Iam-Fei Pun

Upper-Ocean Thermal Structure and the Western North Pacific Category 5 Typhoons. Part I: Ocean Features and the Category 5 Typhoons’ Intensification

Abstract Category 5 cyclones are the most intense and devastating cyclones on earth. With increasing observations of category 5 cyclones, such as Hurricane Katrina (2005), Rita (2005), Mitch (1998), and Supertyphoon Maemi (2003) found to intensify on warm ocean features (i.e., regions of positive sea surface height anomalies detected by satellite altimeters), there is great interest in investigating the role ocean features play in the intensification of category 5 cyclones. Based on 13 yr of satellite altimetry data, in situ and climatological upper-ocean thermal structure data, best-track typhoon data of the U.S. Joint Typhoon Warning Center, together with an ocean mixed layer model, 30 western North Pacific category 5 typhoons that occurred during the typhoon season from 1993 to 2005 are systematically examined in this study. Two different types of situations are found. The first type is the situation found in the western North Pacific south eddy zone (SEZ; 21°–26°N, 127°–170°E) and the Kuroshio (21°–30...

The Interaction of Supertyphoon Maemi (2003) with a Warm Ocean Eddy

Understanding the interaction of ocean eddies with tropical cyclones is critical for improving the understanding and prediction of the tropical cyclone intensity change. Here an investigation is presented of the interaction between Supertyphoon Maemi, the most intense tropical cyclone in 2003, and a warm ocean eddy in the western North Pacific. In September 2003, Maemi passed directly over a prominent (700 km 500 km) warm ocean eddy when passing over the 22°N eddy-rich zone in the northwest Pacific Ocean. Analyses of satellite altimetry and the best-track data from the Joint Typhoon Warning Center show that during the 36 h of the Maemi–eddy encounter, Maemi’s intensity (in 1-min sustained wind) shot up from 41 ms 1 to its peak of 77 m s 1 . Maemi subsequently devastated the southern Korean peninsula. Based on results from the Coupled Hurricane Intensity Prediction System and satellite microwave sea surface temperature observations, it is suggested that the warm eddies act as an effective insulator between typhoons and the deeper ocean cold water. The typhoon’s self-induced sea surface temperature cooling is suppressed owing to the presence of the thicker upper-ocean mixed layer in the warm eddy, which prevents the deeper cold water from being entrained into the upper-ocean mixed layer. As simulated using the Coupled Hurricane Intensity Prediction System, the incorporation of the eddy information yields an evident improvement on Maemi’s intensity evolution, with its peak intensity increased by one category and maintained at category-5 strength for a longer period (36 h) of time. Without the presence of the warm ocean eddy, the intensification is less rapid. This study can serve as a starting point in the largely speculative and unexplored field of typhoon–warm ocean eddy interaction in the western North Pacific. Given the abundance of ocean eddies and intense typhoons in the western North Pacific, these results highlight the importance of a systematic and in-depth investigation of the interaction between typhoons and western North Pacific eddies.

Upper-Ocean Thermal Structure and the Western North Pacific Category 5 Typhoons. Part II: Dependence on Translation Speed

Abstract Using new in situ ocean subsurface observations from the Argo floats, best-track typhoon data from the U.S. Joint Typhoon Warning Center, an ocean mixed layer model, and other supporting datasets, this work systematically explores the interrelationships between translation speed, the ocean’s subsurface condition [characterized by the depth of the 26°C isotherm (D26) and upper-ocean heat content (UOHC)], a cyclone’s self-induced ocean cooling negative feedback, and air–sea enthalpy fluxes for the intensification of the western North Pacific category 5 typhoons. Based on a 10-yr analysis, it is found that for intensification to category 5, in addition to the warm sea surface temperature generally around 29°C, the required subsurface D26 and UOHC depend greatly on a cyclone’s translation speed. It is observed that even over a relatively shallow subsurface warm layer of D26 ∼ 60–70 m and UOHC ∼ 65–70 kJ cm−2, it is still possible to have a sufficient enthalpy flux to intensify the storm to category 5...

Validation and Application of Altimetry-Derived Upper Ocean Thermal Structure in the Western North Pacific Ocean for Typhoon-Intensity Forecast

This paper uses more than 5000 colocated and near-coincident in-situ profiles from the National Oceanic and Atmospheric Administration/Global Temperature and Salinity Profile Program database spanning over the period from 2002 to 2005 to systematically validate the satellite-altimetry-derived upper ocean thermal structure in the western North Pacific ocean as such ocean thermal structure information is critical in typhoon-intensity change. It is found that this satellite-derived information is applicable in the central and the southwestern North Pacific (covering 122-170degE, 9-25degN) but not in the northern part (130-170degE, 25-40degN). However, since > 80% of the typhoons are found to intensify in the central and southern part, this regional dependence should not pose a serious constraint in studying typhoon intensification. Further comparison with the U.S. Naval Research Laboratorys North Pacific Ocean Nowcast/Forecast System (NPACNFS) hydrodynamic ocean model shows similar regional applicability, but NPACNFS is found to have a general underestimation in the upper ocean thermal structure and causes a large underestimation of the tropical cyclone heat potential (TCHP) by up to 60 kJ/cm2. After validation, the derived upper ocean thermal profiles are used to study the intensity change of supertyphoon Dianmu (2004). It is found that two upper ocean parameters, i.e., a typhoons self-induced cooling and the during-typhoon TCHP, are the most sensitive parameters (with R 2~0.7) to the 6-h intensity change of Dianmu during the study period covering Dianmus rapid intensification to category 5 and its subsequent decay to category 4. This paper suggests the usefulness of satellite-based upper ocean thermal information in future research and operation that is related to typhoon-intensity change in the western North Pacific

“Category‐6” supertyphoon Haiyan in global warming hiatus: Contribution from subsurface ocean warming

With the extra-ordinary intensity of 170 kts, supertyphoon Haiyan devastated the Philippines in November 2013. This intensity is among the highest ever observed for tropical cyclones (TCs) globally, 35 kts well above the threshold (135kts) of the existing highest category of 5. Though there is speculation to associate global warming with such intensity, existing research indicate that we have been in a warming hiatus period, with the hiatus attributed to the La Nina-like multi-decadal phenomenon. It is thus intriguing to understand why Haiyan can occur during hiatus. It is suggested that as the western Pacific manifestation of the La Nina-like phenomenon is to pile up warm subsurface water to the west, the western North Pacific experienced evident subsurface warming and created a very favorable ocean pre-condition for Haiyan. Together with its fast traveling speed, the air-sea flux supply was 158% as compared to normal for intensification.

Slow translation speed causes rapid collapse of northeast Pacific Hurricane Kenneth over cold core eddy

Category 4 Hurricane Kenneth (HK) experienced unpredicted rapid weakening when it stalled over a cold core eddy (CCE) on 19–20 September 2005, 2800 km SE of Hawaii. Maximum sea surface temperature (SST) cooling of 8–9°C and a minimum aerially averaged SST of 18.3°C (over 8750 km2) characterized its cool wake. A 3-D mixed-layer model enabled estimation of enthalpy fluxes (latent and sensible heat), as well as the relative importance of slow translation speed (Uh) compared with the preexisting CCE. As Uh dropped below 1.5 m s−1, enthalpy fluxes became negative, cutting off direct ocean energy flux to HK. Although HKs weakening was attributed to wind shear, our results indicate that slow Uh and consequent intense SST cooling were the main causes. The tropical cyclone-intensified CCE experienced rapid growth in magnitude (−6 to −40 cm), increased diameter (60 to 350 km), elevated chlorophyll a for 4 months, and 12 month longevity.

Tropical cyclone‐ocean interaction in Typhoon Megi (2010)—A synergy study based on ITOP observations and atmosphere‐ocean coupled model simulations

A mesoscale model coupling the Weather Research and Forecasting model and the three-dimensional Price-Weller-Pinkel ocean model is used to investigate the dynamical ocean response to Megi (2010). It is found that Megi induces sea surface temperature (SST) cooling very differently in the Philippine Sea (PS) and the South China Sea (SCS). The results are compared to the in situ measurements from the Impact of Typhoons on the Ocean in the Pacific (ITOP) 2010 field experiment, satellite observations, and ocean analysis field from Eastern Asian Seas Ocean Nowcast/Forecast System of the U.S. Naval Research Laboratory. The uncoupled and coupled experiments simulate relatively accurately the track and intensity of Megi over PS; however, the simulated intensity of Megi over SCS varies significantly among the experiments. Only the experiment coupled with three-dimensional ocean processes, which generates rational SST cooling, reasonably simulates the storm intensity in SCS. Our results suggest that storm translation speed and upper ocean thermal structure are two main factors responsible for Megis distinct different impact over PS and over SCS. In addition, it is shown that coupling with one-dimensional ocean process (i.e., only vertical mixing process) is not enough to provide sufficient ocean response, especially under slow translation speed (~2–3 m s−1), during which vertical advection (or upwelling) is significant. Therefore, coupling with three-dimensional ocean processes is necessary and crucial for tropical cyclone forecasting. Finally, the simulation results show that the stable boundary layer forms on top of the Megi-induced cold SST area and increases the inflow angle of the surface wind.

Air-sea fluxes for Hurricane Patricia (2015): Comparison with supertyphoon Haiyan (2013) and under different ENSO conditions

Hurricane Patricia formed on October 20th, 2015 in the Eastern Pacific and, in less than 3 days, rapidly intensified from a Tropical Storm to a record-breaking hurricane with maximum sustained winds measured around 185 knots. It is almost 15 knots higher than 2013s supertyphoon Haiyan (the previous strongest tropical cyclone (TC) ever observed). This research focuses on analyzing the air-sea enthalpy flux conditions that contributed to hurricane Patricias rapid intensification, and comparing them to supertyphoon Haiyans. Despite a stronger cooling effect, a higher enthalpy flux supply is found during Patricia, in particular due to warmer pre-TC sea surface temperature conditions. This resulted in larger temperature and humidity differences at the air-sea interface, contributing to larger air-sea enthalpy heat fluxes available for Patricias growth (24% more than for Haiyan). In addition, air-sea fluxes simulations were performed for hurricane Patricia under different climate conditions to assess specifically the impact of local and large-scale conditions on storm intensification associated with six different phases and types of El Nino Southern Oscillation (ENSO) and long-term climatological summer condition. We found that the Eastern Pacific El Nino developing and decaying summers, and the Central Pacific El Nino developing summer are the three most favorable ENSO conditions for storm intensification. This still represents a 37% smaller flux supply than in October 2015, suggesting that Patricia extraordinary growth is not achievable under any of these typical ENSO conditions but rather the result of the exceptional environmental conditions associated with the build-up of the strongest El Nino ever recorded.

Influence of the Size of Supertyphoon Megi (2010) on SST Cooling

AbstractSupertyphoon Megi (2010) left behind two very contrasting SST cold wake cooling patterns between the Philippine Sea (1.5°C) and the South China Sea (7 °C). Based on various radii of radial winds, we found that the size of Megi doubles over the South China Sea when it curving northwards. On average, the radius of maximum wind (RMW) increased from 18.8 km over the Philippine Sea to 43.1 km over the South China Sea; the radius of 64 kt typhoon-force wind (R64) increased from 52.6 km to 119.7 km; the radius of 50 kt damaging-force wind (R50) increased from 91.8 km to 210 km; and the radius of 34 kt gale-force wind (R34) increased from 162.3 km to 358.5 km. To investigate the typhoon size effect, we conduct a series of numerical experiments on Megi-induced SST cooling by keeping other factors unchanged, i.e., typhoon translation speed and ocean subsurface thermal structure. The results show that if it were not for Megi’s size increase over the South China Sea, the during-Megi SST cooling magnitude woul...

Satellite-Derived Ocean Thermal Structure for the North Atlantic Hurricane Season

AbstractThis paper describes a new model (method) called Satellite-derived North Atlantic Profiles (SNAP) that seeks to provide a high-resolution, near-real-time ocean thermal field to aid tropical cyclone (TC) forecasting. Using about 139 000 observed temperature profiles, a spatially dependent regression model is developed for the North Atlantic Ocean during hurricane season. A new step introduced in this work is that the daily mixed layer depth is derived from the output of a one-dimensional Price–Weller–Pinkel ocean mixed layer model with time-dependent surface forcing.The accuracy of SNAP is assessed by comparison to 19 076 independent Argo profiles from the hurricane seasons of 2011 and 2013. The rms differences of the SNAP-estimated isotherm depths are found to be 10–25 m for upper thermocline isotherms (29°–19°C), 35–55 m for middle isotherms (18°–7°C), and 60–100 m for lower isotherms (6°–4°C). The primary error sources include uncertainty of sea surface height anomaly (SSHA), high-frequency fluc...