Shilpa Manandhar

GPS derived PWV for rainfall monitoring

Precipitable Water Vapor (PWV) is a good source to monitor precipitation. It is defined by the amount of water vapor present in atmosphere. Traditionally, radiosondes and microwave radiometers were used to derive PWV. However, these devices have poor temporal resolutions and high operational costs. Therefore, GPS signal delay is now widely used for such purposes. The main aim of this paper is to study relationship between GPS derived PWV and precipitation. We present an analysis which shows that PWV increases before any rainfall event, while it decreases after the rainfall event. We also derive a threshold PWV that detects the occurrence of rainfall, once PWV exceeds the threshold value. PWV and rainfall data of June 2010 and 2011 are used for validation.

Detecting rainfall onset using Sky Images

Ground-based sky cameras (popularly known as Whole Sky Imagers) are increasingly used now-a-days for continuous monitoring of the atmosphere. These imagers have higher temporal and spatial resolutions compared to conventional satellite images. In this paper, we use ground-based sky cameras to detect the onset of rainfall. These images contain additional information about cloud coverage and movement and are therefore useful for accurate rainfall nowcast. We validate our results using rain gauge measurement recordings and achieve an accuracy of 89% for correct detection of rainfall onset.

Weather Radar to detect and differentiate clouds from rain events

For more than 60 years, Radar data have been used to estimate rainfall rate. But the paradigm is shifting and its applications are being exploited in fields of cloud detection. This paper also takes a step in doing so. Firstly, Radar reflectivity (ZHH) is compared to the occurrence of rainfall and a particular relation between the two is observed. Results are presented to show that rainfall events occur when ZHH is greater than 20 dBZ. Thus it is a matter of interest and also the purpose of this study, to point out the causes behind ZHH less than 20 dBZ, if not rain. Secondly, ZHH is compared to ground based cloud images and it is found that cloud can be the possible reason for lower values of reflectivity. Hence, this paper exploits the usefulness of weather Radar in fields of cloud detection.

Weather radar to detect cloud occurrence level

In this paper, we have presented a feasibility study of weather RADAR in detecting the cloud occurrence level. Weather RADAR is generally used for locating precipitation. However in our study, we have successfully used RADAR data to study the clouds and its related properties. Our study shows that cloud height can be accurately detected using RADAR data with indication of the type of cloud (low, mid or high level). We validate our study with images captured from ground-based sky cameras. Our technique shows better performance in terms of spatial and temporal resolutions, as compared to other cloud detecting models that uses radiosonde data.

Cloud radiative effect study using sky camera

The analysis of clouds in the earths atmosphere is important for a variety of applications, viz. weather reporting, climate forecasting, and solar energy generation. In this paper, we focus our attention on the impact of cloud on the total solar irradiance reaching the earths surface. We use weather station to record the total solar irradiance. Moreover, we employ collocated ground-based sky camera to automatically compute the instantaneous cloud coverage. We analyze the relationship between measured solar irradiance and computed cloud coverage value, and conclude that higher cloud coverage greatly impacts the total solar irradiance. Such studies will immensely help in solar energy generation and forecasting.

Investigation of gaseous attenuation on a Ka-band propagation link in tropical region

In this paper, gaseous attenuation along a Ka-band slant path link is investigated and analyzed for the tropical country of Singapore. The signal is chosen at a frequency of 18.9 GHz with an elevation angle 44.5°. In order to estimate the gaseous attenuation along this slant path, Recommendation ITU-R P.676-10 model is applied. Four-year gaseous attenuation is processed and analyzed. The results show that for Ka-band, the water vapor attenuation is much larger than the dry air attenuation and the total maximum gaseous attenuation can be up to 0.84 dB. The seasonal variation of gaseous attenuation is mainly due to the alteration of dry/wet and hot/cool phases. In addition, the gaseous attenuation is found to keep increasing from Year 2012 to 2015, which might be due to the global warming effect.

A Simplified Model for the Retrieval of Precipitable Water Vapor From GPS Signal

In this paper, a simplified latitude and day-of-year (DoY)-based model is proposed for the retrieval of precipitable water vapor (PWV) from global positioning system (GPS) signal. Conventionally, PWV, the total amount of water in a vertical column of a unit cross-sectional area, is estimated from the GPS signal delay and a dimensionless conversion factor PI. This PI value is found to rely on a water vapor weighted mean temperature (Tm) value which varies widely across the day, month, and year for different regions. It is, therefore, both time specific and site specific. Analysis of the PI value and its effect on the retrieved PWV from the data obtained for tropical, subtropical, and temperate regions show that although the PI value is time and site specific, the change in the median value of PI for different years is minimal and is dependent only on factors like the latitude coordinates of the particular site and the DoY. Therefore, using the data obtained from 174 different sites, a latitude-coordinate and DoY-based PI value model for the retrieval of PWV is proposed in this paper. The proposed model has been successfully validated using data from different databases: the International GNSS Service Global Positioning System National Aeronautics and Space Administration (IGS GPS NASA) database, the International GNSS Service Global Positioning System Global Geodetic Observing System (IGS GPS GGOS) database, and the very-long-baseline interferometry (VLBI) database. Results show strong agreement between PWV values calculated using the proposed model and those calculated using the temperature dependent models with 99%, 98%, and 93% of error within ±1 mm for IGS GPS NASA, IGS GPS GGOS, and VLBI databases, respectively. Moreover, the proposed model allows for the ease of PWV retrieval, which is useful in meteorological studies and also applicable in satellite communications.

Model comparison for estimating cloud liquid water content and attenuation in tropical region

In this paper, comparison and analysis of two cloud models are performed for prediction of cloud liquid water content and cloud attenuation in tropical region. To this aim, one-year (2012) radiosonde data, collected in the tropical country of Singapore, are processed to derive the 0°C/-20°C isothermal heights, calculate cloud liquid water content and then estimate cloud attenuation. Comparing with ITU-R P.840-6 model, our results indicate that the well-known Salonen Uppala (SU) model intends to underestimate the cloud liquid water content and corresponding cloud attenuation in tropical region. In addition, a newly proposed Water Vapor Pressure (WVP) cloud detection model provides relatively large cloud attenuation and shows its feasibility to predict cloud attenuation for tropical region.