Cyclone Gonu - Meteorological History

Meteorological History

Toward the end of May 2007, the monsoon trough spawned a low pressure area in the eastern Arabian Sea. By May 31, an organized tropical disturbance was located about 645 km (400 mi) south of Mumbai, India, with cyclonic convection, or thunderstorm activity, and a well-defined mid-level circulation. The disturbance initially lacked a distinct low-level circulation; instead it consisted of strong divergence along the western end of a surface trough of low pressure. A favorable upper-level environment allowed convection to improve, and by late on June 1, the system developed to the extent that the India Meteorological Department (IMD) classified it as a depression. It tracked westward along the southwestern periphery of a mid-level ridge over southern India. Convection continued to organize, and early on June 2 the Joint Typhoon Warning Center (JTWC) classified it Tropical Cyclone 02A about 685 km (425 mi) southwest of Mumbai.

Upon first forming, the system contended with the entrainment of dry air to the northwest of the storm, which was expected to limit intensification. The storm steadily intensified, and early on June 2 the IMD upgraded it to deep depression status. Later in the day the IMD classified the system as Cyclonic Storm Gonu about 760 km (470 mi) southwest of Mumbai, India. As a mid-latitude trough developed over Pakistan, Gonu turned to the north and northeast, though it resumed a westward track after ridging built to the north of the storm. With a solid area of intense convection, it rapidly intensified to attain severe cyclonic status early on June 3, and with good outflow the JTWC upgraded it to the equivalent of a minimal hurricane. The dry air ultimately had a smaller impact on the intensification than previously estimated. A well-defined eye developed in the center of convection, and after moving over a local increase in ocean heat content, Gonu rapidly deepened.

Late on June 3, the IMD upgraded the storm to Very Severe Cyclonic Storm Gonu. With low amounts of vertical wind shear and favorable upper-level outflow, Gonu strengthened further to attain peak 1-min sustained winds of 270 km/h (165 mph) and gusts to 315 km/h (195 mph), about 285 km (175 mi) east-southeast of Masirah Island on the coast of Oman. The IMD upgraded it to Super Cyclonic Storm Gonu late on June 4, with peak 3-min sustained winds reaching 240 km/h (150 mph) and an estimated pressure of 920 mbar.

After the storm maintained peak winds for about six hours, the IMD downgraded Gonu to very severe cyclonic storm status late on June 4. Its eye became cloud-filled and ragged, and the cyclone gradually weakened due to cooler water temperatures and drier air as it approached the Arabian Peninsula. Due to land interaction with Oman, the inner core of deep convection rapidly weakened, and over a period of 24 hours the intensity decreased by 95 km/h (60 mph). According to the IMD, Cyclone Gonu crossed the eastern-most tip of Oman still as a very severe cyclonic storm early on June 6. Although the winds continued to gradually decrease, overall organization increased slightly in the hours prior to landfall; Gonu maintained a well-defined low-level structure with a weak eye.

After emerging into the Gulf of Oman, the cyclone briefly re-intensified slightly. However, increasing wind shear and entrainment of dry air from the Arabian Peninsula continued to remove deep convection from its eastern semicircle. On June 6, the cyclone turned to the north-northwest, and later that day the JTWC downgraded Gonu to tropical storm status. The IMD followed suit by downgrading Gonu to severe cyclonic storm status and later to cyclonic storm status early on June 7. Gonu crossed the Makran coast in Iran six hours later and the IMD stopped issuing advisories on the cyclone.