Precipitable water as a predictor of LCL height

Based on the precipitable water observations easily available from in situ and remote sensing sensors, a simple approach to define the lifting condensation level (LCL) is proposed in this study. High-resolution radiosonde and microwave radiometer observations over peninsular Indian region during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment Integrated Ground Observational Campaign (CAIPEEX-IGOC) during the monsoon season of 2011 are used to illustrate the unique relationship. The inferences illustrate a linear relationship between the precipitable water (PW) and the LCL temperature. This relationship is especially valuable because PW is easily available as a derived parameter from various remote sensing and ground-based observations. Thus, it could be used to estimate the LCL height and perhaps also the boundary layer height. LCL height and PW correlations are established from historical radiosonde data (1984–2012). This finding could be used to illustrate the boundary layer-cloud interactions during the monsoon and is important for parameterization of boundary layer clouds in numerical models. The relationships are illustrated to be robust and seem promising to get reasonable estimates of the LCL height over other locations as well using satellite observations of PW.

     

Anomalous electric field changes and high flash rate beneath a thunderstorm in northeast India

In spite of many experimental and theoretical studies the relationships between storm dynamics, severe weather, and lightning activity have been least understood. Measurements of electric field made under a severe thunderstorm at a northeastern Indian station, Guwahati, India are reported. Lightning flash rate increases drastically to about 84 flashes per minute (fpm) during the active stage which lasted for about 7 minutes, from about 15 flashes per minute during the initial phase of thunderstorm. Sudden increase in lightning flash rate (‘lightning jump’) of about 65 fpm/min is also observed in the beginning of the active stage. The dissipating stage is marked by slow and steady decrease in lightning frequency. Despite very high flash rate during the active stage, no severe weather conditions are observed at the ground. It is proposed that the short duration of the active stage might be the reason for the non-observance of severe weather conditions at the ground. Analysis of Skew-t graph at Guwahati suggests that vertical distribution of Convective Available Potential Energy (CAPE) also may play some role in non-occurrence of severe weather at ground in spite of large lightning flash rate and lightning jump observed in this thunderstorm. Further, all electric field changes after a lightning discharge indicates the presence of strong Lower Positive Charge Centers (LPCC) in the active and dissipation stages. This suggests that LPCC plays an important role in initiation of lightning discharges in these stages.     

Atmospheric aerosol formation and its growth during the cold season in India

The effects of molecular diffusivity of H₂SO₄ and NH₃ vapours on nucleated particles of SO₄²⁻ and NO₃⁻ species are reported. Condensation sink and source rate of H₂SO₄ and NH₃ vapours, growth rates and ratios of real to apparent nucleation rates are calculated for SO₄^– and NO₃⁻ aerosols using fractional contributions of them in total aerosol size-distribution during the measurement period at Pune, reported in Chate and Pranesha (2004). The percentage of nucleated SO₄²⁻ and NO₃⁻ aerosols of mid-point diameter 13 nm are 2% and 3% respectively of the total particles (13 nm ≤ D _p ≤ 750 nm) for both H₂SO₄ and NH₃ diffusion. In the diameter range 75 nm ≤ D _p ≤ 133 nm, it is 48% and 45% of SO₄²⁻ and NO₃⁻ aerosols, respectively for NH₃ diffusion and 43% and 36% of SO₄²⁻ and NO₃⁻ for H₂SO₄ diffusion. Increase in percentage of nucleated particles of these species corresponding to mid-point diameter 133 nm around 0900 h IST is significantly higher than that of mid-point diameter 13 nm and it is due to photo-chemical nucleation, coagulation and coalescence among nucleated clusters. The ratios of real to apparent formation rates for SO₄²⁻ and NO₃⁻ aerosols are 12% and 11% respectively, corresponding to mid-point diameter 13 nm, 17% and 13%, for midpoint diameter 133 nm and 12% and 9.5%, for mid-point diameter 750 nm. The results indicate that nucleation involving H₂SO₄ and acidic NH₃ diffusion on SO₄²⁻ and NO₃⁻ particles is the most relevant mechanism in this region.     

Investigations of the development of thunderstorm with hail. Part 3. Numerical simulation of cloud evolution

The three-dimensional nonstationary model of a convective cloud is used for investigating a thunderstorm with hail which developed over Pyatigorsk on May 29, 2012 and produced a severe hailstorm. The values of cloud characteristics (liquid water content, ice content, vertical velocity, etc.) are obtained. The importance ofconsidering wind shear is noted. The simulation results are used to analyze the transformation of precipitation field and the electric charge structure of the analyzed cloud during its development.     

Airborne measurements of submicron aerosols across the coastline at Bhubaneswar during ICARB

Airborne measurements of the number concentration and size distribution of aerosols from 13 to 700 nm diameter have been made at four vertical levels across a coastline at Bhubaneswar (20°25′N, 85°83′E) during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) programme conducted in March–April 2006. The measurements made during the constant-level flights at 0.5, 1, 2 and 3 km altitude levels extend ～100 km over land and ～150km over ocean. Aerosol number concentrations vary from 2200 to 4500 cm^?3 at 0.5 km level but are almost constant at ～ 6000 cm^?3 and ～ 800 cm^?3 at 2 and 3 km levels, respectively. At 1km level, aerosol number concentration shows a peak of 18,070 cm^?3 around the coastline. Most of the aerosol size distribution curves at 0.5 km and 1 km levels are monomodal with a maxima at 110nm diameter which shifts to 70 nm diameter at 2 and 3 km levels. However, at the peak at 1 km level, number concentration has a bimodal distribution with an additional maximum appearing in nucleation mode. It is proposed that this maxima in nucleation mode at 1 km level may be due to the formation and transport of new particles from coastal regions.     

Investigation of Cu cong Seeding Effect during Rainfall Augmentation in India

The results are presented of complex investigation of clouds in India which were seeded with a glaciogenic reagent from the aircraft. The seeding effect was assessed by the comparison of radar characteristics of the seeded clouds with the characteristics of clouds within the radar field of view. The maximum reflectivity increase in the seeded clouds in comparison with clouds in the natural cycle, an increase in the precipitation flux and specific mass of precipitated hail are observed. The merging of seeded clouds was observed during the study. It is shown that this process affects cloud characteristics resulting in the significant (by several times) increase in the precipitation flux. The measurements of electric discharges registered by the lightning detection network demonstrated that there were no lightning strikes in the seeded clouds, although the calculations indicated an increase in the electric activity of the clouds.     

Below-cloud rain scavenging of atmospheric aerosols for aerosol deposition models

Below-cloud aerosol scavenging is generally estimated from field measurements using advanced instruments that measure changes in aerosol distributions with respect to rainfall. In this study, we discuss various scavenging mechanisms and scavenging coefficients from past laboratory and field measurements. Scavenging coefficients derived from field measurements (representing natural aerosols scavenging) are two orders higher than that of theoretical ones for smaller particles (D_p < 2 μm). Measured size-resolved scavenging coefficients can be served as a better option to the default scavenging coefficient (e.g. a constant of 10^?4 s^?1 for all size of aerosols, as used in the CALPUFF model) for representing below-cloud aerosol scavenging. We propose scavenging correction parameter (C_R) as an exponential function of size-resolved scavenging coefficients, winds and width in the downwind of the source–receptor system. For a wind speed of 3 m s^?1, C_R decrease with the width in the downwind for particles of diameters D_p < 0.1 μm but C_R does not vary much for particles in the accumulation mode (0.1 < D_p < 2 μm). For a typical urban aerosol distribution, assuming 3 m s^?1 air-flow in the source–receptor system, 10 km downwind width, 2.84 mm h^?1 of rainfall and using aerosol size dependent scavenging coefficients in the C_R, scavenging of aerosols is found to be 16% in number and 24% in volume of total aerosols. Using the default scavenging coefficient (10^?4 s^?1) in the CALPUFF model, it is found to be 64% in both number and volume of total aerosols.     

Diurnal variation of surface currents at a tropical station

The surface measurements of different atmospheric electrical and meteorological parameters made at a tropical station were analyzed, to study their diurnal variation pattern with specific emphasis on convection current and the meteorological conditions responsible for its generation. The analysis shows that most of the time the displacement current is very small. The convection current is positive for most of the time of the day indicating transport of negative charge to the earth by convection. In spite of very low winds during night, the convection current is found to be more during night than during day. Large space charge density gradient near the earth's surface during stable and stratified atmosphere at night may be a reason for large convection current during that time. This study demonstrates that eddy diffusion during day time and large space charge gradient during night time are responsible for generation of convection current at this location.