A Diagnostic Method for Fog Forecasting Using Numerical Weather Prediction (NWP) Model Outputs

An attempt has been made in the present study to forecast fog with adiagnostic method using the outputs of global NWP model. The diagnosticmethod is based on the combination of thresholds of meteorologicalvariables involved in fog formation. The thresholds are computed usingthe observations during fog. These thresholds are applied to the output of aglobal NWP model for forecasting fog. The occurrence of fog is a commonphenomenon during winter season over the northern plains of India. Thediagnostic method is used to predict fog occurrences over three stations innorth India. The proposed method is able to predict both occurrences andnon-occurrences of fog at all the three stations. It is found that 94% of thefog events forecasted by the model using the diagnostic method have beenactually observed at the selected stations. The performance of method inpredicting fog is found best over Delhi with the highest accuracy (0.61) andprobability of detection (0.60). The study signifies that diagnostic approachbased on the output of a global model is a useful tool for predicting fogover a single location

Scale interactions near the foothills of Himalayas during CAIPEEX

Scale interactions associated with small scale (<100 km) dynamics might play a crucial role in the distribution of aerosol in the Himalayan foothills region. Turbulence measurements from a horizontal flight path during Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) are used to illustrate the scale interactions in the vertically sheared flow below the high-level subtropical westerly jet, which is important in the transport of pollution. Data analysis reveals the three dimensional property of large eddies that scale 10-12 km near the slopes, which could bring pollution from the valley to the Tibetan Plateau through a circulation adhering to the slopes. This circulation has a subsidence region away from the slopes and may also contribute to the buildup of pollution in elevated layers over the Plains. The vertical velocity and temperature spectra from research flight data showed clear indications of (-5/3) slope in the mesoscale range. The isotropic behavior of the velocity spectra was noticed for cloud-free traverses, while this behavior is distorted for cloudy conditions with the enhancement of energy at smaller scales as well as with low frequency gravity wave generation. A high-resolution cloud allowing model simulation over the flight path is used to examine the representation of these dynamical interactions in the numerical model. Based on the analysis of observational data and model inferences, a conceptual understanding of the flow in the region close to the foot hills and its role in the distribution of aerosol and cloud condensation nuclei is presented

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

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

Aerosol size distributions and their temporal variations in relation to urban activity

140-150Multiwavelength columnar aerosol optical depth measurements have been made at a few traffic junctions in the city of Pune, India, to examine the nature of aerosol size distributions close to urban activity. Aerosol loading inferred from Angstrom turbidity coefficient (β) showed that it is highest at site 5 and is least at site 1. Wavelength exponent (α) showed that the above two sites have larger sized particles in the atmosphere than those at sites 2, 3 and 4 which are in the core of the urban area. Daytime temporal variations in both α and β broadly followed those in traffic activity. Aerosol size distributions retrieved by inversion technique were predominantly (60%) single mode or two-slope type. Bimodal type size distributions were also observed at the traffic junctions on about 25% of the time. Time evolution of aerosol size distribution at the traffic junctions varied from day to day and also showed the influence of urban activity