Multi-scale Organization of Convection in a Global Numerical ...

Journal of the Meteorological Society of Japan, Vol. 87, No. 2, pp. 335--345, 2009.

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DOI:10.2151/jmsj.87.335

NOTES AND CORRESPONDENCE Multi-scale Organization of Convection in a Global Numerical Simulation of the December 2006 MJO Event Using Explicit Moist Processes

Tomoe NASUNO Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan

Hiroaki MIURA Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA

Masaki SATOH Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan Center for Climate System Research, University of Tokyo, Kashiwa, Japan

Akira T. NODA and Kazuyoshi OOUCHI Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan (Manuscript received 2 June 2008, in final form 17 December 2009)

Abstract Multi-scale convective organization in a Madden-Julian Oscillation (MJO) event that occurred during December 2006 and January 2007 was studied by global numerical experiments using explicit moist physics. The simulations successfully reproduced the eastward-propagating (@5 m s
1 ) convective envelope of the MJO with a zonal scale of 5000–10,000 km, which included eastward-propagating (10–15 m s
1 ) disturbances (EPDs) and westward-propagating cloud clusters (CCs) with zonal scales of 1000–2000 km and O (100 km), respectively. The simulated EPDs were composed of CCs, with new clusters growing to the east of older ones. When the large-scale circulation associated with the MJO intensified, the EPDs formed well-organized squall-type clusters (rainbands). The dynamical structure of the simulated EPDs was reminiscent of moist Kelvin waves. Relevance of westward-propagating wave disturbances including cross-equatorial flow to convective organization in the EPDs was also suggested.

Corresponding author: Tomoe Nasuno, Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showamachi, Kanagawa, 236-0001, Yokohama, Japan. E-mail: [email protected] 6 2009, Meteorological Society of Japan

1. Introduction The Madden-Julian Oscillation (MJO; Madden and Julian 1971, 1972) is a phenomenon in which cooperation between large-scale dynamics and or-

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ganized convection is essential (Madden and Julian 2005; Wang 2005). Because multi-scale organization of convection is a key aspect of the MJO (Nakazawa 1988), it has been investigated extensively in the context of the link between convection and large-scale dynamics (Nakazawa 1988; Sui and Lau 1992; Takayabu 1994; Dunkerton and Crum 1995; Chen et al. 1996; Houze et al. 2000). Recently, intensive field observations, such as the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE; Webster and Lukas 1992) and the Study of the MJO-convection Onset (MISMO; Yoneyama et al. 2008), and numerical experiments with 2D cloudresolving models considering planetary-scale domains (Oouchi 1999; Grabowski and Moncrie¤ 2001; Oouchi and Yamasaki 2001; Tulich et al. 2007) have increased our knowledge of the multiscale mechanisms in the MJO and large-scale tropical convective disturbances. General circulation models (GCMs) applying the Multi-scale Modeling Framework (MMF, Randall et al. 2003), in which 2D cloud-resolving simulations are operated on each grid in place of conventional convective parameterization, have achieved significant improvement in simulating the MJO. Global explicit simulations of convection with convection-permitting grid sizes (1–10 km) have been another useful approach to investigate the multi-scale mechanisms in the MJO (Hartmann and Hendon 2007; Nasuno et al. 2007). Miura et al. (2007; referred to hereafter as M07) first performed a set of global month-long (week-long) 7-km (3.5-km) mesh numerical experiments of an MJO event that occurred during December 2006 and January 2007. M07 demonstrated the capability of the model to reproduce the general evolution of this MJO event, as well as to capture the inner structure of the eastward-propagating convective envelope, which was not fully observable in the real atmosphere. Inoue et al. (2008) and Masunaga et al. (2008) then quantitatively evaluated the simulated cloud statistics in comparison with satellite data, and Fudeyasu et al. (2008) and Satoh (2008) assessed the reproducibility of tropical cyclones and diurnal variation of convection for that case. These studies showed that the simulation was of su‰cient quality for investigations of various mechanisms in it, although one cannot ask for exact prediction of individual clouds. This paper focuses on the multi-scale organization of convection in the M07 simulation.

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2. Simulation and data The numerical experiments were performed using the Nonhydrostatic ICosahedral Atmospheric Model (NICAM; Satoh et al. 2008). The model was initialized using the National Centers for Environmental Prediction (NCEP) Global Tropospheric Analyses data, and free integration was performed for 30 (7) days for the 14- and 7-km (3.5-km) mesh runs, starting at 0000 UTC, 15 (25) December 2006 (hereafter, DX14, DX7, and DX3.5, respectively). Moist convection was explicitly calculated without using convective parameterization even for the DX14. Sea surface temperature (SST) was interpolated from weekly Reynolds SST (Reynolds and Smith 1994). To evaluate the simulations, outgoing long-wave radiation (OLR) from National Oceanic and Atmospheric Administration satellites (NOAA-OLR; 2.5 grid, daily average), the black body temperature ðTBB Þ merged from 11 infrared (IR) channels aboard five geostationary satellites of the Climate Prediction Center (CPC-IR; @4-km, hourly), and the NCEP Global Tropospheric Analyses data (1 grid, 6-hourly) were used. 3. Results Figure 1 shows Hovmo¨ller diagrams of NOAAOLR and the simulated OLR in DX14, DX7, and DX3.5. The simulation data were averaged daily on a 2.5 grid to match the satellite data. Deviations from the temporal (for the period of simulation) and zonal averages are also depicted. The general evolution of this MJO event, such as eastward propagation of the 5000–10,000 km convective envelope at approximately 5 m s
1 over 60 –180 E and the merging of the eastward- and westwardpropagating signals around the dateline, was reproduced in all the simulations. Quantitatively, cloud statistics were better represented with higher resolution (Inoue et al. 2008). Figures 1e and 1f compare the CPC-IR and DX7 OLR. In both panels, cloud clusters (referred to hereafter as CCs) of O (100 km) sizes occurred in the large-scale convective envelope. In the convective envelope, the CCs formed to the east of the previous CCs and migrated westward as they developed, with lifetimes of 0.5–2 days. These findings are consistent with the well-known hierarchical structure of convection in the MJO (Nakazawa 1988; Sui and Lau 1992; Takayabu 1994; Chen et al. 1996). In this MJO event, eastward-propagating

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Fig. 1. Hovmo¨ller diagrams of (a) NOAA-OLR and OLR in (b) DX14, (c) DX7, and (d) DX3.5 (W m
2 ), averaged in 7.5 N–7.5 S. Contour lines depict deviations from temporal and zonal averages for each case. (e) CPC-IR (K) and (f ) the DX7 OLR (W m
2 ), averaged in 1 N–1 S. The colors in (e) and (f ) are graded according to the cumulative frequencies of IR TBB (from Multi-functional Transport Satellite [MTSAT]1R) and the DX7 OLR analyzed by Inoue et al. (2008, their Fig. 2).

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convective signals with 10–15 m s
1 phase velocity were pronounced (M07; Figs. 1e, 1f ). Thus, major components of the convective envelope of the MJO were simulated, although the individual CCs were not accurately predicted, with quantitative biases in the model (Inoue et al. 2008; Masunaga et al. 2008). Figure 2a presents a Hovmo¨ller diagram of lowlevel zonal velocity (deviation from 4-day running average) together with surface precipitation and OLR for DX7. The eastward-propagating disturbances with 10–15 m s
1 phase velocity (referred to as EPDs in this paper) are evident in the precipitation. The precipitation in the EPDs was mostly initiated over the maritime continent (100 – 110 E), and CCs developed at approximately 2day intervals. The EPDs were accompanied with zonal wind anomalies with zonal scale of 1000– 2000 km. Several EPDs had intense, long-lasting precipitation, especially after the onset of low-level westerlies1 (e.g., those starting at 140 E on 5 January, 125 E on 6 January, and 110 E on 11 January 2007, named A, B, and C, respectively). The vertical structure of zonal velocity is shown in Figs. 2b and 2c. As pointed out by M07, the model overpredicted the intensity of the circulation associated with the MJO, including stronger and deeper low-level westerlies over the maritime continent and the western Pacific. However, the primary pattern of the large-scale circulation with low-level zonal convergence in 150 –160 E was reproduced. Deviations from the time-averaged structure (Fig. 2c) were composited to examine vertical structure of the EPDs (Fig. 3; the base point was 119.4 E). Composites were regressed on the longitude of maximum zonal convergence near the surface associated with EPD B (at 123 E on 1200 UTC 6 January–153 E on 1800 UTC 8 January). Since the phase velocities of the EPDs were nearly uniform, this method extracted the structures of all the EPDs existing in the period. 1 Time-longitude plots of low-level zonal velocity in comparison with the NCEP data were given in Fig. S1 of M07.

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The EPDs are identified by the deep convective regions at zonal intervals of 10 –20 (Fig. 3). In the lower troposphere, the easterly (westerly) anomalies coincide with the warm (cold) anomalies. The phase lines are tilted westward with height in z ¼ 4–10 km, with cold anomalies at the rear (western) side of the deep convection (upward motion) and a warm anomaly in and ahead (to the east) of the deep convection (Fig. 3b). Upward motion in convection was positively correlated with warm anomaly in the upper troposphere. These features are common to the structure of the convectively coupled gravity waves (Oouchi 1999; Grabowski and Moncrie¤ 2001; Tulich et al. 2007). Above z ¼ 10 km, structure with a larger-scale (> 5000 km) masked over the EPDs. EPD B dominated other EPDs in the degree of low-level convergence, temperature anomalies, and amount of total condensate (Fig. 3). M07 showed that well-developed CCs in the EPDs sometimes developed linear morphology. This was true of EPDs A, B, and C. Figures 4a, b show the composite vertical structure of the linear CC in the EPD B (Fig. 4c). The composite was made in the same procedure as in Fig. 3, except for the period; 1800 UTC 6 January–1200 UTC 8 January, when the linear CC was maintained. The structure is reminiscent of a squall line. Distinct from EPD (Fig. 3), vertical motion and temperature anomalies better correlated, and low–level convergence coincided with negative pressure anomaly (Fig. 4a, 4b; @119 E). Zonal and vertical velocities were roughly 180 out of phase (Fig. 4a), suggesting convective momentum transport. At the gust front, new clouds were successively generated by the uplifting of inflowing air by cold outflow. A well-defined cold pool (117 –119.5 E) at the rear side of the gust front was maintained by evaporation of rain water. The magnitude and depth of the cold pool relative to the sounding ahead (120 –130 E) were approximately 2 K and 1 km, respectively (Fig. 4b), which leads to a phase speed of @12 m s
1 (derived from the theory of long-lived squall lines by Rottuno et al. 1988). This is comparable to the propagation speed of EPD B (Fig. 2a). The mean zonal wind

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Fig. 2. Hovmo¨ller diagram of (a) zonal velocity at z ¼ 1.5 km in DX7 (m s
1 ). Four-day running means were subtracted. White and black lines in (a) indicate the surface precipitation rate at 2 mm hr
1 intervals and OLR (negative anomalies from the 4-day running mean) at 15 W m
2 intervals, respectively. Heightlongitude sections of zonal wind in (b) NCEP data and (c) DX7, averaged for 1200 UTC 5 January–1800 UTC 8 January 2007. Averages in 3 N–3 S are depicted.

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Fig. 3. Height-longitude sections of composite (a) zonal velocity (m s
1 ), (b) temperature (K), and (c) total condensate (1.0e-5 kg kg
1 ) in DX7. The 3 N–3 S averages were composited on the gust front for 1200 UTC 5 January–1800 UTC 8 January. The time-mean values were subtracted before compositing. Contour lines in (a), (b), and (c) indicate temperature (0.2 K intervals), total condensate (1.0e-5 kg kg
1 ), and vertical velocity (3.0e-2 m s
1 intervals), respectively. Zonal-vertical wind vectors are also depicted in (a) with the vertical component multiplied by 50.

(125 –155 E average of Fig. 2c) includes jet-type profile with westerly shear below z ¼ 2 km (Fig. 4a, left panel). The linear CC propagated parallel to the shear vector by generating new clouds on the upwind side of the low-level inflow. The lowlevel inflow is more enhanced in front of EPD B (120 –130 E; Fig. 4a, left panel) than in other re-

gions, throughout the eastward propagation in the large-scale gradient of zonal velocity (Fig. 2c). This explains the selective growth of the squalltype CC. Other CCs in EPDs were more weakly organized (Figs. 2a, 3, 4c) and did not necessarily move eastward by such systematic formation of clouds.

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Fig. 4. (a), (b) Same as Fig. 3 except showing composites for 1800 UTC 6 January–1200 UTC 8 January. The left panels of (a) and (b) show vertical profiles of zonal wind (closed circle: composite average in 120 –130 E; open circle: time-mean average in 60 –140 E) and temperature (closed circle: composite averages in 120 –130 E; open circle: 117 –119.5 E), respectively. (c) Horizontal distribution of zonal velocity at z ¼ 1.5 km (m s
1 ) on 0000 UTC 7 January 2007 in DX7, with contour lines indicating the surface precipitation rate at 1 and 2 mm hr
1 (0000 UTC–0130 UTC average), and wind vectors at z ¼ 1.5 km.

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Fig. 5. Same as Fig. 2a except showing meridional velocity in (a) NCEP data at 850 hPa and (b) DX7 at z ¼ 1.5 km (m s
1 ). Contour lines in (a) indicate CPC-IR (4-day running mean subtracted, negative anomalies) at
5 K and 10 K intervals. (c) Same as Fig. 4c, except showing meridional velocity.

Figure 4c shows a plan view of the simulated low-level zonal velocity on 0000 UTC 7 January 2007. The meridionally oriented (i.e., perpendicular to shear vector) CCs in EPDs A and B were at their decaying and mature stages, respectively. The CCs were accompanied by an enhanced low-level westerly on the west side (Figs. 3a, 4c). Noticeably,

the low-level westerly had meridional structure; it was more enhanced in the Northern Hemisphere around EPD B (130 –140 E) and in the Southern Hemisphere around EPD A (145 –155 E). Together with the cross-equatorial southerly (northerly) at 110 –125 E (140 –150 E), the low-level westerly formed a meandering pattern. This sug-

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gests that another kind of disturbance is relevant to the organization of convection in the EPDs. Figure 5 shows Hovmo¨ller diagrams of meridional velocity in a similar manner to Fig. 2a. Westward-propagating signals that came across the dateline were evident both in the NCEP data (Fig. 5a) and in DX7 (Fig. 5b). They traveled into the western flank of the large-scale circulation associated with the MJO, with modification at the leading edge of the MJO. The CCs in EPDs coincided more with the phase of weak meridional wind than with the phase of peak intensity (Figs. 5a, 5b). This is true to the squall-type CCs shown in Fig. 4c. They matured in the phase where southerly turned to northerly (Figs. 5b, 5c). 4. Summary and discussion Multi-scale convective organization in an MJO event that occurred between December 2006 and January 2007 was studied in a series of numerical experiments using a global nonhydrostatic model with explicit moist processes. The simulations reproduced the observed multi-scale structure of the MJO; the 5000–10,000 km scale convective envelope included EPDs with zonal scales of 1000– 2000 km, which were composed of westwardpropagating CCs of O (100 km). The EPDs traveled at 10–15 m s
1 , substantially faster than the eastward migration of convective envelope at 5 m s
1 . The EPDs in this simulated MJO event were similar to those observed by Dunkerton and Crum (1995) and by Ichikawa and Yasunari (2007) in zonal scale, phase speed, and period. This was in contrast to the ‘‘super cloud cluster’’ found in a previous study, which had a zonal scale of 3000– 5000 km and a period of 10 days (Nakazawa 1988). The simulated EPDs resembled the convectively coupled gravity waves simulated by 2D cloud-resolving models (Oouchi 1999; Grabowski and Moncrie¤ 2001; Tulich et al. 2007), or moist Kelvin waves2 in vertical phase structure. The EPDs migrated by growth of new CCs to the east of older ones. When the large-scale circulation associated with the MJO was established, simulated CCs formed long-lived squall-type systems. The structure of the squall-type CCs included welldefined cold pools and gust fronts, where clouds were systematically formed. Also, they were accompanied with mesoscale circulations distinct from 2 Structures of the simulated EPDs were unclear in meridional velocity (not shown).

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1000–2000-km scale gravity waves (e.g., rearinflow, convective/mesoscale updrafts). The propagation speed of a simulated squall-type CC was 12 m s
1 (Fig. 2a), close to the estimation based on the theory of a long-lasting squall line by Rotunno et al. (1988). Rainbands and squall lines have been frequently observed in the active phase of the MJO (Takahashi and Uyeda 1995; LeMone et al. 1998). Roles of these long-lived wellorganized convective systems in the MJO are of special interest (Houze et al. 2000). According to LeMone et al. (1998)’s arguments, the occurrence of the simulated squall-type CCs can be explained by intensification of low-level westerly shear (relative to the EPDs) in the evolution of the MJO, and humid environment over the western Pacific region (M07). It was also found that CCs in the EPDs were better organized in the phase of enhanced low-level westerlies associated with westward-propagating wave disturbances that traveled across the Pacific with a few-day period. They were similar to previously observed mixed Rossby-gravity-wave-type disturbances in flow field with cross-equatorial flow and to tropical depression-type disturbances in zonal scale (Takayabu and Nitta 1993; Wheeler and Kiladis 1999). They may possibly be a hybrid of the two types (Takayabu and Nitta 1993). Further investigations including analyses of zonal wavenumber-frequency spectra and structure are underway. Acknowledgments The authors acknowledge Drs. Tetsuo Nakazawa, Yukari N. Takayabu, Taroh Matsuno, Hiroyuki Yamada, Kazuaki Yasunaga, Hironori Fudeyasu, Hisayuki Kubota, Shang-Ping Xie, and Timothy Dunkerton for valuable discussions. Two anonymous reviewers are also greatly acknowledged. This study was supported by Core Research for Evolutional Science and Technology of the Japan Science and Technology Agency (CREST, JST). The simulations were performed with the Earth Simulator at the Earth Simulator Center of the Japan Agency for Marine-Earth Science and Technology. References Chen, S. S., R. A. Houze, and B. E. Mapes, 1996: Multiscale variability of deep convection in relation to large-scale circulation in TOGA COARE. J. Atmos. Sci., 53, 1380–1409.

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