 3(238–288N).Each strip is further separated into three boxes with different elevations (solid boxes in Fig.1):938–1038E,1038–1128E,and 1128–1208E.To describe the seasonal variations of the diurnal cycle,PFs are grouped into different seasons,strips,and regions.The PF samples in each strip and box during different seasons are listed in Table 2.The hour-dependent sampling bias of the TRMM PR for each box is also calculated and removed in a way sim-ilar to that used to correct for latitude-dependent sampling bias.
Spatial distributions of the diurnal cycles of PFs (or
rainfall)over each strip are presented in Hovmo
¨ller di-agrams similar to those of Carbone et al.(2002),but here they are done with 2-hourly and 28-longitude
bins.
F I
G .3.The 3B42-based (left)mean monthly rainfall in mm month 21and (right)latitude-bias-corrected total precipitation feature numbers in 28328boxes during (a),(b)pre-mei-yu,(c),(d)mei-yu,and (e),(f)midsummer during 1998–2008.
Diurnal cycles of occurrence frequency are derived from the summation of each de?ned parameter in eight local time bins (3-h bin)over each selected box during each
speci?c season for 11yr.In both the Hovmo
¨ller diagram and the frequency distribution diagram,the 1:2:1?lter is applied.The 1:2:1?lter is de?ned as the mean value of values in three consecutive hour–longitude bins with different weight [i.e.,N i 5(N i 2112N i 1N i 11)/4].The diurnal cycle of the vertical precipitation structures are presented by the time–height contoured frequency by altitude diagrams (CFADs;3-hourly and 1-km bin)of the occurrence frequency of the total area of the PFs with radar re?ectivity $20dB Z in each box.The oc-currence frequency is unconditional.It is de?ned as the fraction of the total area to the total PR sampling area in a speci?c box.In the study of nocturnal (2330–0530LT)and after-noon (1130–1730LT)rainfall and convection,the local time (LT)is de?ned as UTC plus 7h.In the TRMM 3B42dataset,the 3-hourly data point is de?ned as the total of the 1.5h behind and ahead;for example,0000UTC is 2230–0130UTC.Therefore,2330–0530LT covers both 1800and 2100UTC,while 1130–1730LT covers both 0600and 0900UTC.
Using the ERA-Interim dataset,composite wind ?elds at 850and 500hPa during different seasons are gen-erated separately to show the seasonal transition on the large-scale ?ow.The same ERA-Interim dataset is also used to construct vertical pro?les of seasonal mean hor-izontal wind over the region (288–328N,1058–1158E),showing diurnal phase propagation.Vertical cross sections of the U component at 0100LT (1800
UTC)
F I
G .4.(top)Hovmo
¨ller diagram of precipitation features during pre-mei-yu over different strips:(a)strip 1,(b)strip 2,and (c)strip 3.(bottom)The elevation along the longitude of each
strip.
F I
G .5.As in Fig.4,but for the mei-yu season.
and 1300LT (0600UTC)are further created along the latitude of 308N.Finally,the anomaly wind ?elds at 0100and 1300LT at 850hPa are de?ned by the de-viation of the 0100and 1300LT wind ?elds from the daily mean wind ?eld.
3.Climatology of large-scale ?ow and rainfall https://www.360docs.net/doc/9b16261330.html,rge-scale ?ow at 850and 500hPa
Figure 2presents the seasonal transitions of wind ?elds at 850and 500hPa over East Asia based on
the
F I
G .6.As in Fig.4,but for the midsummer
season.
F I
G .7.Diurnal variations of various parameters in pre-mei-yu over different regions:(a)–(h)regions A–
H shown in Fig.1(marked by
capital letters).
ERA-Interim dataset.Seasonal transitions of the at-mospheric?ow are quite evident at both levels.Be-fore the onset of the monsoon,anticyclonic circulation (the subtropical high)dominates over southeastern Asia,with westerly?ow over the tropical South China Sea and southwesterly over southern China at both the850-and500-hPa levels.The subtropical ridge is located at about208N at850hPa and158N at500hPa and extends across the Indo-China Peninsula.At500hPa, strong westerlies prevail throughout East Asia north of208N.
After the onset of the monsoon,the subtropical anticyclonic circulation is replaced by strong south and southwesterly?ow out of the deep tropics that brings a large amount of relatively warm and moist tropical air to the East Asian monsoon region.Also, the westerlies in the midtroposphere weaken and retreat northward.By midsummer,these westerlies re-treat to northern China and the subtropical ridge prog-resses up to southern China.Although the southwesterly winds in the lower troposphere over southern China weaken during midsummer,they extend to northern China.One feature to note is that near the surface,east-erlies?owing into the Sichuan basin,although persistent,weaken with the progress of the seasons.As can be seen in later sections and in the literature(Yu et al.2009;Chen et al.2010),the diurnal variations of the low-level wind change seasonally.
b.Rainfall and precipitation features
In general,the seasonal changes of the spatial distri-bution of rainfall from the3B42product and the pre-cipitation features(Fig.3)are consistent with that of the large-scale?ow,especially at850hPa(Fig.2). During pre-mei-yu,the heaviest rainfall is broadly dis-tributed across southeast China between228and308N with a rainfall maximum of300mm month21,and only 100mm month21close to the foothills of the TP.During the mei-yu,rainfall over the South China Sea,the Bay of Bengal,and the foothills of TP increases markedly while remaining mostly south of the Yangtze River.The heaviest rainfall maxima located over southern China and Taiwan are mostly contributed by the mei-yu rain-bands(Xu et al.2009).Both rainfall and precipitation systems?nally shift to the north of the Yangtze River with the northward penetration of the southwesterly?ow in
midsummer.
F IG.8.As Fig.7,but for the mei-yu season.
4.Diurnal variations of storm population,total rain,deep convection,and lightning a.Downstream phase propagation 1and its geographic and seasonal variability
Generally,diurnal cycles of storm population and pre-cipitation are dominated by afternoon maxima over high terrain,but in some seasons and locations they exhibit a phase delay,moving eastward into the lowlands,result-ing in nocturnal maxima,except in midsummer (Figs.4–6).This con?rms ?ndings based on Geosynchronous Meteo-rological Satellite (GMS)retrievals by Asai et al.(1998)and Wang et al.(2005),and is similar to the phase propagation east of the Rocky Mountains during the warm season (Carbone et al.2002).Another general feature is that the phase propagation is most evident in strip 2in Figs.4b and 5b,regions D and E during pre-mei-yu and mei-yu,but disappears in midsummer (Fig.6b).
b.Phase differences of precipitation,deep convection,and lightning
The interpretation of the Hovmo
¨ller diagrams (Figs.4–6)showing phase propagation (or not)in certain locations and seasons can be clari?ed by also showing the diurnal cycles of the precipitation feature number,total rainfall,MCS number,?ash counts,and deep convection,all of which are shown in Figs.7–9.There are too many interesting details to describe in this paper,so the following discussion is con-?ned to the regimes that seem least ambiguous.
Before discussing the phase propagation (or its ab-sence)in detail,we examine the relatively simple situ-ation on the Tibetan Plateau (regions A and D).During all three seasons,these two regions have strong after-noon maxima in PF number,lightning ?ash counts,and deep convection,followed some hours later by maxima in MCS number and total rain (Figs.7a,d,8a,d,and 9a,d).This common sequence of events is strong evi-dence of the well-known local control of convection over the TP rather than large-scale dynamics (Uyeda et al.2001;Qie et al.2003).The time lag is reasonable for the organization of local convection into larger meso-scale systems,from which both convective and strati-form rain may fall for several
hours.
F I
G .9.As in Fig.7,but for the midsummer season.
1
In this paper,phase propagation refers to the apparent total phase speed in the zonal direction,without implying anything about storm propagation with respect to the wind at any level.
pre-mei-yu,(middle)mei-yu,and(right)midsummer.
F IG.11.As in Fig.10,but for regions(a)–(c)F and(d)–(e)H.
The Sichuan basin shows a marked phase propagation during pre-mei-yu and mei-yu (Figs.4e and 5e),all but disappearing in midsummer (Fig.6e).However,the speci?c sequence of the peaking of each parameter is dif?cult to interpret in detail.The only clear result is that during pre-mei-yu and mei-yu,consistent with the notion of phase propagation of mesoscale phenomena eastward from the TP,all measures have nocturnal maxima,but they are not as sharp as those seen during the afternoon on the TP.The contrast in midsummer is striking (Fig.9e).While there is no evident phase propagation in the total number of PFs (Fig.6e),which actually have a weak afternoon maximum that suggests some locally generated convection,there are nocturnal maxima of deep convection,lightning,and early morning maxima of MCSs and total rain,suggesting that both deep convection and mesoscale systems have a mech-anism favoring the nocturnal generation in the Sichuan basin,which is further explored in sections 5and 6.
c.Diurnal variations of the vertical structure of precipitation A great advantage of the TRMM Precipitation Radar data,in spite of the large attenuation at its 2-cm wave-length,is its ability to reveal vertical pro?les of radar re?ectivity,especially in the upper troposphere.If the at-tenuation correction (Iguchi et al.2000)is applied through a large depth of strong echo,its accuracy does suffer,but the height reached by the 20-dB Z echo is not affecte
d.Here,we construct the relative frequency of the radar echo area $20dB Z at each altitude over selected regions and seasons (Figs.10and 11).
Over the Sichuan basin (box E),consistent with pre-vious ?ndings,extremely deep convection is present with 20dB Z to 14km from 0000to 0600LT during pre-mei-yu (Fig.10a).But during mei-yu and midsummer,the data reveal a very different diurnal cycle in
vertical
F I
G .12.The 3B42-based seasonal total rainfall (mm)occurring during (left)midnight–early morning (2330–0530LT)
and (right)afternoon (1130–1730LT)during (a),(b)pre-mei-yu,(c),(d)mei-yu,and (e),(f)midsummer.
structure.The ?rst part of the early morning maxima still extends up to 15km but this apparently evolved from an earlier (1800–2100LT)peak.The second part of the early morning peak (0300–0600LT)only goes up to 10km during mei-yu and to 8km during midsummer,perhaps indicating a predominance of decaying MCSs and stratiform rain,consistent with Fig.9e.
Box G demonstrates a completely different situation.In all three seasons,there is a prominent late afternoon and evening peak of extremely deep radar echo,consis-tent with Figs.7g,8g,and 9g,and their lightning peaks,while much lower echo tops occur during early morning.The diurnal cycles of the radar echo structure down-stream of the above regions (boxes F and H)are different from E and G,but do not present any clear phase prop-agation from upstream.Multiple maxima are observed
except during midsummer,when the afternoon peak of the very deep echo dominates.In all seasons,region F has an early morning peak with shallow echo tops,which,together with the morning rainfall maxima in Figs.7f,8f,and 9f,implies a role for decaying MCSs with stratiform rain.Region H has a complex structure in the pre-mei-yu,evolving toward a simple domination of an afternoon maximum in midsummer,also shown in Figs.7h,8h,and 9h.
5.Nocturnal versus afternoon peaks in rainfall,PF numbers,and lightning a.Spatial distribution of rainfall
As mentioned above,the MCS population and total PR volumetric rain have nocturnal peaks over the
foothills
F I
G .13.Percentage of precipitation features to the total PFs occurring in a 18318box during (left)midnight–early morning (2330–0530LT)and (right)afternoon (1130–1730LT)during (a),(b)pre-mei-yu,(c),(d)mei-yu,and (e),(f)midsummer.
of the TP and over central eastern China in certain sea-sons.The spatial distribution of nocturnal rainfall based on TRMM 3B42rainfall data (Fig.12)is quite consistent with the above ?ndings and with previous studies (Yu et al.2007b;Li et al.2008).During pre-mei-yu,there is a band-shaped nocturnal maximum extending from the Sichuan basin to the coast.In contrast,afternoon rainfall maxima exist along and within 500km of the coast of south and central China,with a prominent minimum in the Sichuan basin.The east–west nocturnal rainfall center may be related to the propagation of precipitation systems downstream of the eastern TP as demonstrated previously.The band-shaped nocturnal maximum downstream of the eastern TP is less evident during mei-yu and disap-pears in midsummer.Nocturnal rainfall maxima still appear over the south and southeast foothills of the TP,but closer to the foothills.The spatial pattern of afternoon
rainfall is quite similar in mei-yu and midsummer,except that signi?cant rainfall extends northward and spreads through all of eastern China,while the afternoon mini-mum persists in the Sichuan basin.
b.Distribution of PFs and lightning features Spatial distributions of the occurrence of precipitating features during 2330–0530LT versus 1130–1730LT during different seasons are presented in Fig.13.Figure 14shows the distribution of PFs by numbers of lightning ?ashes during selected periods in different seasons.Over the TP,afternoon storms dominate in every season,similar to the classic diurnal characteristics over high terrain (Fig.13).
In the foothills of the southeastern TP and eastward for 1000km,higher percentages of nocturnal PFs occur,especially during the pre-mei-yu.Many of these
nocturnal
F I
G .14.Distribution of precipitation features categorized by lightning ?ash rate occurring during the (left)mid-night–early morning (2330–0530LT)and (right)afternoon (1130–1730LT)in (a),(b)pre-mei-yu,(c),(d)mei-yu,and (e),(f)midsummer.Values of the different colors are shown in the color bar.
precipitation features have lightning of moderate to high ?ash rate,indicating either intense convection,large me-soscale convective systems,or both.But during mei-yu this nocturnal peak is less obvious.Lightning downstream of the TP is even less frequent in the afternoon during mei-yu than nocturnal lightning during the pre-mei-yu. This seasonal difference in lightning activity has been described in Xu et al.(2010)and is consistent with their ?nding of higher radar re?ectivity from the TRMM PR in the mixed phase region.
6.Discussion:Possible mechanisms for the variability
of phase propagation and diurnal cycles
Previous sections show that the nocturnal and after-noon peaks of rainfall and other parameters show both different seasonal and geographic distributions.Here, we ask how the seasonal changes in zonal wind pro?le and how the diurnal cycle in the low-level?ow?eld may help explain the more striking features of the rainfall cycle,most notably,the dominance of the nocturnal and early morning rainfall in the Sichuan basin.
a.Relating the phase propagation to the
zonal wind pro?le
The most obvious phase propagation has been shown to be in strip2,the latitude belt from288to328N extending from the high TP through the Sichuan basin and eastward. To supplement the large-scale?ow?elds at just two levels (Fig.2)for this region,Fig.15shows the mean zonal wind pro?le for the region of strongest phase propagation for each season.Both pre-mei-yu and mei-yu have substantial zonal wind components above the700-hPa level,as well as strong wind shear,especially in pre-mei-yu.In mid-summer,the mean zonal wind in this belt essentially dis-appears.
Carbone et al.(2002)and others have demonstrated a recurrent pattern of phase propagation of rainfall maxima in the central United States downstream of the Rocky Mountains,related to the maintenance of deep convection and mesoscale systems,initially phase locked to afternoon convection over the Rockies,resulting in nocturnal maxima well downstream,moving in the gen-eral direction of‘‘steering-level winds’’near700hPa,but typically about8m s21faster.It is well beyond the scope of this paper to distinguish between the many possible reasons for a speci?c speed with respect to the wind at any particular level.But we do note that the wind pro?le in and east of the Sichuan basin(Fig.15)is fairly close to that of Carbone et al.(2002).Further,the observed phase prop-agation in the Sichuan basin is between about12and 15m s21in mei-yu(from Figs.4e and5e)compared with Carbone et al.’s14m s21.It is an open question whether our results should be interpreted as having a phase speed 10–12m s21faster than our700-hPa wind,or whether our steering-level winds should be taken closer to600hPa(the TP is higher than the Rocky Mountains),in which case our results also imply a phase speed about8m s21faster. b.Relating the diurnal cycle of precipitation features
and rainfall to the diurnal cycle of low-level winds Despite the rather coarse1.58resolution of the ERA-Interim reanalysis data used in this paper,the changes between the1800UTC(0100LT)and0600UTC(1300LT) low-level wind?elds are suf?ciently large to help explain the diurnal cycle of the observed rainfall and pre-cipitation systems.Figure16shows the west–east verti-cal cross section of the zonal winds through the center of the Sichuan basin for each season,while Fig.17shows the anomaly wind?elds from the seasonal mean for0100 and1300LT at850hPa for a region that includes the Sichuan basin.
Above700hPa,there is little change in the tropo-spheric zonal wind pro?le between0100and1300LT, but there is a large change in the low-level winds.Spe-ci?cally,the low-level easterly?ow near850hPa con-verges in the center of the Sichuan basin at0100LT in all seasons,near1058E,but at1300LT in all seasons that convergence is along the slope of the Tibetan Plateau near1038E.This convergence of about1m s21may seem small,but it is consistent with rising motion on the slopes during the afternoon,and rising motion in the center of the basin during the https://www.360docs.net/doc/9b16261330.html,ing the slightly better resolution of1.08of the National Oceanic
and F IG.15.Vertical pro?le of mean zonal wind?elds over the region 288–328N,1058–1158E during pre-mei-yu,mei-yu,and midsummer during1998–2008.A plus sign(1)indicates the data points from Carbone et al.(2002)over the United States.
Atmospheric Administration (NOAA)Global Forecast System (GFS),He and Zhang (2010)demonstrated a very similar magnitude of diurnal wind change,consistent with a similar phase propagation and diurnal cycle of rainfall along the terrain slope in northern China.
Several authors [including Carbone et al.(2002)and He and Zhang (2010)]have pointed to the existence of a nocturnal low-level in?ow of moist air that may assist the formation and maintenance of the nocturnal pre-cipitation in many regions of the world.The anomaly wind ?eld in this case (Fig.17)supports this interpre-tation in the Sichuan basin as well.In all three seasons,the 850-hPa ?ow changes from southerly to northerly with a vector change of about 2m s 21between 0100and 1300LT.In addition,the change from convergence to divergence between 0100and 1300LT is obvious,and at least as large a magnitude as that shown by He and Zhang (2010)for their study region in northern China (their Fig.5).While the absolute wind speeds hardly
qualify for the label ‘‘low-level jet’’in the seasonal mean,the low-level ?ow change is consistent with a more favorable ?ow of potentially unstable air into the Sichuan basin at night.
7.Conclusions
The principal ?ndings of this study include the fol-lowing points:
1)There is a phase propagation of diurnal cycles of precipitating storms,total rainfall,convection,and lightning from the foothills of the eastern TP down-stream,which is most evident over the Sichuan basin during the pre-mei-yu and mei-yu seasons,absent dur-ing midsummer,and consistent with a lack of zonal winds and zonal shear during midsummer.
2)Before midsummer,the eastern TP foothills are dom-inated by nocturnal rainfall,but the early morning
peak
F I
G .16.Cross section of mean zonal wind speed (m s 21;dashed is positive,dotted is negative with intervals of 20.5m s 21)along 298–308N,during pre-mei-yu,mei-yu,and midsummer during 1998–2008.Shown are data for (left)0100and (right)1300LT.
of precipitation is only in phase with deep convection,MCSs,and lightning during pre-mei-yu.
3)Most of the nocturnal precipitation is in phase with MCSs and possibly contributed to by long-lived MCSs evolving from late afternoon or early night convection,but these early morning MCSs show larger percentages of deep convection in pre-mei-yu.4)In midsummer,most of East Asia is dominated by af-ternoon precipitation and convection,with the Sichuan basin and northern part of eastern China as the excep-tions where early morning rainfall prevails,in spite of most deep convection and lightning occurring during the afternoon.Most of the midnight and early morning rainfall is contributed by MCSs.
5)In all three seasons,there is strong low-level con-vergence in the Sichuan basin during night time with southerly winds ?owing into the basin,consistent with the nocturnal maximum of rainfall,while di-vergence during the day is consistent with a marked rainfall minimum.
Acknowledgments.This research was supported by NASA Precipitation Measurement Mission Grant NAG5-13628under the direction of Dr.Ramesh Kakar.Special thanks are given to the TRMM Science Data and In-formation System (TDSIS)group led by Drs.Erich Stocker and John Kwiatkowski at NASA Goddard Space Flight Center,Greenbelt,Maryland,for data processing assistance.Many thanks also go to Dr.Chuntao Liu at the University of Utah,Salt Lake City,Utah,for science dis-cussions.Careful and constructive reviews by Dr.Richard Carbone and two anonymous reviewers resulted in sig-ni?cant improvements to the manuscript and are greatly appreciated.
REFERENCES
Asai,T.,S.Ke,and Y.-M.Kodama,1998:Diurnal variability of
cloudiness over East Asia and the western Paci?c Ocean as revealed by GMS during the warm season.J.Meteor.Soc.Japan,76,
675–684.
F I
G .17.Anomaly wind ?elds at 850hPa during (top to bottom)pre-mei-yu,mei-yu,and midsummer during 1998–2008at (left)0100and (right)1300LT.Wind speed is indicated by the arrow on the right-hand side.
Awaka,J.,T.Iguchi,H.Kumagai,and K.Okamoto,1997:Rain type classi?cation algorithm for TRMM precipitation radar.
Proc.1997Int.Geoscience and Remote Sensing Symp.,Sin-gapore,IEEE,1633–1635.
Berrisford,P.,D.P.Dee,K.Fielding,M.Fuentes,P.Kallberg, S.Kobayashi,and S.M.Uppala,2009:The ERA-Interim Archive.ERA Rep.Series,No.1,ECMWF,16pp. Carbone,R.E.,and J.D.Tuttle,2008:Rainfall occurrence in the United States in the warm season:The diurnal cycle.J.Cli-mate,21,4132–4136.
——,——,D.A.Ahijevych,and S.B.Trier,2002:Inferences of predictability associated with warm season precipitation epi-sodes.J.Atmos.Sci.,59,2033–2056.
Cecil,D.J.,S.J.Goodman,and D.J.Boccippio,2005:Three years of TRMM precipitation features.Part I:Radar,ra-diometric,and lightning characteristics.Mon.Wea.Rev.,133, 543–566.
Chen,C.-S.,W.-C.Chen,Y.-L.Chen,P.-L.Lin,and https://www.360docs.net/doc/9b16261330.html,i, 2005:Investigation of orographic effects on two heavy rainfall events over southwestern Taiwan during the Mei-yu season.
Atmos.Res.,73,101–130.
Chen,G.,W.Sha,and T.Iwasaki,2009:Diurnal variation of pre-cipitation over southeastern China:Spatial distribution and its seasonality.J.Geophys.Res.,114,D13101,doi:10.1029/ 2008JD011103.
Chen,G.T.-J.,1983:Observational aspects of the Meiyu phenomena in subtropical China.J.Meteor.Soc.Japan,61,306–312.——,1994:Large-scale circulations associated with the East Asian summer monsoon and the mei-yu over South China and Taiwan.
J.Meteor.Soc.Japan,72,959–983.
——,2004:Research on the phenomena of Meiyu during the past quarter century:An overview.The East Asian Monsoon,
C.-P.Chang,Ed.,World Scienti?c,357–403.
Chen,H.,R.Yu,J.Li,W.Yuan,and T.Zhou,2010:Why nocturnal long-duration rainfall presents an eastward-delayed diurnal phase of rainfall down the Yangtze River valley.J.Climate,23, 905–917.
Chen,Y.-L.,1993:Some synoptic-sacle aspects of the surface fronts over southern China during TAMEX.Mon.Wea.Rev.,121, 50–64.
Dai,A.,and K.E.Trenberth,2004:The diurnal cycle and its de-piction in the Community Climate System Model.J.Climate, 17,930–951.
——,F.Giorgi,and K.E.Trenberth,1999:Observed and model simulated precipitation diurnal cycle over the contiguous United States.J.Geophys.Res.,104,6377–6402.
Dee,D.P.,and S.Uppala,2009:Variational bias correction of satellite radiance data in the ERA-Interim reanalysis.Quart.
J.Roy.Meteor.Soc.,135,1830–1841.
Ding,Y.,1992:Summer monsoon rainfalls in China.J.Meteor.Soc.
Japan,70,373–396.
——,and J.C.-L.Chan,2005:The East Asian summer monsoon: An overview.Meteor.Atmos.Phys.,89,117–142. Domros,M.,and G.Peng,1988:The Climate of China.Springer-Verlag,360pp.
Fujinami,H.,S.Nomura,and T.Yasunari,2005:Characteristics of diurnal variations in convection and precipitation over the southern Tibetan Plateau during summer.SOLA,1,49–52. He,H.,and F.Zhang,2010:Diurnal variations of warm-season precipitation over northern China.Mon.Wea.Rev.,138, 1017–1025.
Hirose,M.,and K.Nakamura,2005:Spatial and diurnal variation of precipitation systems over Asia observed by the TRMM
Precipitation Radar.J.Geophys.Res.,110,D05106,doi:10.
1029/2004JD004815.
Houze,R.A.,Jr.,1993:Cloud Dynamics.Academic Press,573pp. Huang,H.-L.,C.C.Wang,G.T.-J.Chen,and R.E.Carbone,2010: The role of diurnal solenoidal circulation on propagating rainfall episodes near the eastern Tibetan plateau.Mon.Wea.
Rev.,138,2975–2989.
Huffman,G.J.,and Coauthors,2007:The TRMM Multisatellite Precipitation Analysis(TMPA):Quasi-global,multiyear, combined-sensor precipitation estimates at?ne scales.J.Hy-drometeor.,8,38–55.
Iguchi,T.,T.Kozu,R.Meneghini,J.Awaka,and K.Okamoto, 2000:Rain-pro?ling algorithm for the TRMM Precipitation Radar.J.Appl.Meteor.,39,2038–2052.
Kummerow,C.,W.Barnes,T.Kozu,J.Shiue,and J.Simpson,1998: The Tropical Rainfall Measuring Mission(TRMM)sensor package.J.Atmos.Oceanic Technol.,15,809–817.
Li,J.,R.Yu,and T.Zhou,2008:Seasonal variations of the diurnal cycle of rainfall in the southern contiguous China.J.Climate, 21,6036–6043.
Lin,X.,D.A.Randall,and L.D.Fowler,2000:Diurnal variability of the hydrologic cycle and radiative?uxes:Comparisons be-tween observations and a GCM.J.Climate,13,4159–4179. Liu,C.,and E.J.Zipser,2008:Diurnal cycles of precipitation,clouds, and lightning in the tropics from9years of TRMM observations.
Geophys.Res.Lett.,35,L04819,doi:10.1029/2007GL032437.——,——,D.J.Cecil,S.W.Nesbitt,and S.Sherwood,2008a:A cloud and precipitation feature database from nine years of TRMM observations.J.Appl.Meteor.,47,2712–2728.——,——,G.G.Mace,and S.Benson,2008b:Implications of the differences between daytime and nighttime CloudSat obser-vations over the tropics.Geophys.Res.Lett.,113,D00A04, doi:10.1029/2008JD009783.
Murakami,T.,1958:The sudden change of upper westerlies near the Tibetan Plateau at the beginning of summer season.
J.Meteor.Soc.Japan,36,239–247.
——,and Y.Ding,1982:Wind and temperature changes over Eurasia during the early summer of1979.J.Meteor.Soc.Japan, 60,182–196.
Nesbitt,S.W.,and E.J.Zipser,2003:The diurnal cycle of rainfall and convective intensity according to three years of TRMM measurements.J.Climate,16,1456–1475.
——,——,and D.J.Cecil,2000:A census of precipitation features in the Tropics using TRMM:Radar,ice scattering,and light-ning observations.J.Climate,13,4087–4106.
——,D.J.Gochis,and https://www.360docs.net/doc/9b16261330.html,ng,2008:The diurnal cycle of clouds and precipitation along the Sierra Madre Occidental ob-served during NAME-2004:Implications for warm season precipitation estimation in complex terrain.J.Hydrometeor., 9,728–743.
Ohsawa,T.,H.Ueda,T.Hayashi,A.Watanabe,and J.Matsumoto, 2001:Diurnal variations of convective activity and rainfall in tropical Asia.J.Meteor.Soc.Japan,79,333–352.
Qie,X.,R.Toumi,and T.Yuan,2003:Lightning activities on the Tibetan Plateau as observed by the lightning imaging sensor.
J.Geophys.Res.,108,4551,doi:10.1029/2002JD003304. Shen,Y.,A.Xiong,Y.Wang,and P.Xie,2010:Performance of high-resolution satellite precipitation products over China.
J.Geophys.Res.,115,D02114,doi:10.1029/2009JD012097. Tao,S.,and L.Chen,1987:A review of recent research on the East Asian summer monsoon in China.Monsoon Meteorology,
C.-P.Chang and T.N.Krishnamurti,Eds.,Oxford University
Press,60–92.
Trenberth,K.E.,A.Dai,R.M.Rasmussen,and D.B.Parsons, 2003:The changing character of precipitation.Bull.Amer.
Meteor.Soc.,84,1205–1217.
Uyeda,H.,and Coauthors,2001:Characteristics of convective clouds observed by a Doppler radar at Naqu on Tibetan Plateau during the GAME-Tibet IOP.J.Meteor.Soc.Japan,79,463–474. Wallace,J.M.,1975:Diurnal variations in precipitation and thunderstorm frequency over the conterminous United States.
Mon.Wea.Rev.,103,406–419.
Wang,C.C.,G.T.J.Chen,and R.E.Carbone,2004:A climatology of warm-season cloud patterns over East Asia on GMS in-frared brightness temperatures observations.Mon.Wea.Rev., 132,1606–1629.
——,——,and——,2005:Variability of warm season cloud epi-sodes over East Asia based on GMS infrared brightness tem-perature observations.Mon.Wea.Rev.,133,1478–1500.
Xu,W.,E.J.Zipser,and C.Liu,2009:Rainfall characteristics and convective properties of mei-yu precipitation systems over South China,Taiwan and the South China Sea.Part I:TRMM observations.Mon.Wea.Rev.,137,4261–4275.——,——,——,and H.Jiang,2010:On the relationship between lightning frequency and thundercloud parameters of regional
precipitation systems.J.Geophys.Res.,115,D12203,doi:10.1029/ 2009JD013385.
Yu,R.,Y.Xu,T.Zhou,and J.Li,2007a:Relation between rainfall duration and diurnal variation in the warm season precipita-tion over central eastern China.Geophys.Res.Lett.,34,L13703, doi:10.1029/2007GL030315.
——,T.Zhou,A.Xiong,Y.Zhu,and J.Li,2007b:Diurnal varia-tions of summer precipitation over contiguous China.Geo-phys.Res.Lett.,34,L01704,doi:10.1029/2006GL028129.——,J.Li,and H.Chen,2009:Diurnal variation of surface wind over central eastern China.Climate Dyn.,33,1089–1097. Zhao,Z.,L.R.Leung,and Y.Qian,2005:Characteristics of diurnal variations of precipitation in China for the recent years.
CLIVAR Exchanges,No.34,International CLIVAR Project Of?ce,Southampton,United Kingdom,24–26.
Zhou,T.,R.Yu,H.Chen,A.Dai,and Y.Pan,2008:Summer pre-cipitation frequency,intensity,and diurnal cycle over China:A comparison of satellite data with rain gauge observations.
J.Climate,21,3997–4010.
Zipser,E.J.,D.J.Cecil,C.Liu,S.W.Nesbitt,and D.P.Yorty, 2006:Where are the most intense thunderstorms on Earth?
Bull.Amer.Meteor.Soc.,87,1057–1071.