Evolution of the Population of Precipitating Convective Systems over the Equatorial Indian Ocean in Active Phases of the Madden Julian Oscillation

Transcription

1 SEPTEMBER 2013 Z U L UAGA AND HOUZE 2713 Evolution of the Population of Precipitating Convective Systems over the Equatorial Inian Ocean in Active Phases of the Maen Julian Oscillation MANUEL D. ZULUAGA AND ROBERT A. HOUZE JR. University of Washington, Seattle, Washington (Manuscript receive 15 November 2012, in final form 22 February 2013) ABSTRACT Three-imensional raar reflectivity fiels from a ual-wavelength Doppler polarimetric raar (S-PolKa) eploye in the equatorial Inian Ocean are use to evaluate the composition of the population of convective clou elements uring active phases of the MJO. Rainfall in active perios was intermittent, occurring in 11 episoes of about 2 4 ays, separate by several nonrainy ays. Data for these 2-ay perios were composite relative to the time of maximum rainfall. Analysis of the S-PolKa ata shows the makeup of the convective population uring the rainfall episoes. Four types of echo structures were analyze statistically for the 11 rainfall episoes: shallow convective echoes (SCE), eep convective cores (DCC), wie convective echo cores (WCC), an broa stratiform (BSR) echo regions. SCE an DCC events were most frequent before the maximum rainfall, with the peak frequency of SCE leaing that of DCCs. WCCs were most frequent uring the rainfall maximum, an BSR regions were most frequent in the later part of the rainfall episoe. Composite ECMWF Interim Re-Analysis (ERA-Interim) ata an 3-hourly atmospheric sounings inicate that the 2 4-ay episoes were relate to the passage of equatorial waves. In the early part of a rainfall episoe, the wave-passage conitions were unstable, favoring eep penetrating convective elements, while in the later perio the wave ivergence profile was commensurate with convective systems in late anvil-proucing stages. These results support the stretche builing-block notion of the life cycle of tropical convection an confirm satellitebase interpretations of SCE, DCC, WCC, an BSR statistics in the composition of the convective population. 1. Introuction The Maen Julian oscillation (MJO; Maen an Julian 1971, 1972) accounts for a major component of the intraseasonal variability of the tropical general circulation, an it is closely connecte with the variation of the convective clou population over the equatorial Inian Ocean (Zhang 2005). This stuy uses a series of observational atasets collecte uring Dynamics of the Maen Julian Oscillation (DYNAMO) fiel project an the Atmospheric Raiation Measurement Program (ARM) s MJO Investigation Experiment (AMIE). These partner campaigns collecte ata over the Inian Ocean an western Pacific, where the MJO is best efine. The goal was to obtain information that will allow scientists to iscern etails of the convective clou fiel an to relate these etails to the large-scale circulation of the MJO. Corresponing author aress: Manuel D. Zuluaga, Department of Atmospheric Sciences, University of Washington, Box , Seattle, WA mzuluaga@uw.eu The 4-month perio of ata collection in DYNAMO/ AMIE was long enough to provie etaile observation of cycles of the convective population uring occurrences of the MJO over the Inian Ocean. However, the area of etaile ata collection was small, an ultimately the results must be relate to the large-scale circulation of the tropics. To connect the measurements of convective an mesoscale clou systems in DYNAMO/AMIE to the MJO, it will be necessary to upscale the fiel ata to larger scales of motion. One important pathway for this upscaling is to relate the local observations at the fiel sites to satellite ata, which are global in reach. Orinary infrare or passive microwave imagery alone is limite in what it can provie in terms of insight into convective clous in the MJO because it oes not provie threeimensional structural information. Satellites with active raars on boar can, however, provie climatological information on the structure of eep convective systems. The Tropical Rainfall Measuring Mission (TRMM) satellite (Simpson an Aler 1988; Kummerow et al. 1998, 2000) has a 2-cm-wavelength precipitation raar on DOI: /JAS-D Ó 2013 American Meteorological Society

2 2714 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 70 boar, an ClouSat (Stephens et al. 2002) features a millimeter-wavelength clou raar. One of the limitations of using these tools is the ifficulty in etermining the probable life cycles of the convective entities proucing satellite-base statistics because TRMM an ClouSat sample any given location by using snapshots wiely separate in time. One goal of the present stuy is the use of the excellent time continuity of the fielprogram measurements to improve interpretations of the statistics obtaine with such satellite snapshot ata. Yuan an Houze (2010) an Yuan et al. (2011) have use ClouSat combine with infrare an passive microwave ata to etermine the climatology of mesoscale convective systems over the whole tropics. However, they were able only to ientify mesoscale convective systems in the mature stage of evelopment. Their technique cannot ientify eveloping or issipating systems. The TRMM satellite raar has been use to characterize the climatology of eep convection over continental regions an oceanic regions of South Asia an South America (Houze et al. 2007; Romatschke an Houze 2010; Romatschke et al. 2010; Rasmussen an Houze 2011; Barnes an Houze 2013). The TRMM methoology use in these stuies ientifies four types of echo objects: shallow isolate echoes (SCE), eep convective cores (DCC), wie convective cores (WCC), an broa stratiform regions (BSR). These papers assume that the four types of echo entities signify eep convective clou systems in early, mile, an late stages of evelopment. While this assumption seems reasonable in light of the literature on convective clou systems, it has never been verifie by surface-base raar ata. In this stuy we use groun-base raar ata obtaine with the National Center for Atmospheric Research (NCAR) S-PolKa raar eploye in the central Inian Ocean near the equator on Au Atoll in the Malives to etermine the occurrence of SCE, DCC, WCC, an BSR echoes relative to the evelopment in time of rainfall episoes in active phases of the MJO. Our results will show that the frequency istributions showing the occurrence of SCE, DCC, WCC, an BSR echoes peak at a sequence of times, respectively, an that the large-scale environment reflects these shifts in the membership of the clou population uring any given rainfall episoe. The results presente in this work thus confirm that that satellite-base statistics on the frequencies of occurrence of the four types of echo objects ientifie in TRMM raar ata inee inicate where eep convection forms, matures, moves, an issipates. Perhaps most importantly, this stuy also provies physical insight into the connection between the convective clou population an the larger-scale ynamics of the active phases of the MJO. In particular we will show the multiscale nature of the precipitation in the MJO. The MJO itself is the largest scale. It consists of active phases of about 2 3-weeks uration when convective rain occurs an another 2 3 weeks of ry weather. In DYNAMO it became clear that the rainfall perio oes not have a continuous nature but is episoic with perios of rain lasting about 2 4 ays separate by several ays of suppresse conitions, all within an active MJO perio. Unerstaning the precipitating clou population of the MJO then reuces to the unerstaning of the 2 4-ay episoes of rainfall. The 2 4-ay episoes in turn are larger in scale than iniviual convective elements. Even the largest mesoscale convective systems (MCSs) (Houze 2004) have lifetimes shorter than 2 4 ays. The 2 4-ay time scale is characteristic of certain types of equatorial waves (Kilais et al. 2009). We explore the possibility that the population of convective clous evolves in each 2 4-ay perio uner the control of intermeiate-scale equatorial wave. We take avantage of the DYNAMO souning ata along with reanalysis fiels to etermine how the raar-observe convective population associate with the two to four episoes of rainfall occurring uring MJO active phases evolves in concert with 2 4-ay wave passages. Equatorial waves of these time scales inclue 2-ay inertiogravity waves an mixe Rossby gravity waves (e.g., Matsuno 1966; Haertel an Kilais 2004; Kuang 2008; Kilais et al. 2009). In this paper, we will present evience that the convective clou population behaves in a way that is consistent with the stretche builing block notion of convective interaction with the largescale environment, as propose by Mapes et al. (2006). Those authors hypothesize that there is a natural selection in the atmosphere for wave packets whose phase structure prouces a local, Eulerian sequence of clouzone-supporting anomalies that aligns with the convective clou system life cycle (p. 26). We will show that the large-scale conitions varying over the DYNAMO/ AMIE area on the 2 4-ay time scale correspon to a stretching of the convective life cycle, an that evolution of the statistical preference for SCE, DCC, WCC, an BSR raar echoes is synchronize with this synopticscale variation as woul be expecte if the larger-scale wave phases were selectively aligning with convective life-cycle stages. 2. Datasets The S-PolKa raar was eploye on the islan of Hithaoo (0.68S, 73.18E), locate at the Au Atoll in the Malives, from 1 October 2011 through 15 January The S-PolKa scanning strategy inclue surveillance (SUR) scans for mapping the entire area of raar

3 SEPTEMBER 2013 Z U L UAGA AND HOUZE 2715 FIG. 1. (a) Beam elevation angles use for the surveillance scans of the S-PolKa raar uring DYNAMO/AMIE. The number at the en of each line gives the angle (8). (b) Azimuth angles use uring the elevation angle (RHI) scans. (c) RHI scan recoring angles for the S-PolKa raar. Dot ashe lines in (a) an (c) enote the initial istances for the range of raar ata use to etermine vertical heights (see text). coverage (3608 of azimuth) out to a range of 150 km. Data were recore in azimuth bins of 18. The SUR scans were conucte at elevation angles ranging from 0.58 to (Fig. 1a). Figure 1a shows that the maximum elevation angle for the SUR is limite in altitue such that it can etermine the heights of echoes greater than a given height only beyon a certain range. For example, to resolve reflectivity fiels up to 8-km height, one can only consier ranges between 41 km an the maximum raar range (150 km). The ot ashe lines in Fig. 1a inicate three such ranges of km for echo heights of 8 km, km for 10-km height, an km for 12-km heights. The SUR scans were followe by a set of elevation angle scans at a sequence of fixe azimuths (such elevation scans are known as RHI scans because in the early ays of raar they were isplaye on a screen known as a range height inicator ). Data were recore in elevation angle bins of RHI scans were esigne to recor ata with fine vertical resolution over the two azimuthal sectors shown in Fig. 1b. The first encompasse most of the northeastern quarant covering azimuths from 48 to 828 an was esigne to recor ata over a wie oceanic sector that was nearly free of surface clutter. The secon, a narrower sector over an oceanic southeastern quarant that covere azimuths from 1148 to 1408, was chosen to match the location of the vertically pointing KAZR raar place at the Department of Energy (DOE) ARM site on Gan Islan (0.78S, 73.28E), which was about 9 km from the S-PolKa site. RHI scans were conucte by moving the antenna continuously in the vertical irection so that ata coul be recore at angle increments from up to a 458 elevation (Fig. 1c). As for the SUR scans, only certain ranges of ata coverage were consiere (the initial range is inicate by the ot ashe line in Fig. 1c) because these ranges resolve reflectivity fiels up to a given echo height. The entire sequence of SUR an RHI scans was repeate at 15-min intervals. Figure 1b shows that the horizontal area of RHI scans covers only about km 2. This limite area coverage prevents the ientification of broa stratiform regions, which by efinition in the present stuy are at least km 2 in area, with the RHI scans alone. To be able to analyze the behavior of BSRs within the RHI sectors we first ientify BSRs using the SUR scans (which cover about km 2 ) an then we match the location of a BSR in a SUR scan to the corresponing raials in the RHI scan obtaine nearest in time to the SUR scan. The raar reflectivity ata use in this stuy was the final, quality-controlle S-PolKa ataset from the S-ban raar. The ata were fully calibrate for noise correction an with atmospheric attenuation applie. Details of the S-PolKa parameters an quality-control proceures are on the website spol/. The polar-coorinate ata for the SUR an RHI scans were interpolate from the original polar-coorinate format to a three-imensional Cartesian gri using the NCAR Sorte Position Raar Interpolator (SPRINT; Mohr an Vaughan 1979). The gri element size of the Cartesian ata is 0.5 km km in the horizontal an 0.5 km in the vertical. The existence of SCE, DCC, WCC, an BSR echo volumes was etermine by analysis of the Cartesian interpolate reflectivity fiels. A rain classification technique base on the

4 2716 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 70 methoology of Churchill an Houze (1984) as moifie by Steiner et al. (1995) an most recently by Yuter an Houze (1997) was use. The classification algorithm was applie to the interpolate reflectivity fiel at the 2.5-km level in orer to objectively separate the raar echoes into convective an stratiform components. Parameters 1 of the Yuter an Houze (1997) version of the algorithm were tune specifically for the S-Polka reflectivity fiel observe uring DYNAMO by testing the algorithm results against RHI ata to minimize false inications of convective an stratiform echoes. This tuning proceure is escribe in Steiner et al. (1995). The raar-erive precipitation amounts use in this stuy are compute using a hybri polarimetrically tune version of the reflectivity rain-rate relation of R 5 (az) 1/b,wherea an b (M. Katsumata 2011, personal communication). Details of the algorithm an the parameters use for the calculation of rain amounts are provie at ynamo/spol/parameters/rain_rate/rain_rates.html. The parameters of the reflectivity rain-rate relation are still being refine. However, for this stuy the absolute amounts of rainfall are not critical. More important are the time variation of the rainfall an the subivision of rain into convective an stratiform components. Neither the time variation nor the convective/stratiform separation is sensitive enough to the exact rain rate to have any effect on our present results. Meteorological conitions in the vicinity of the raar location were obtaine from two sources. An ARM souning was launche every 3 h at the Gan site. Time series of win, temperature, an specific humiity were interpolate in space from these sounings at 100-m vertical resolution an were also interpolate in time to fill in missing ata points. European Centre for Meium- Range Weather Forecast (ECMWF) Interim Re-Analysis (ERA-Interim) ata (Berrisfor et al. 2009) were use to ocument the synoptic conitions over the region surrouning S-PolKa. Specifically, we use three-imensional 6-hourly ata for zonal, meriional, an vertical win components; ivergence fiel; potential temperature; an specific humiity on a gri. Time series anomalies of each atmospheric fiel were compute by subtracting the time mean at each pressure level from the time series ata for the ARM souning an ERA- Interim ata, respectively. 1 The best classification results were foun running the algorithm at the 2.5-km level, an using a 5 10, b 5 64, an Z ti 5 40 as values of the parameters in Eq. (B1) of Yuter an Houze (1997). The threshols were set to 11 km as backgroun raius an 5 km as the convective raius. 3. Definitions of eep convective cores, wie convective cores, an broa stratiform regions Following the methoology of Houze et al. (2007), Romatschke an Houze (2010), an Romatschke et al. (2010), who examine ata from the TRMM raar, we ientify convective an stratiform echoes that evelop extreme characteristics of intensity, height, or horizontal extent. Specifically, we ientify an extract from the raar echo patterns of DCC, WCC, an BSR echoes as they were efine in those previous stuies. In this stuy, we also inclue a new category for shallow convective elements. First, we locate the regions ientifie as convective an stratiform echoes by the tune Yuter an Houze (1997) algorithm. Within the convective echo region we etermine the following entities: SCEs are contiguous three-imensional reflectivity echo volumes that have maximum heights lower than 1km below the 08C level, which is estimate using the 3-hourly ARM/Gan souning. This efinition is consistent with that use by TRMM 2A23, version 7, rain prouct (Awaka et al. 2009) an iscusse previously by Schumacher an Houze (2003). The SCEs represent small precipitating clous of shallow to moerate epth that are locate in regions governe primarily by warm rain processes. Within the convective echoes seen by S-PolKa, we also ientify the most intense echo cores by extracting all contiguous three-imensional echo volumes containing equivalent raar reflectivity values exceeing 30 BZ. For these intense three-imensional echo objects, we etermine the following subsets: DCCs are intense echo cores whose maximum heights are at least 8 km MSL. This category inclues young an vigorous convective cells with strong uprafts. WCCs are intense echo cores whose horizontal areas excee 800 km 2. This category captures regions where intense convective cells have aggregate into mesoscale areas of active vigorous cells. They are often part of large MCSs (Houze 2004) in an intensifying stage of evelopment where iniviual convective cells merge together. Deep an wie convective cores (DWCCs) are intense echo cores that fall into both of the previous two categories, i.e., they are both eep an wie. The threshol of 30 BZ use here to efine intense echo cores is lower than that originally use in Houze et al. (2007) an Romatschke et al. (2010) of 40 BZ.The higher value was useful for continental regions; however, echoes reaching this intensity are rarer over oceans.

5 SEPTEMBER 2013 Z U L UAGA AND HOUZE 2717 TABLE 1. Number of echo structures observe in each extreme convective category for ifferent height area combinations uring both S-PolKa reflectivity ata scanning strategies. Bolface numbers correspon to the selecte combination of height area use for the analysis. Height (km) Area SUR RHI (km 2 ) DCC WCC DWCC DCC WCC DWCC FIG. 2. Hourly time series of total (re), convective (green), an stratiform (blue) accumulate rain over the region of raar coverage. Black ashe line shows the running-mean series of 24-h total accumulate rain, which was use to locate the maxima in rain accumulation for the purpose of compositing the ata. Barnes an Houze (2013) have foun that the 30-BZ threshol is more useful over the tropical oceanic environments consiere here. The height an area threshols use here for WCCs an BSRs were chosen after sensitivity analysis of ifferent height area combinations as will be explaine later. Finally, within stratiform echo regions, we ientify one subcategory: BSRs are those contiguous stratiform raar echoes without any reflectivity threshol that extens over a horizontal area of at least km 2.Thiscategory represents parts of mature stage MCSs compose mainly of large stratiform rainy areas (Houze 2004). 4. Occurrence of eep convective echo types uring MJO precipitation episoes a. Overall frequency of events During October 2011 January 2012 of the DYNAMO/ AMIE campaign, three main perios of enhance precipitation occurre over the S-PolKa area in association with active phases of the MJO over the Inian Ocean. Figure 2 shows the hourly time series of accumulate rain over the region covere by the S-PolKa raar for these three perios: October, November, an December. During these enhance perios, the rainfall was highly variable on the synoptic time scale (i.e., a few ays). The precipitation came in 11 episoes of significant rain accumulation, each with a mixture of convective an stratiform rainfall. These episoes tene to have a 2-ay uration an to be separate generally by 2 4 ays uring active MJO phases; October events tene to occur at 2-ay intervals, while in November an December they occurre at 4 6-ay intervals. Table 1 shows the total occurrence of convective raar echo entities for the eep an wie categories efine in section 2b an sample uring the three enhance precipitation perios of the MJO. The statistics are presente using both SUR an RHI scanning strategies an were evaluate for ifferent combinations of vertical heights an horizontal areas for the most intense convective echo cores efine in section 3. Similarly, Table 2 shows the total occurrence of broa stratiform raar echo events for ifferent combinations of horizontal areas. Comparison of the numbers an proportions of echo elements obtaine using the ifferent combinations of heights an areas is ifficult because both the SUR an RHI scans covere ifferent areas an ha ifferent vertical resolutions. Consequently, the numbers of echo elements ientifie using the two scanning strategies are not istribute in the same proportions. However, the evaluation of ifferent combinations of height area threshols gives us confience when comparing such combinations with the results presente in the next section. Results for the istribution of the frequencies of occurrence of the ifferent forms of TABLE 2. As in Table 1, but for broa stratiform regions an using SUR scans only. Area (km 2 ) BSR

6 2718 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 70 FIG. 3. Composites of the frequency of occurrence of each of the ifferent types of raar echo structure efine in section 2 uring 24 h before (negative time) an after (positive time) the composite maximum in rain accumulation (ash otte curve) calculate with (a) SUR an (b) RHI scanning strategy ata. The right y axis is for the colore curves. The rainfall accumulation composite is compute by centering each of the 11 rain episoes in Fig. 2 on the time of the maximum of its running-mean curve in Fig. 2. eep convection (not shown) are not especially sensitive to the choices of threshols. The threshol choices that we have mae le to a total of 1662 (1456) DCC echo entities that reache 8 km or greater in height when evaluate using SUR (RHI) scans. In the same way, 531 (170) WCC echoes covere areas larger than 800 km 2, with 289 (239) events falling into both DCC an WCC categories, an 122 BSRs with areas of at least km 2. For the shallow convective category a total of 1090 events were ientifie when using SUR scans an 578 when using RHI scans. A PDF analysis showing the frequency istribution of the horizontal areas covere by the SCE elements yiels an area average of about 8 km 2 with a stanar eviation of about 5 km 2. This result inicates that these echoes are convective in scale an are similar to the shallow isolate convective echoes ientifie in TRMM ata Schumacher an Houze (2003) an Barnes an Houze (2013). b. Composites of echo occurrence relative to time of maximum rainfall Figure 3 shows the frequency of occurrence of each type of echo structure foun at each hour from 24 h before (negative values) to 24 h after (positive values) the maximum in rain accumulation for each of the 11 rain episoes seen in Fig. 2. The frequency is calculate as the percentage of the total number of events foun in each convective category. In aition, Fig. 3 shows the composite of rain accumulation for the 11 rainfall episoes.thezerotimeforeachepisoeistheclosesttimeto the maximum value in the 24-h-running-mean rainfallaccumulation curve in Fig. 2. Table 3 lists these ates an

7 SEPTEMBER 2013 Z U L UAGA AND HOUZE 2719 TABLE 3. Date an time of the selecte maxima in rain accumulation for the 11 rainiest events uring the October 2011 January 2012 perio of the DYNAMO campaign. Date Time (UTC) 16 Oct Oct Oct Oct Oct Oct Nov Nov Nov Dec Dec times. We calculate the composites in Fig. 3 from both SUR an RHI scanning strategies. The results from the two atasets are remarkably similar in the istribution of the frequency of raar echo structures. Figure 3 shows that SCEs are present at all times but that they peak in occurrence about 12 h prior to the time maximum accumulation. The number of SCEs ecreases as the DCC category maximizes about 5 10 h before the time of maximum rain accumulation. The SCEs slowly increase beginning about 12 h after the peak rainfall. After the DCC echoes reach their maximum frequency (about 25 to 210 h), the frequency of events categorize as both eep an wie (DWCCs) increases an reaches a maximum just before the time of maximum in rain accumulation. The peak occurrence of WCCs comes next, with a maximum about 1 2 h after the time of maximum rainfall. There is a seconary peak in the frequency of occurrence of DCCs in RHI scans not observe in the SUR scans, possibly occurring in the RHI sector as a sampling artifact. At 0 h, the broa stratiform regions begin to appear an reach a maximum aroun 18h. The progression of raar echo structures seen in Fig. 3 shows how the convective population changes its character uring the 2-ay rainfall episoe. The appearance first of a mixture between SCEs an DCCs, while the rainfall initially accumulates, inicates that the precipitating clou population is ominate by shallow an eep convective elements in relatively early stages of evelopment. Then the population of convective elements becomes ominate by intense convective cells (i.e., DCCs) that are separate from each other but are prouce by strong eep penetrating local uprafts, inicative of the larger-scale environment now allowing the cells to exten to higher levels. As time progresses, relative to the maximum rainfall, such iniviual towers aggregate to form interconnecte regions of intense convection. The maximum occurrence of WCC echoes following the maximum of DCCs an lagging the maximum in DWCCs inicates that as the rain episoe progresses the population of precipitating clous becomes ominate by intense cells aggregating into larger units, likely to form MCSs. The higher frequency of the wier systems coinciing with the time of highest accumulation of rain is consistent with the well-known fact that rain accumulation is closely relate to areal coverage by rain (e.g., Atlas et al. 1990). It is also well known that as MCSs mature an issipate, the convection composing them prouces mesoscale areas of stratiform rain that last for hours an may or may not be attache to the remaining active convection. The maximum frequency of occurrence of BSR echoes in the eclining perio of the rain episoe inicates that the large-scale environment uring this later perio supporte the growth of systems to their most extreme mesoscale stage in which large stratiform regions can occur in association with MCSs. The graual increase of SCE echo elements in later stages of the rain episoe is consistent with the precipitating clou population returning to its backgroun state as the synoptic-scale wave passes but the MJO active phase continues. 5. Environmental conitions associate with the evolution of raar echo structures uring MJO precipitation episoes To investigate the large-scale environmental conitions supporting the 11 rain episoes occurring uring the active MJO phases of DYNAMO/AMIE, we composite ata from the Gan sounings an from ERA-Interim variables. The souning variables provie measurements of the atmospheric conitions in the immeiate vicinity of the raar site. We use ERA-Interim variables over a region aroun the raar site (1.58S 1.58N, E) to inicate conitions over a broaer region surrouning the raar. a. Environmental time composites for rainfall episoes Figure 4 shows time height composite plots of potential temperature an specific humiity relative to the maximum in rain accumulation at 0 h for each of the 11 rain episoes seen in Fig. 2 an inicate in Table 3. In aition, a composite of convective available potential energy (CAPE) anomalies for the time variation of the sounings is inclue in plot. At aroun 215 h, a shallow layer of positive potential temperature anomalies appears in both time sections (Figs. 4a,b). In aition, the potential temperature anomaly ecreases with height at aroun 210 to 26 h in Fig. 4a an aroun 215 to 211 h in Fig. 4b. At the same time that the surface reaches a maximum in potential temperature anomalies, the lower-level

8 2720 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 70 FIG. 4. Composite time height sections of potential temperature (shaing, K) an specific humiity (black contours, gkg 21 ; soli contours inicate positive values) anomalies from 224 to 124 h aroun the maxima in rain accumulation. The profiles are calculate using (a) 3-hourly DOE ARM rawinsone observations an (b) ERA-Interim ata. The re curve is the composite time series of CAPE anomalies with values inicate by the right-han axis in each plot. specific humiity increases an reaches an anomalous maximum aroun 29to26 h when calculate using the Gan souning an aroun 212 to 29 h for ERA-Interim ata. These ata all point to estabilization favoring eeper convection before the maximum in rain accumulation an, accoringly, the CAPE increases uring this time. After the maximum in low-level specific humiity, the potential temperature changes to negative anomaly values while the specific humiity anomaly increases with height. When the time reaches the 0 h corresponing to the maximum in rain, the atmosphere cools an becomes moist at milevels an ry at lower levels. The CAPE ecreases, an the environment persists until the surface an the atmospheric milevels reach a minimum of specific humiity aroun 115 h. Figure 5 shows time height composite plots of ivergence an vertical velocity calculate using ERA-Interim ata in a similar manner to Fig. 4. At 220 h, negative anomalies of surface ivergence appear an ecrease to a minimum (i.e., peak low-level convergence) at aroun 210 h. During this same time perio, pressure velocity ecreases, inicating enhance upwar motion with a maximum at the 750-hPa level between 210 an 0 h. After this time, vertical velocity anomalies increase, an the center of convergence seen at the surface before the maxima in rain shifts upwar, reaching the 500-hPa level with a maximum value at 112 h. At the same time, the lower levels of the atmosphere become anomalously ivergent with maximum values aroun 110 to 115 h. This sequence of ivergence an vertical velocity sloping upwar from negative towar positive lag over the 2-ay composite perio is consistent with the sequence expecte from a westwar-propagating equatorial inertiogravity wave [e.g., see Fig. 4 of Haertel an Kilais (2004), Fig. 8 of Kuang (2008), or Fig. 11 of Kilais et al. (2009)]. Sometimes the pattern might be ominate by synoptic-scale easterly waves with longer perios, but their signatures woul be expecte to be similar but stretche to a somewhat longer perio. We calculate separate composites (not shown) for the October, November, an December rainfall episoes. They confirm that the events that occurre in October inee ominate the anomaly patterns. Other types of waves contributing to the composite, such as 4-ay mixe Rossby gravity waves or synoptic easterly waves ue to instability of the win in the ITCZ region likely occurre in November an December but were too few in number to be easily isolate in the composite. Figure 6 shows time height plots for zonal an meriional win anomalies composite for 224 to 124 h from the maximum in rain accumulation as composite in Fig. 4. Figure 6a, calculate using the Gan sounings, shows an anomalous low-level easterly win at aroun 220 h. Aroun 212 h, the win anomaly reverses to westerly. The anomalous westerly win is maximum at 850 hpa, just before the maximum in rain accumulation. At the same time, anomalous northerly win increases with height reaching a maximum at aroun 700 hpa. After the maximum in rain accumulation, the anomalous westerly win ecreases an reverses again towar an anomalous easterly component at aroun 120 h. The anomalies of zonal an meriional win calculate using ERA-Interim ata (Fig. 6b) behave in a remarkably similar manner, except that the maximum in westerly win anomaly is reache several hours after the maximum in rain accumulation. The upper-level zonal win anomalies (i.e., aroun 200 hpa) in both Figs. 6a an 6b show a similar progression from easterly to westerly anomalies, but the magnitue attains higher values at a later time than their surface counterpart. An increase in low-level westerly win anomalies as the most intense rainfall propagates has been observe in association

9 SEPTEMBER 2013 Z U L UAGA AND HOUZE 2721 FIG. 5. As in Fig. 4b, but for ivergence (shaing, s 21 ) an vertical velocity (contours, hpa s 21 ; soli lines inicate positive values) anomalies calculate using ERA-Interim ata. with the passage of MJO relate isturbances across the region (Lin an Johnson 1996; Chen et al. 1996; Zhang an McPhaen 2000; Beneict an Ranall 2007). These statistical composite structures uring the time perio of the rainfall episoe are similar to the way that iniviual eep convective clou systems behave, but on a longer time scale than an iniviual clou system. An iniviual MCS forms when eep cells occurring separately in early stages aggregate to form mesoscale units of eep convection in the MCS s milife stages, an prouce a stratiform rain area in its mature to later stages (Houze 2004). The eep cells of the MCS tap into the unstable air in the lower layers of the troposphere. The result is lower-tropospheric convergence an growing convective cells, which then evolve into precipitating stratiform clou mass that eventually stabilizes the lower levels. An iniviual MCS typically evelops an ascening, sloping layer of airflow feeing it from the convergent zone. The sloping in the convective region of the MCS fees a large stratiform region, characterize by milevel convergence. The milevel inflow escens in connection with sublimation an evaporation of hyrometeors, thus constituting a sloping ownraft. This mesoscale ownraft lies uner the upwar-sloping ascent layer. These MCS behaviors are well known (Houze 2004), an they are known to occur over tropical oceans. Sloping of mesoscale up- an ownrafts has been seen in MCSs over the western Pacific warm pool (Kingsmill an Houze 1999). These circulation features are like those shown by Moncrieff (1992) to be a natural ajustment to instability in sheare flows. Moncrieff s theory was pose in a two-imensional framework, but the results of Kingsmill an Houze ocument that the same types of circulation features occur even if the MCSs o not take the form of two-imensional squall lines. In Figs. 4 6, we see a sequence of events similar to that of an iniviual MCS, but on a time scale corresponing to a 2 4-ay equatorial wave much longer than the time scale of an MCS. In aition, the peak statistical frequencies of occurrence of SCE, DCC, WCC, an BSR raar echoes follow a temporal sequence in line with these souning ata. That is, over the 2-ay perio of a precipitation episoe, the convective statistics (not the iniviual convective clou systems) inicate that the synoptic-scale conitions in the earlier part of the 2-ay episoe favor MCSs in their earlier stages of evelopment an that as the 2-ay perio continues the large-scale conitions are increasingly favorable for the persistence an/or amplification of the later stratiform stages of evelopment of iniviual MCSs. We may view this behavior of the echo statistics against the backgroun of the evolution of the synoptic-scale environment. The synoptic-scale wave pattern constituting the environment of a convective population uring a 2-ay precipitation episoe embee in the active phase of the MJO systematically favors convection in early, mile, an late stages of evelopment, respectively. FIG. 6. As in Fig. 4, but for composite time height sections of zonal (shaing, m s 21 ) an meriional (contours, m s 21 ; soli lines inicate positive values) win anomalies.

10 2722 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 70 Figures 4 6 show this progression over the 2-ay perio of the episoe. Thus, we see that three scales are involve. The largest scale is the MJO, which establishes the possibility of a convectively isturbe environment. The intermeiate scale is the 2-ay perio of a precipitation episoe, where the time scale is probably controlle by an inertio-gravity wave or other synoptic-scale easterly wave passage. The smallest scale is the iniviual convective element. These elements are nearly always present an unergoing life cycles shorter than 2 ays. During the passage of the intermeiate-scale wave, the population of convective elements ajusts, with the early phase favoring a higher proportion of convective elements in earlier stages of evelopment, the mile phase favoring a larger proportion of MCSs in mile stages of evelopment, an the later phase allowing a larger proportion of the population to consist of MCSs reaching later stages of evelopment. These composite time series are thus consistent with the stretche builing-block notion of Mapes et al. (2006), accoring to which the changing makeup of the ensemble of precipitating clous over a time perio greater than the time scale of iniviual clou systems, such as the MJO rainfall episoe consiere here, coincies with a temporal sequence of wave-etermine thermoynamic an ynamic variables analogous to but on a longer time scale than the life cycle sequence of an iniviual MCS. It seems unlikely that without a wave originating from some larger-scale ynamics, that such an orerly 2-ay sequence of clou population statistics coul occur. However, as note by Mapes et al. (2006), the convective population may well fee back positively to the wave structure an make this type of convectively couple wave eminently more observable. Since the convective population is weighte towar a particular stage of convective evelopment in a given larger-scale wave phase environment, it stans to reason that the particular stage of the convective population woul imprint itself on the larger-scale wave in a positively reinforcing sense. b. Spatial istribution of environmental properties at ifferent times uring rainfall episoes To investigate how the synoptic conitions vary in space uring the time perios of the precipitation episoes stuie here, we have employe the ERA-Interim ata (section 2). Figure 7 shows longitue height composites of the reanalysis zonal vertical win, ivergence, an specific humiity anomalies average over the 1.58S 1.58N latitue ban at three representative times uring the composite rainfall episoe. The vertical win is exaggerate 1000 times to allow a comparison with the horizontal win. FIG. 7. Latituinal-average composites of ERA-Interim ivergence (shaing, s 21 ), specific humiity (contours, g kg 21 ; soli blue lines inicate positive values), an zonal vertical win (vectors) anomalies for (a) 224, (b) 26, an (c) 112 h from the maxima in rain accumulation. The vertical line at 73.18E inicates the location of the S-PolKa raar. At 24 h before the maximum in rain accumulation (Fig. 7a), low-level easterly win an low specific humiity were present in the vicinity of the raar. At the same time, west of the raar location (aroun 658E), there is a region of low-level convergence an positive anomalies of specific humiity. As time progresses from 224 to 26 h, the region west of the raar favoring moistening an low-level convergence moves eastwar towar the raar region. By 26 h, the atmosphere aroun the raar location is moist, with strong vertical motion an low-level convergence (Fig. 7b). At this time, the upwar motion associate with upper-level ivergence penetrates to the highest level (up to 400 hpa) aroun the same time as the frequency of DCC events is greatest (Fig. 3). These behaviors are analogous to the early stage

11 SEPTEMBER 2013 Z U L UAGA AND HOUZE 2723 of MCS evelopment, but on a larger scale, an these time perios correspon to when DCCs an WCCs are more prevalent. From 26 to 112 h, the lower atmosphere cools an ries, an the low-level convergence ecreases at the raar site (not shown). The region of high moisture, low-level convergence, an upwar motion continues moving eastwar, while in the region near the raar ownwar motion an ivergence set in (Fig. 7c). Between 16 an112 h the lower atmosphere has the lowest potential temperature an specific humiity anomalies. These later-perio characteristics are analogous to MCSs in later stages of evelopment with large stratiform rain areas. This time perio is when BSRs are most frequent (Fig. 3). 6. Conclusions This stuy has ientifie an examine the most extreme convective entities etecte by the NCAR S-PolKa raar uring the DYNAMO/AMIE fiel campaign. We have analyze the S-PolKa three-imensional reflectivity fiel to ientify four basic types of echo objects, which have been use in previous stuies to analyze TRMM satellite raar observations of eep convection over other regions of the earth (Houze et al. 2007; Romatschke an Houze 2010; Rasmussen an Houze 2011; Barnes an Houze 2013). The satellite ata are in the form of snapshots, which make the results ifficult to interpret with certainty. Our stuy applies this approach to temporally continuous groun-base raar ata, an the results verify interpretations mae in stuies using TRMM ata. In aition, the results provie insight into the nature of the convective clou populations proucing MJO rainfall episoes over the Inian Ocean. From October 2011 through early January 2012, three main active phases of the MJO occurre, an within these active phases, the rain in the vicinity of the S-PolKa raar was concentrate in 11 episoes separate by 2 4 ays much shorter perios than the time scale of an MJO active stage, yet longer than the time scale of iniviual convective elements or systems. This 2 4-ay moulation likely is ue to westwar-propagating synoptic-scale equatorial waves. We performe a running mean to highlight the 11 episoes an ientify their times of peak rainfall accumulation over the area observe with S-PolKa. We have shown that each of the 11 rain episoes was characterize by an ensemble of eep convection that went through a sequence of statistical states such that each statistical state was ominate by convective systems in successively later stages of evelopment: Shallow convective echoes were almost always present before, uring, an after the time of the peak in rain accumulation. However, their frequency was maximum about 12 h prior to the time of maximum rain accumulation, then reache a minimum at the peak of the rain episoe, an after, their frequency of occurrence recovere towar the en of the 2-ay perio. The number of eep convective cores tene to increase as the frequency of shallow echoes ecrease. The eep convective cores reache a maximum at 5 10 h beforethepeakinrainfall accumulation. The number of wie convective cores was at a maximum aroun the time of maximum rain accumulation. The frequency of broa stratiform regions maximize several hours after the maximum of rain accumulation. At any given time the clou population prouces echo elements of all four types, but the frequencies of each echo type maximize in the above orer. Since mesoscale convective systems (Houze 2004) begin with an outbreak of convective cells, then grow upscale to mesoscale proportions as the cells aggregate into horizontally contiguous mesoscale units, this sequence of statistical states relative to the time of maximum rainfall accumulation implies that prior to the rain maximum the environment favors a population of intense convective elements that has a preominant proportion of intense convective elements in formative an growing stages. During the perio of peak rainfall, the environment allows the population of convective elements to have a higher proportion of wie convective cores (i.e., more convection forming into MCSs). As the rain accumulation rops off in the later part of the rainfall episoe, the environment is permitting the clou population to have a higher proportion of echo elements in the form of broa stratiform regions, inicating that in this later perio the environment is allowing a greater proportion of the echo elements in the form of MCSs to reach their later stages of evelopment. The convective population likely fees back to the environment, reinforcing the conitions controlling the nature of the population at any given time relative to the center time of the rain episoe. To gain insight into this concurrent systematic variation of the environment an convective population statistics over the 2-ay rain episoes occurring in the active perio of MJOs, we composite both the Gan souning an ERA-Interim variables to investigate the connection of the changing clou population to the large-scale environment uring the 11 MJO rainfall episoes. The results show unstable conitions in the early part of the rainfall episoe, when SCE an DCC echoes were most frequent. By the time of the maximum rainfall, the clou population was ominate by WCC echoes, which are a signature of the presence of a maximum number of

12 2724 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 70 mature MCSs. The large-scale vertical motion was a maximum at this time, with a pronounce convergence signature at low levels. During the perio of eclining rain accumulation over the raar area, stabilization occurre, an the large-scale mean motion was ownwar in the mi- to low troposphere, with strong ivergence at low levels. These stages are analogous to the life-cycle stages of an iniviual MCS, but they manifest on a longer time scale an over a bigger area than an iniviual system. These stages probably correspon to equatorial waves superimpose on the MJO environment. The ivergence sequence of the composite is consistent with the sequence expecte of a westwar-propagating inertiogravity wave (Haertel an Kilais 2004; Kuang 2008; Kilais et al. 2009) with other longer-perio synopticscale waves likely mixe into the composite but har to separate within the limite sample of ata. Thus, the large-scale environment an convective population exhibit characteristics consistent with the stretche builingblock hypothesis of Mapes et al. (2006). Coincient with the membership of the clou population changing from an ensemble containing a preominance of young eep but relatively separate convective cells to a population ominate by mature MCSs to a population with oler MCSs with large stratiform regions, the large-scale conitions take on the aspect of a stretche analog to the typical MCS life cycle. Hours before the maximum in rain accumulation, when the ensemble is ominate by SCEs an DCCs, the surface westerly win an convergence increase, the atmosphere warms an becomes moister, an the CAPE reaches a peak; that is, the environment is favorable for eep convection to occur. When WCCs become most frequent near the time of the peak of the rainfall episoe, upwar motion maximizes. Then as BSRs have their greatest frequency, stabilization occurs, evaporative cooling increases, an low-level convergence an upwar motion ecrease an reverse to ownwar motion an low-level ivergence. This simultaneous behavior of the convective population an synoptic-scale environment is entirely consistent with Mapes et al. s (2006) hypothesis of a natural selection in the atmosphere for waves whose phases prouce a local Eulerian sequence of structures aligning with convective life-cycle behavior. Stuies using TRMM satellite raar ata to examine the climatology of eep convection have compile statistics of DCCs, WCCs, an BSRs an have speculate that when statistical maxima of these echo types occur in a sequence of geographical locations, the locations of the maxima inicate regions where mesoscale convective systems ten to form, mature, an issipate, respectively. However, since the TRMM orbits obtain only snapshots of the raar echo pattern at wiely separate times, it has not been possible to verify these speculations. The time continuity of the groun-base S-PolKa raar use in the present stuy has been able to etermine that the statistical maxima of DCC, WCC, an BSR frequencies o inee occur in a temporal sequence consistent with mesoscale convective system life-cycle stages. In a concurrent stuy, Barnes an Houze (2013) are using 14 years of TRMM raar snapshots to ientify isolate shallow echoes, DCCs, WCCs, an BSRs over broa expanses of the Inian an western Pacific Oceans uring ifferent phases of the MJO. Our stuy provies groun valiation for that climatological investigation by making it possible to interpret the concentrations of each of the echo types seen by the TRMM raar as representative of life-cycle stages of the eep convection proucing precipitation in the MJO. Such satellite atasets are a primary pathway for upscaling results from the DYNAMO/ AMIE fiel project to the scale of the MJO an in turn for proviing observational information for moeling stuies aime at unerstaning the coupling of convection to the large-scale circulation of the MJO. The S-PolKa raar ata collecte in DYNAMO/ AMIE have the potential to a information that will further unerpin satellite interpretations of the MJO raar echo climatology. A further strength of the S-PolKa raar is that it can use its S-ban ual-polarimetric capability to ientify the ominant hyrometeor types proucing the raar echoes. The scanning strategy of the S-PolKa raar (section 2; Fig. 1c) was esigne to optimize the information on hyrometeor types by applying techniques such as that of Vivekananan et al. (1999) to euce the most probable vertical profiles of hyrometeor types in each echo seen by the raar. In a future article, we will apply such methos to the echo types ientifie in this stuy. That future article will enhance the basic echo-type results of the present stuy with information on the microphysical processes most responsible for proucing the precipitation in the early, mile, an late stages of the active phases of the MJO rainfall episoes. The results of the present an future stuies of ata collecte by the S-PolKa raar in DYNAMO/AMIE will thus provie information on the nature of MJO convection that will help guie numerical simulations of global an regional moels use to simulate an forecast MJO behavior. Acknowlegments. This research was supporte by National Science Founation Grants AGS an AGS , National Aeronautics an Space Aministration Grant NNX10AH70G, an Department of Energy Grants DE-SC /ER an DE-SC We are grateful to the NCAR science team for proviing the quality-controlle raar ata use in this stuy. S. Brozik