Showing papers by "Animesh Maitra published in 2012"

Rain-Induced Scintillations and Attenuation of Ku-Band Satellite Signals at a Tropical Location

Abstract: The phenomenon of scintillations in relation to rain attenuation of Ku-band satellite signals has been studied at a tropical location. The standard deviation (σ) of scintillations increases with attenuation up to a value in the range of 6-7 dB, beyond which σ decreases with attenuation. A technique is proposed to obtain some effective values of structure constant (Cn2) of refractive-index variation from the experimental observations of σ and rain rate (R). The value of Cn2 also increases with attenuation up to values in the 6-7-dB range and decreases beyond that value. The eddies in turbulent raining medium grow with rain rate, and consequently with attenuation, causing an increase in the outer scale (LO) of turbulence and thus increasing σ until LO reaches the size of the first Fresnel zone. In a further development, the contribution of LO toward Cn2 decreases, resulting in the decrease of fast fluctuations with rain attenuation.

A Comparative Study of Cloud Liquid Water Content from Radiosonde Data at a Tropical Location

Abstract: In this paper, some features of cloud liquid water content with respect to rain and water vapor are presented. Cloud liquid water density profile is obtained from radiosonde observation with Salonen's model and Karsten's model at Kolkata, a tropical location in the Indian region. Cloud liquid water contents (LWC) are obtained from these profiles which show a prominent seasonal variation. The monsoon months exhibit much higher values of LWC than in other months. However Salonen's model yields higher LWC values than that obtained with Karsten's model. The variation of daily total rainfall with LWC shows a positive relationship indicating the role of LWC in controlling the rainfall. Also the variation pattern of LWC with integrated water vapor (IWV) content of the atmosphere indicates that a threshold value of water vapor is required for cloud to form and once cloud is formed LWC increases with IWV.

Time series predictor of Ku band rain attenuation over an Earth–space path at a tropical location

Abstract: SUMMARY A channel model is proposed to predict the time series of Ku band rain attenuation during a rain event at a tropical location. The model is based on considering the Gaussian distribution of the conditional occurrence of rain attenuation with a particular value of the attenuation occurring before. The mean (μ) and standard deviation (σ) for the distribution are modeled with the experimental data. The measured attenuation at a particular time instant is used to obtain μ and σ and to predict the attenuation after certain interval. The channel model has tested well giving the predicted attenuation that agrees with the measured value with a mean error within 15% higher than 1 dB. Validity of the model is also tested with the first-order and second-order statistics of attenuation occurrence, on a long-term basis. The method can also be applied even if attenuation measurements are missing for certain period of time with increased error. Copyright © 2011 John Wiley & Sons, Ltd.

Some characteristics of earth-space path propagation phenomena at a tropical location

Abstract: Two significant propagation characteristics of Ku-band satellite signal, namely rain attenuation and depolarization have been studied over Kolkata (22°34'N, 88°29'E), a tropical location. The existing Simple Attenuation Model (SAM) for rain attenuation estimation has been improved by incorporating the effective rain rate at different rain rates. The impact of rain drop size distribution (DSD) on determining the extent of depolarization at the tropical location is investigated. The lognormal distribution has been found to be more accurate than the gamma distribution to model rain drop sizes in the present case. Rain DSD varies for different types of rain, even during different phases of rain event and is found to influence the cross-polar enhancement of Ku-band signal. The annual variation of cross-polar enhancement indicates the dominance of different types of rain during different seasons of the year.

Time series prediction of rain rate during rain events at a tropical location

Abstract: A new model is proposed to predict rain rate series during a rain event at a tropical location. The model is based on Gaussian distribution of conditional occurrence of rain rate with a particular value of the rain occurring earlier. The mean and standard deviation of the distribution are modelled with the measured data. The rain rate at a particular time instant is predicted from the knowledge of previous samples. The predictor has tested well with a maximum mean error within 20%. This model produces predicted rain rate series whose first and second-order statistics fit very accurately with those of the experimental data. The method can also be applied with increased error, even if rain rate measurements are missing for certain periods of time. Comparisons with other models are also shown.

Application of synthetic storm technique to predict time series of rain attenuation from rain rate measurement for a tropical location

Abstract: A comparison of measured attenuation series with the attenuation series obtained from rain rate measurement by using synthetic storm technique is made for Ku band signal at a tropical location. Validity of the model is tested for the long-term statistics — the cumulative distribution of attenuation occurrence and fade duration. Applicability of the model is also shown on event by event basis for the present location. The mean absolute error is within 1 dB. It has been shown that the long term statistics of predicted rain attenuation is insensitive to storm translation speed. No significant differences are found when cumulative distributions of predicted attenuation values are compared for different data sampling interval. It has been observed that for 80% events cross corelation coefficient is above 0.85 at zero lag.

Nowcasting of tropical rain using dual frequency atmospheric brightness temperatures at Kolkata

Abstract: Nowcasting of thunderstorm has important implication on minimizing the loss of properties and life Conventional approaches for such purpose using radiometer measured brightness temperature of only water vapor line usually tend to overestimate the occurrence probability In this paper, we demonstrate that use of brightness values of oxygen absorption lines in addition with the water vapor absorption lines can provide unique signature of heavy rain