Indian Ocean Tropical Cyclones Research Articles

Impact of Kalpana-1-Derived Water Vapor Winds on Indian Ocean Tropical Cyclone Forecasts

Abstract The water vapor winds from the operational geostationary Indian National Satellite (INSAT) Kalpana-1 have recently become operational at the Space Applications Centre (SAC). A series of experimental forecasts are attempted here to evaluate the impact of water vapor winds derived from Kalpana-1 for the track and intensity prediction of two Bay of Bengal tropical cyclones (TCs), Sidr and Nargis, using the Weather Research and Forecasting (WRF) modeling system. The assimilation of water vapor winds has made some impact in the initial position errors as well as track forecasts when compared with the corresponding control experiments for both TCs. However, no statistically significant improvement is noticed in the simulations of TC intensities [i.e., minimum sea level pressure (MSLP) and maximum surface winds forecasts when satellite winds are used for assimilation]. Moreover, the performance of Kalpana-1 winds is evaluated by repeating the same sets of experiments using Meteosat-7 winds derived at the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and compared with observed data. The simulation of initial position errors, and track and intensity forecasts using the assimilation of water vapor winds from both satellites are comparable. Though, these results are preliminary with respect to the Kalpana-1 winds, the present study can provide some insight to the WRF model users over the Indian Ocean region.

Southern hemisphere tropical cyclone intensity forecast methods used at the Joint Typhoon Warning Center, Part II: statistical-dynamical forecasts

The development and performance of a statistical - dynamical tropical cyclone intensity forecast model, which was developed for the United States of America’s Joint Typhoon Warning Center (JTWC), is described. This model, called the Southern Hemisphere Statistical Typhoon Intensity Prediction Scheme (SH STIPS), mirrors similar capabilities created for use in the western North Pacific and North Indian Ocean tropical cyclone basins. The model is created by fitting an optimal combination of factors related to climatology and persistence, intensification potential, vertical wind shear, dynamic size/intensity forecasts and atmospheric stability. All of these factors except the climatology and persistence information are derived from global forecast model analyses and forecasts. In July 2005 the SH STIPS model began a real-time evaluation period. The forecasts from the SH STIPS model have outperformed the combined climatology and persistence based forecast and thus are skillful in independent testing since that time. Since October 2006, SH STIPS has been the primary member in an operational consensus forecast of tropical cyclone intensity change provided to the JTWC. Documentation is provided for potential users of forecasts based on this methodology and for researchers interested in developing similar capabilities in the future.

On the peculiar storm track of TC Favio during the 2006–2007 Southwest Indian Ocean tropical cyclone season and relationships to ENSO

The 2006–2007 Southern Indian Ocean tropical cyclone season proved to be devastating for the already flood-ravaged coastline of Mozambique. The landfall of Tropical Cyclone Favio on 22 February 2007 led to several deaths and considerable damage. A unique aspect of Tropical Cyclone Favio was its westward track around the southern tip of Madagascar and then northwestwards through the Mozambique Channel to make landfall in southern Mozambique – most tropical cyclones in this region track southeastwards after approaching Madagascar and recurve back into the South Indian Ocean. In addition to its track, Tropical Cyclone Favio was highly unusual in that it made landfall in Mozambique during an El Nino year, an occurrence that has only happened one other time in the known record. It is shown that the impacts of the 2006–2007 El Nino over the South Indian Ocean (in terms of sea surface temperature and atmospheric circulation) were very different from previous El Nino events, thus helping to create more favourable conditions for landfall than would otherwise be expected. An analysis relating the unusual behaviour of Favio to the large scale climate mode patterns in 2006–2007 is presented. It is concluded that the unique track and anomalous landfall of Tropical Cyclone Favio are related and that seasonally-based statistical models for predicting the risk of landfall in Mozambique should be adjusted to accommodate anomalous circulation patterns that occur on synoptic to intraseasonal scales.

The Impact of Variational Assimilation of SSM/I and QuikSCAT Satellite Observations on the Numerical Simulation of Indian Ocean Tropical Cyclones

Abstract In this study, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) with three-dimensional variational data assimilation (3DVAR) is utilized to investigate the influence of Special Sensor Microwave Imager (SSM/I) and Quick Scatterometer (QuikSCAT) observations on the prediction of an Indian Ocean tropical cyclone. The 3DVAR sensitivity runs were conducted separately with QuikSCAT wind vectors, SSM/I wind speeds, and total precipitable water (TPW) to investigate their individual impact on cyclone intensity and track. The Orissa supercyclone over the Bay of Bengal during October 1999 was used for simulation and assimilation experiments. Assimilation of the QuikSCAT wind vector improves the initial position of the cyclone’s center with a position error of 33 km, which was 163 km in the background analysis. Incorporation of QuikSCAT winds was found to increase the air–sea heat fluxes over the cyclonic region, which resulted in the improved simulated intensity when compared with the simulation made without QuikSCAT winds in the initial conditions. The cyclone track improved significantly with assimilation of QuikSCAT wind vectors. The track improvement resulted from relocation of the initial cyclonic vortex after assimilation of QuikSCAT wind vectors. Like QuikSCAT, assimilation of SSM/I wind speeds strengthened the cyclonic circulation in the initial conditions. This increase in the low-level wind speeds enhanced the air–sea exchange processes and improved the simulated intensity of the cyclone. The lack of information about the wind direction from SSM/I prevented it from making much of an impact on track prediction. As compared to the first guess, assimilation of the SSM/I TPW shows a moistening of the lower troposphere over most of the Bay of Bengal except over the central region of the cyclone, where the assimilation of SSM/I TPW reduces the lower-tropospheric moisture. This decrease of moisture in the TPW assimilation experiment resulted in a weak cyclone intensity.

Modulation of South Indian Ocean Tropical Cyclones by the Madden–Julian Oscillation and Convectively Coupled Equatorial Waves

Abstract The subseasonal modulation of tropical cyclone (TC) genesis by large-scale atmospheric wave modes is studied using data from the south Indian Ocean region. The modes considered are the Madden–Julian oscillation (MJO), and the convectively coupled equatorial Rossby (ER), Kelvin, and mixed Rossby–gravity (MRG) waves. Analysis of all TCs west of 100°E reveals a large and statistically significant modulation by the MJO and ER waves, a small yet significant modulation by Kelvin waves, and a statistically insignificant modulation by MRG waves. Attribution of the observed TC modulation was made through examination of the wave-induced perturbations to the dynamical fields of low-level vorticity, vertical shear, and deep convection. Possible thermodynamic influences on TC genesis were neglected. Different combinations of the three dynamical fields were necessary for successful attribution for each of the large-scale wave modes. For example, for the MJO, the modulation was best attributable to its perturbations to both the vorticity and shear fields, while for the ER wave, it was its perturbations to the convection and vorticity fields that appeared to best be able to explain the modulation. It appears that there is no single factor that can be used for the attribution of all subseasonal TC variability. Finally, it is shown that the modulation of TCs by at least the MJO and ER waves is large enough to warrant further investigation for prediction on the weekly time scale.

Tropical Cyclone Eline and Its Unusual Penetration and Impacts over the Southern African Mainland

Abstract February–March 2000 saw devastating floods in Mozambique, Zimbabwe, and South Africa. Due to the huge damage and loss of life, global media attention was extensive. Less well known is that one of the weather systems that contributed to these floods (ex-Tropical Cyclone Eline) tracked almost 2000 km across southern Africa toward the cool southeast Atlantic and led to substantial rainfall over arid to semiarid southern Namibia (over two standard deviations above average for these two months and the wettest summer since 1976). Less than 5% of southwest Indian Ocean tropical cyclones actually make landfall on the east coast of southern Africa and even fewer significantly penetrate into the interior, because of the relatively dry 1–1.5-km-high interior plateau that covers most of the region. It is argued that the precursor synoptic conditions together with large-scale circulation and SST anomalies over the Indian Ocean associated with a strengthening La Nina were highly favorable for this unusual evol...

The effects of the Southwest Indian Ocean tropical cyclones on Ethiopian drought

Tropical cyclones are one of the prominent weather systems that are generated over the tropical oceans. The cyclones that develop in the Southwest Indian Ocean (SWIO) usually travel west then southwest and finally recurve to southeast, generally before reaching the East African coast. However, it is shown in this study how SWIO-tropical cyclones/depressions can indirectly affect Ethiopian weather. Using correlation and composite analyses, interannual and intraseasonal rainfall variations in Ethiopia were compared with the frequency and time of occurrence of the tropical cyclones. Years showing the consecutive occurrence of several tropical depressions over the SWIO coincide with the drought years of Ethiopia. Reciprocally, years of abnormally low frequency of tropical cyclones are associated with heavy rainfall in Ethiopia. Belg rainfall (‘small rains’, February–May) is much more influenced by the cyclonic activity than Kiremt rainfall (‘big rains’, June–September), which occurs outside the cyclonic season of the Southeast Indian Ocean. On a daily basis, rainfall activity during the Belg period is significantly reduced when a tropical depression is observed in the SWIO, before picking up again a few days later. No systematic time-lag was found between the cyclone occurrence and the decrease in daily rainfall amounts. However, at interannual time-scales, a higher (lower) frequency of tropical depressions during the months of November–January tends to be followed by abnormally low (high) Belg rainfall. One of the mechanisms suggested to explain this seasonal persistence in the frequency of tropical depressions involves sea-surface temperatures in the SWIO, from where many of the depressions originate. The atmospheric processes accounting for the teleconnection between Ethiopia rainfall and tropical depressions were examined. Monthly anomalies of upper air winds for years showing frequent occurrence of tropical depressions proved significant not only over the SWIO, but also the Northern Hemisphere above Northeast Africa. In addition to a slightly enhanced cross-equatorial flow, diverting moisture advection towards the active centre of the system, much stronger than usual equatorial easterlies in the upper troposphere and a reduced southward excursion of the subtropical jet stream result in weaker convective activity over Ethiopia. © 1998 Royal Meteorological Society

Some Large-Scale Characteristics Associated with Tropical Cyclone Development in the North Indian Ocean during FGGE

Abstract This paper discusses the meteorological conditions associated with tropical cyclone formation in the north Indian Ocean during the 1979 FGGE year. Seven developing systems are composited together using FUGE Ill-b analyses to show the common circulation features surrounding developing cloud clusters. Three systems are further discussed to show different environmental influences on the low-level buildup of circulation during formation. The characteristics of these three disturbances’ 200 mb outflow patterns and a general discussion of north Indian Ocean tropical cyclone activity are also given. Results show that tropical cyclone formation generally follows the initial increase of strong low-level winds on one side (either equatorial or polar) of a precyclone disturbance. This early buildup of wind appears to result from environmentally forced asymmetric wind surge action. Some of this increase appears to result from inward advection of velocity, but part of the increase seems to occur in situ. Thes...

Statistical prediction of tropical storm motion over the Bay of Bengal and Arabian Sea

Statistical analog models for the prediction of tropical cyclone motion have been developed for all of the world's tropical cyclone basins. The popularity and success of these models is related to their ability to optimize the variance reducing potential of climatology and persistence, At the same time, they effectively recognize certain repetitious synoptic patterns without having to use fields of sometimes unavailable and often unreliable synoptic data over the tropics.
 The analog method can be simulated by the use of continuous function derived through the use of multivariate regression analysis. This method simulates all aspects of the analog process including the derivation of probability ellipses while avoiding certain operational problems inherent to the purely analog approach.
 This paper describes the derivation of such a model for the north Indian Ocean tropical cyclone basin. It is patterned after an earlier paper (Neumann and Randrianarison 1976) which describes the development of a similar model for the southwest Indian Ocean tropical cyclone basin.