Energetics of lower tropospheric ultra-long waves: A key to intra-seasonal variability of Indian monsoon

Analysis of fifty four (1951-2004) years of daily energetics of zonal waves derived from NCEP/ NCAR wind (u and v) data and daily rainfall received over the Indian landmass (real time data) during southwest monsoon season (1 June-30 September) indicate that energetics (momentum transport and kinetic energy) of lower tropospheric ultra-long waves (waves 1 and 2) of low latitudes hold a key to intra-seasonal variability of monsoon rainfall over India. Correlation coefficient between climatology of daily (122 days) energetics of ultra-long waves and climatology of daily rainfall over Indian landmass is 0.9. The relation is not only significant but also has a predictive potential. The normalised plot of both the series clearly indicates that the response period of rainfall to the energetics is of 5-10 days during the onset phase and 4-7 days during the withdrawal phase of monsoon over India. During the established phase of monsoon, both the series move hand-in-hand. Normalised plot of energetics of ultra-long waves and rainfall for individual year do not show marked deviation with respect to climatology. These results are first of its kind and are useful for the short range forecast of rainfall over India

Energetics of lower tropospheric planetary waves over mid latitudes: Precursor for Indian summer monsoon

Based on NCEP/NCAR reanalysis data, kinetic energy and momentum transport of waves 0 to 10 at 850 hPa level are computed from monthly mean zonal (u) and meridional (v) components of wind from equator to 90°N. Fourier technique is used to resolve the wind field into a spectrum of waves: Correlation analysis between All India Seasonal Monsoon Rainfall (AISMR) and energetics of the waves indicates that effective kinetic energy of waves 1, 3 and 4 around 37.5°N in February has significant correlation (99.9) with the subsequent AISMR. A simple linear regression equation between the effective kinetic energy of these three waves and AISMR is developed. Out of 47 years' (1958-2004) data, 32 years (1958-1989) are utilized for developing the regression model and the remaining 15 years (1990-2004) are considered for its verification. Predicted AISMR is in close agreement with observed AISMR. The regression equation based on the dynamics of the planetary waves is thus useful for Long Range Forecasting (LRF) of AISMR. Apart from the regression equation, the study provides qualitative predictors. The scatter diagram between AISMR and effective kinetic energy of waves 1, 3 and 4 around 37.5°N indicates that if the kinetic energy is more (less) than 5 m2s-2, the subsequent monsoon will be good (weak). Stream function fields indicate that high latitude trough axis along 40°E (70°E) leads to a good (weak) monsoon over India

The cloud aerosol interaction and precipitation enhancement experiment (CAIPEEX): Overview and preliminary results

While the demand for enhancing rainfall through cloud seeding is strong and persistent in the country, considerable uncertainty exists on the success of such an endeavour at a given location. To understand the pathways of aerosol-cloud interaction through which this might be achieved, a national experiment named Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) in two phases, was carried out. The rationale of CAIPEEX, the strategy for conducting the experiment, data quality and potential for path-breaking science are described in this article. Pending completion of quality control and calibration of the CAIPEEX phase-II data, here we present some initial results of CAIPEEX phase-I aimed at documenting the prevailing microphysical characteristics of aerosols and clouds and associated environmental conditions over different regions of the country and under different monsoon conditions with the help of an instrumented research aircraft. First-time simultaneous observations of aerosol, cloud condensation nuclei (CCN) and cloud droplet number concentration (CDNC) over the Ganges Valley during monsoon season show very high concentrations (> 1000 cm-3) of CCN at elevated layers. Observations of elevated layers with high aerosol concentration over the Gangetic valley extending up to 6 km and relatively less aerosol concentration in the boundary layer are also documented. We also present evidence of strong cloud- aerosol interaction in the moist environments with an increase in the cloud droplet effective radius. Our observations also show that pollution increases CDNC and the warm rain depth, and delays its initiation. The critical effective radius for warm rain initiation is found to be between 10 and 12 μm in the polluted clouds and it is between 12 and 14 μm in cleaner monsoon clouds

Application of satellite-derived OLR data in the prediction of the onset of Indian summer monsoon

Outgoing Longwave Radiation (OLR) data, obtained from NOAA polar orbiting satellites for the months March-June, for 31 years is utilized to investigate low-frequency oscillation in pre-monsoon convection over southwest peninsular Indian region. The analysis reveals a characteristic fall in OLR over the region, about 5-9 weeks prior to the onset of Indian summer monsoon in almost all the years under study. Such fall is shown to be related with a characteristic peak in convective activity associated with the movement of Inter-Tropical Convergence Zone over the region. The results suggest an indirect non-linear relationship between the time of occurrence of the pre-monsoon convective peak and the onset date. A regression equation is developed to predict the onset date. The results of estimation of present method and that of an earlier method (based on conventional synoptic observations) are compared. The performance of both these methods is validated for two independent years (2007 and 2008). The study highlights the potential application of the developed method for the prediction of onset of Indian summer monsoon well in advance using the remotely sensed satellite-derived OLR data. © 2009 Springer-Verlag