Hiroshi Taniguchi | University of Hawaii (original) (raw)

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Papers by Hiroshi Taniguchi

Monthly Weather Review, 2009

This study discloses detailed Madden–Julian oscillation (MJO) characteristics in the two 30-day i... more This study discloses detailed Madden–Julian oscillation (MJO) characteristics in the two 30-day integrations of the global cloud-system-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) using the all-season real-time multivariate MJO index of Wheeler and Hendon. The model anomaly is derived by excluding the observed climatology because the simulation is sufficiently realistic. Results show that the MJO has a realistic evolution in amplitude pattern, geographical locations, eastward propagation, and baroclinic- and westward-tilted structures. In the central Indian Ocean, convection develops with the low-level easterly wind anomaly then matures where the low-level easterly and westerly anomalies meet. Anomalous moisture tilts slightly with height. In contrast, over the western Pacific, the convection grows with a low-level westerly anomaly. Moisture fluctuations, leading convection in eastward propagation, tilt clearly westward with height. The frictional moisture converg...

Climate Dynamics, 2013

The present study assesses the forecast skill of the Madden-Julian Oscillation (MJO) observed dur... more The present study assesses the forecast skill of the Madden-Julian Oscillation (MJO) observed during the period of DYNAMO (Dynamics of the MJO)/CINDY (Cooperative Indian Ocean Experiment on Intraseasonal Variability in Year 2011) field campaign in the GFS (NCEP Global Forecast System), CFSv2 (NCEP Climate Forecast System version 2) and UH (University of Hawaii) models, and revealed their strength and weakness in forecasting initiation and propagation of the MJO. Overall, the models forecast better the successive MJO which follows the preceding event than that with no preceding event (primary MJO). The common modeling problems include too slow eastward propagation, the Maritime Continent barrier and weak intensity. The forecasting skills of MJO major modes reach 13, 25 and 28 days, respectively, in the GFS atmosphere-only model, the CFSv2 and UH coupled models. An equal-weighted multi-model ensemble with the CFSv2 and UH models reaches 36 days. Air-sea coupling plays an important role for initiation and propagation of the MJO and largely accounts for the skill difference between the GFS and CFSv2. A series of forecasting experiments by forcing UH model with persistent, forecasted and observed daily SST further demonstrate that: (1) air-sea coupling extends MJO skill by about 1 week; (2) atmosphere-only forecasts driven by forecasted daily SST have a similar skill as the coupled forecasts, which suggests that if the high-resolution GFS is forced with CFSv2 forecasted daily SST, its forecast skill can be much higher than its current level as forced with persistent SST; (3) atmosphere-only forecasts driven by observed daily SST reaches beyond 40 days. It is also found that the MJO-TC (Tropical Cyclone) interactions have been much better represented in the UH and CFSv2 models than that in the GFS model. Both the CFSv2 and UH coupled models reasonably well capture the development of westerly wind bursts associated with November 2011 MJO and the cyclogenesis of TC05A in the Indian Ocean with a lead time of 2 weeks. However, the high-resolution GFS atmosphere-only model fails to reproduce the November MJO and the genesis of TC05A at 2 weeks' lead. This result highlights the necessity to get MJO right in order to ensure skillful extended-range TC forecasting. Keywords MJO forecasting skill Á GFS, CFSv2, and UH global models Á DYNAMO/CINDY field campaign Á Air-sea coupling Á Atmosphere-only forecast Á MJO-TC interactions Á Extended-range TC forecasting This paper is a contribution to the Topical Collection on Climate Forecast System Version 2 (CFSv2). CFSv2 is a coupled global climate model and was implemented by National Centers for Environmental Prediction (NCEP) in seasonal forecasting operations in March 2011.

Journal of Fluid Mechanics, 2006

Unstable modes of a linear shear flow in shallow water on an equatorial β-plane are obtained over... more Unstable modes of a linear shear flow in shallow water on an equatorial β-plane are obtained over a wide range of values of a non-dimensional parameter and are interpreted in terms of resonance between neutral waves. The non-dimensional parameter in the system is E ≡ γ 4 /(gHβ 2), where γ , g, H and β are the meridional shear of basic zonal flow, gravitational constant, equivalent depth and the northsouth gradient of the Coriolis parameter, respectively. The value of E is varied within the range −2.50 6 log E 6 7.50. The problem is solved numerically in a channel of width 5γ /β. The structures of the most unstable modes, and the combinations of resonating neutral waves that cause the instability, change according to the value of E as follows. For log E < 2.00, the most unstable modes have zonally non-symmetric structures; the most unstable modes for log E < 1.00 are caused by resonance between equatorial Kelvin modes and continuous modes, and those for 1.00 6 log E < 2.00 are caused by resonance between equatorial Kelvin modes and westward mixed Rossby-gravity modes. The most unstable modes for log E > 2.00 have symmetric structures and are identical with inertially unstable modes. Examinations of dispersion curves suggest that nonsymmetric unstable modes for 1.00 6 log E < 2.00 and inertially unstable modes for log E > 2.00 are the same kind of instability.

Climate Dynamics, 2011

Project Athena is an international collaboration testing the efficacy of high-resolution global c... more Project Athena is an international collaboration testing the efficacy of high-resolution global climate models. We compare results from 7-km mesh experiments of the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) and 10-km mesh experiments of the Integrated Forecast System (IFS), focusing on the Intra-Seasonal Oscillation (ISO) and its relationship with tropical cyclones (TC) among the boreal summer period (21 May-31 Aug) of 8 years (2001-2002, 2004-2009). In the first month of simulation, both models capture the intra-seasonal oscillatory behavior of the Indian monsoon similar to the observed boreal summer ISO in approximately half of the 8-year samples. The IFS simulates the NW-SE-oriented rainband and the westerly location better, while NICAM marginally reproduces mesoscale organized convective systems and better simulates the northward migration of the westerly peak and precipitation, particularly in 2006. The reproducibility of the evolution of MJO depends on the given year; IFS simulates the MJO signal well for 2002, while NICAM simulates it well for 2006. An empirical orthogonal function analysis shows that both models statistically reproduce MJO signals similar to observations, with slightly better phase speed reproduced by NICAM. Stronger TCs are simulated in NICAM than in IFS, and NICAM shows a wind-pressure relation for TCs closer to observations. TC cyclogenesis is active during MJO phases 3 and 4 in NICAM as in observations. The results show the potential of high-resolution global atmospheric models in reproducing some aspects of the relationship between MJO and TCs and the statistical behavior of TCs.

Journal of the Meteorological Society of Japan. Ser. II, 2010

An ensemble simulation of cyclone Nargis was performed using the Non-hydrostatic ICosahedral Atmo... more An ensemble simulation of cyclone Nargis was performed using the Non-hydrostatic ICosahedral Atmospheric Model (NICAM) at 14-km mesh size in order to examine the e¤ect on cyclogenesis of disturbances associated with intraseasonal oscillations. An analysis of observational data reveals that cyclone Nargis formed during the northward propagation of low-level zonal wind, associated with active cloud areas and precipitation from the equator to 20 N in the Bay of Bengal, when the active convective region associated with the Madden-Julian Oscillation (MJO) passed through the bay and then resided over the Maritime continent. The northward migration of low-level zonal wind, outgoing longwave radiation (OLR), and precipitation are successfully simulated in the ensemble results. Each simulated tropical cyclone (TC) genesis also occurs with northward migration and with a timing such that the active convective region associated with the MJO resides over the east side of the Maritime continent. The incipient disturbances that contributed to the initiation of cyclone Nargis formed during the period when the westerly wind burst passed through the Bay of Bengal after the monsoon onset and developed to TCs in the ensemble simulation. However, for an ensemble member for which northward migration as a monsoon onset is not simulated, no TC is formed in the Bay of Bengal. It is also found that the e¤ect of the easterlies across the northern part of the Malay Peninsula is important for TC formation in our simulation.

Monthly Weather Review, 2009

This study discloses detailed Madden–Julian oscillation (MJO) characteristics in the two 30-day i... more This study discloses detailed Madden–Julian oscillation (MJO) characteristics in the two 30-day integrations of the global cloud-system-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) using the all-season real-time multivariate MJO index of Wheeler and Hendon. The model anomaly is derived by excluding the observed climatology because the simulation is sufficiently realistic. Results show that the MJO has a realistic evolution in amplitude pattern, geographical locations, eastward propagation, and baroclinic- and westward-tilted structures. In the central Indian Ocean, convection develops with the low-level easterly wind anomaly then matures where the low-level easterly and westerly anomalies meet. Anomalous moisture tilts slightly with height. In contrast, over the western Pacific, the convection grows with a low-level westerly anomaly. Moisture fluctuations, leading convection in eastward propagation, tilt clearly westward with height. The frictional moisture converg...

Climate Dynamics, 2013

The present study assesses the forecast skill of the Madden-Julian Oscillation (MJO) observed dur... more The present study assesses the forecast skill of the Madden-Julian Oscillation (MJO) observed during the period of DYNAMO (Dynamics of the MJO)/CINDY (Cooperative Indian Ocean Experiment on Intraseasonal Variability in Year 2011) field campaign in the GFS (NCEP Global Forecast System), CFSv2 (NCEP Climate Forecast System version 2) and UH (University of Hawaii) models, and revealed their strength and weakness in forecasting initiation and propagation of the MJO. Overall, the models forecast better the successive MJO which follows the preceding event than that with no preceding event (primary MJO). The common modeling problems include too slow eastward propagation, the Maritime Continent barrier and weak intensity. The forecasting skills of MJO major modes reach 13, 25 and 28 days, respectively, in the GFS atmosphere-only model, the CFSv2 and UH coupled models. An equal-weighted multi-model ensemble with the CFSv2 and UH models reaches 36 days. Air-sea coupling plays an important role for initiation and propagation of the MJO and largely accounts for the skill difference between the GFS and CFSv2. A series of forecasting experiments by forcing UH model with persistent, forecasted and observed daily SST further demonstrate that: (1) air-sea coupling extends MJO skill by about 1 week; (2) atmosphere-only forecasts driven by forecasted daily SST have a similar skill as the coupled forecasts, which suggests that if the high-resolution GFS is forced with CFSv2 forecasted daily SST, its forecast skill can be much higher than its current level as forced with persistent SST; (3) atmosphere-only forecasts driven by observed daily SST reaches beyond 40 days. It is also found that the MJO-TC (Tropical Cyclone) interactions have been much better represented in the UH and CFSv2 models than that in the GFS model. Both the CFSv2 and UH coupled models reasonably well capture the development of westerly wind bursts associated with November 2011 MJO and the cyclogenesis of TC05A in the Indian Ocean with a lead time of 2 weeks. However, the high-resolution GFS atmosphere-only model fails to reproduce the November MJO and the genesis of TC05A at 2 weeks' lead. This result highlights the necessity to get MJO right in order to ensure skillful extended-range TC forecasting. Keywords MJO forecasting skill Á GFS, CFSv2, and UH global models Á DYNAMO/CINDY field campaign Á Air-sea coupling Á Atmosphere-only forecast Á MJO-TC interactions Á Extended-range TC forecasting This paper is a contribution to the Topical Collection on Climate Forecast System Version 2 (CFSv2). CFSv2 is a coupled global climate model and was implemented by National Centers for Environmental Prediction (NCEP) in seasonal forecasting operations in March 2011.

Journal of Fluid Mechanics, 2006

Unstable modes of a linear shear flow in shallow water on an equatorial β-plane are obtained over... more Unstable modes of a linear shear flow in shallow water on an equatorial β-plane are obtained over a wide range of values of a non-dimensional parameter and are interpreted in terms of resonance between neutral waves. The non-dimensional parameter in the system is E ≡ γ 4 /(gHβ 2), where γ , g, H and β are the meridional shear of basic zonal flow, gravitational constant, equivalent depth and the northsouth gradient of the Coriolis parameter, respectively. The value of E is varied within the range −2.50 6 log E 6 7.50. The problem is solved numerically in a channel of width 5γ /β. The structures of the most unstable modes, and the combinations of resonating neutral waves that cause the instability, change according to the value of E as follows. For log E < 2.00, the most unstable modes have zonally non-symmetric structures; the most unstable modes for log E < 1.00 are caused by resonance between equatorial Kelvin modes and continuous modes, and those for 1.00 6 log E < 2.00 are caused by resonance between equatorial Kelvin modes and westward mixed Rossby-gravity modes. The most unstable modes for log E > 2.00 have symmetric structures and are identical with inertially unstable modes. Examinations of dispersion curves suggest that nonsymmetric unstable modes for 1.00 6 log E < 2.00 and inertially unstable modes for log E > 2.00 are the same kind of instability.

Climate Dynamics, 2011

Project Athena is an international collaboration testing the efficacy of high-resolution global c... more Project Athena is an international collaboration testing the efficacy of high-resolution global climate models. We compare results from 7-km mesh experiments of the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) and 10-km mesh experiments of the Integrated Forecast System (IFS), focusing on the Intra-Seasonal Oscillation (ISO) and its relationship with tropical cyclones (TC) among the boreal summer period (21 May-31 Aug) of 8 years (2001-2002, 2004-2009). In the first month of simulation, both models capture the intra-seasonal oscillatory behavior of the Indian monsoon similar to the observed boreal summer ISO in approximately half of the 8-year samples. The IFS simulates the NW-SE-oriented rainband and the westerly location better, while NICAM marginally reproduces mesoscale organized convective systems and better simulates the northward migration of the westerly peak and precipitation, particularly in 2006. The reproducibility of the evolution of MJO depends on the given year; IFS simulates the MJO signal well for 2002, while NICAM simulates it well for 2006. An empirical orthogonal function analysis shows that both models statistically reproduce MJO signals similar to observations, with slightly better phase speed reproduced by NICAM. Stronger TCs are simulated in NICAM than in IFS, and NICAM shows a wind-pressure relation for TCs closer to observations. TC cyclogenesis is active during MJO phases 3 and 4 in NICAM as in observations. The results show the potential of high-resolution global atmospheric models in reproducing some aspects of the relationship between MJO and TCs and the statistical behavior of TCs.

Journal of the Meteorological Society of Japan. Ser. II, 2010

An ensemble simulation of cyclone Nargis was performed using the Non-hydrostatic ICosahedral Atmo... more An ensemble simulation of cyclone Nargis was performed using the Non-hydrostatic ICosahedral Atmospheric Model (NICAM) at 14-km mesh size in order to examine the e¤ect on cyclogenesis of disturbances associated with intraseasonal oscillations. An analysis of observational data reveals that cyclone Nargis formed during the northward propagation of low-level zonal wind, associated with active cloud areas and precipitation from the equator to 20 N in the Bay of Bengal, when the active convective region associated with the Madden-Julian Oscillation (MJO) passed through the bay and then resided over the Maritime continent. The northward migration of low-level zonal wind, outgoing longwave radiation (OLR), and precipitation are successfully simulated in the ensemble results. Each simulated tropical cyclone (TC) genesis also occurs with northward migration and with a timing such that the active convective region associated with the MJO resides over the east side of the Maritime continent. The incipient disturbances that contributed to the initiation of cyclone Nargis formed during the period when the westerly wind burst passed through the Bay of Bengal after the monsoon onset and developed to TCs in the ensemble simulation. However, for an ensemble member for which northward migration as a monsoon onset is not simulated, no TC is formed in the Bay of Bengal. It is also found that the e¤ect of the easterlies across the northern part of the Malay Peninsula is important for TC formation in our simulation.