• S INDIRA RANI

      Articles written in Journal of Earth System Science

    • Impact of SAPHIR radiances on the simulation of tropical cyclones over the Bay of Bengal using NCMRWF hybrid-4DVAR assimilation and forecast system

      DEVANIL CHOUDHURY ANKUR GUPTA S INDIRA RANI JOHN P GEORGE

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      Observing System Experiments (OSEs) were conducted to analyze the impact of assimilation of Megha-Tropique’s (MT) Sounder for Probing Vertical Profiles of Humidity (SAPHIR) radiances on the simulation of tracks and intensity of three tropical cyclones (Kyant, Vardah, and Maarutha) formed over the Bay of Bengal during 2016–2017 North Indian Ocean cyclone period. National Centre for Medium Range Weather Forecast (NCMRWF) Unified Model (NCUM) Hybrid-4DVAR assimilation and forecast system was used for the OSEs. Assimilation of SAPHIR radiances produced an improvement of 9% and 12%, respectively, in the cyclones’ central sea level pressure (CSLP) and the maximum sustained wind (MSW), while an improvement of 38% was seen in the cyclone tracks within the forecast lead time of 120 hrs. Initial assessment shows that the improvement in the cyclone intensity is due to the assimilation of the unique surface peaking channel of SAPHIR (channel-6), whereas the improvement in the cyclone track is due to the assimilation remaining five channels of SAPHIR. Thus, the assimilation of SAPHIR radiances in the NCUM system showed improvement in both intensity and track of the cyclones over the Bay of Bengal; however, more cyclone cases over different ocean basins have to be analyzed to make a robust conclusion. This study specifies the importance of similar microwave humidity instruments in the same frequency range for the detailed exploration of cyclone track and structure.

      $\bf{Highlights}$

      $\bullet$ Impact of SAPHIR humidity channel information in the NCMRWF Hybrid-4DVar assimilation and forecast system is analysed through Observing system experiments (OSEs)

      $\bullet$ Assimilation of SAPHIR humidity information improved both track and intensity of the cyclones compared to the control experiment, and the improvement is visible upto a lead time of 5 days

      $\bullet$ It is noted that the improvement in the cyclone intensity simulation is due to the assimilation of the lowest peaking channel of the SAPHIR, while the track improvement is contributed by other channels as well.

      $\bullet$ This study underlines the importance of SAPHIR like instruments in the low earth orbiting satellites with frequent revisit time to explore the features of cyclones.

    • Assimilation of individual components of radiosonde winds: An investigation to assess the impact of single-component winds from space-borne measurements on NWP

      S INDIRA RANI PRITI SHARMA JOHN P GEORGE M DAS GUPTA

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      Wind observations are critical for the better atmospheric analysis for Numerical Weather Prediction (NWP), particularly over the tropics. High-resolution direct wind observations are essential for defining smaller scales and deeper atmospheric structures. Recently launched Aeolus satellite delivers wind profiles that mostly satisfy these requirements, suitable for NWP assimilation. The main product from Aeolus is the horizontally projected Line of Sight wind component, a single component of wind, approximately zonal in nature over the tropics and more meridional over the Polar region, and the main limitation of this observation. Observing system experiments are conducted with the assimilation of individual components of radiosonde and pilot balloon winds to assess the impact of a single component of wind compared to the assimilation of full wind vector in the NCMRWF global assimilation and forecast system. Denial of the zonal component of wind in the assimilation system produced a larger observation increment (observation – model background) in the meridional wind than the full vector assimilation. In contrast, the observation increment of the zonal wind remains nearly the same, even after removing the meridional wind component from the assimilation system. Assimilation of both zonal and meridional components produced changes in the analysis fields of various meteorological variables; however, the zonal component plays a significant role in the tropics. Both wind components play an important role in controlling the humidity field, whereas only zonal components of wind impact the temperature field in the upper troposphere and lower stratosphere. Though the full vector wind assimilation produces a larger impact in the forecast fields of various meteorological variables, the zonal component has more impact than the meridional component. Verification of analysis and forecast wind against the satellite-derived atmospheric motion vectors clearly show the importance of both the horizontal components of winds in the lower troposphere. In contrast, the zonal component of wind alone has a high impact on the upper troposphere and lower stratosphere.

    • Validation and assimilation of INSAT atmospheric motion vectors: Case studies for tropical cyclones

      PRITI SHARMA S INDIRA RANI M DAS GUPTA

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      INSAT-3DR is the latest geostationary satellite launched by the Indian Space Research Organization (ISRO) as a continuation to the INSAT-3D, for enhanced meteorological observations. National Centre for Medium Range Weather Forecasting (NCMRWF) receives INSAT-3DR Atmospheric Motion Vectors (AMVs) through Global Telecommunication System (GTS) along with the AMVs from other satellites. The INSAT-3DR AMVs are validated against the in-situ observations for a period of 3 months, May–July 2020. The validation results are compared with the AMVs from other satellites like INSAT-3D and Meteosat-8 located over the same geographical area and found that the quality of INSAT-3DR AMVs is comparable. After the successful validation, INSAT-3DR AMVs are assimilated in the NCMRWF Global Forecast System (NGFS) for two cyclone cases, formed during May–June 2020 over the North Indian Ocean. Four Observation System Experiments (OSEs) are designed, with the assimilation of individual and combined AMVs from INSAT (3D and 3DR) and Meteosat-8, to see the impact of AMVs during the cyclones Amphan formed over the Bay of Bengal and Nisarga formed over the Arabian Sea. In general, assimilation of AMVs improved the simulation of both the cyclones Amphan and Nisarga formed during May–June 2020. Introduction of INSAT AMVs slowed down the otherwise fast-moving cyclone Amphan simulated due to the assimilation of Meteosat-8 AMVs. Both intensity and track of the cyclones Amphan and Nisarga are better simulated when the AMVs from INSAT and Meteosat-8 are assimilated together.

    • Improved skill of NCMRWF Unified Model (NCUM-G) in forecasting tropical cyclones over NIO during 2015–2019

      SUSHANT KUMAR ANUMEHA DUBE SUMIT KUMAR S INDIRA RANI KULDEEP SHARMA S KARUNASAGAR SAJI MOHANDAS RAGHAVENDRA ASHRIT JOHN P GEORGE ASHIS K MITRA

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      Operational forecasting of tropical cyclone (TC) track and intensity in the India Meteorological Department (IMD) relies more and more on the numerical weather prediction (NWP) model guidance from national and international agencies particularly, on the medium range (24–120 h). Any improvement in TC forecasts by the NWPmodels enhances the operational forecaster’s confidence and capability. The real-time information from the National Centre for Medium Range Weather Forecasting (NCMRWF) global NWP model (NCUM-G) is routinely used by operational forecasters at IMD as model guidance. The present study documents the improved skill of NCUM-G in forecasting the North Indian Ocean (NIO) TCs during 2015–2019, based on a collection of 1810 forecasts involving 22 TC cases. The study highlights three significant changes in the modelling system during the recent five years, namely (i) increased grid resolution from 17 to 12 km, (ii) use of hybrid 4D-Var data assimilation (DA), and (iii) increased volume of assimilated data. The study results indicate a consistent improvement in the NCUM-G model forecasts during the premonsoon (April–May,AM)and post-monsoon (October–December,OND)TCseasons. In addition to a 44% reduction in the initial position error, the study also reports a statistically significant decrease in the direct position error (DPE) and error in the intensity forecast, resulting in a forecast gain of 24 hrs. Comparing NWP models with IMDs official track error shows that NCUM-G and ECMWF model forecasts feature lower DPE than IMD in 2019, particularly at higher (96, 108, and 120 h) lead times.

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