Abstract

Nanda Bikram Adhikari

Study on precipitation system on the global-scale is important to understand and address the large-scale environmental problems. Satellite observations are unique and only way to capture the global-scale precipitation phenomena. A recent advancement in a millimeter wave space-borne radar technology is contributing to clarify the effect of precipitating system on the global-scale climate. This is being recognized after a successful launch of Tropical Rainfall Measuring Mission (TRMM) satellite that carried the first and only to date space-borne precipitation radar (PR). The vertical profile of rain data collected by the PR with its fine vertical resolution is of immense value to the study of tropical rainfall distribution and its variation. Meanwhile, because of TRMM’s success, a follow-on mission, so-called a Global Precipitation Measurement (GPM) mission is now under discussion to extend TRMM’s observation by enhancing its instrument capability in such a way that it can address some of the key science questions from microphysical to climate time scale. On the other hand, the satellite-based precipitation products suffer from several uncertainties due to, for example, sampling frequency, the extension of the sizes of rain cells, the diurnal cycle of rainfall and uncertainties in the rainrate retrieval algorithms. This study aims to address some of these aspects and explore the performance of a next-generation dual-wavelength (13.6/35 GHz, which is proposed for GPM) radar to observe precipitation system from space.　First, this study analyzes a detectable rain range of the higher-frequency (35 GHz) radar by using TRMM PR data, where the effect of strong rain attenuation with different sensitivity thresholds is investigated. The study is based on the simulation where we the frequency distribution of the received power of the 35-GHz radar is first calculated. As a result, for example, it is noticed that if the receiver minimum noise threshold is equivalent to about 10 dBZ, the missing fraction of near-surface rain due to attenuation degrading the signal below the noise threshold is about 15% over land and 3% over ocean compared to the PR.
　Next attempt of this study is set to examine relative sensitivity of a variety of major error sources in the retrieval of rainrate by a conventional dual-wavelength radar technique (DWRT) as well as radar reflectivity (Ze) ? rainrate (R) method. The analysis is based on simulations where we utilize a large set of disdrometer-measured raindrop size distribution (DSD) data. By utilizing vertical rain-field structure (VRS) data collected by TRMM PR, the simulation, first, statistically examined the significance of the VRS effect in the DWRT. As a result, this effect is found to be negligible in the DWRT. Next, we attempted to gauge relative sensitivities of the DWRT and Ze₁₄ (at 13.6 GHz)-R method to the natural fluctuation of DSD. Statistical error analyses suggest some distinct lower bounds of rainrate retrieval accuracies of the two estimates. For instance, if the minimum sensitivity of 35-GHz radar is equivalent to about 10 dBZ and the rainrate is about 10 mm h^-1, DWRT shows ~51% of improvement in the accuracy for 3-km range resolution, while it has ~44% improvement for 1-km range resolution compared to the Ze₁₄?R method. The simulation also analyzed the effects of nonuniform rain-field (NUR) and mismatching in the observed field-of-views (FOV) of the radars in DWRT. It is noticed that the NUR effect introduces small amount of enhancement in the errors comparing to that due to the effect of the DSD variation and/or the coupling of the Mie scattering effect, while mismatching in the FOV significantly enhances errors as well as biases in the DWRT estimates.
　Lastly, taking into account of the important issues for the future space-borne dual-wavelength radar, an assessment on the performance of the dual-wavelength (13.6/35.0 GHz) precipitation radar to observe snow and rain from space is drawn based on a simplified model, so-called non-coalescence and non-breakup model, for falling hydrometeors. The simulation, first, generates vertical profiles of dual-wavelength radar integral parameters using the measured-DSD data at ground. The simulation results are then compared with some typical observations. Despite many uncertainties and assumptions, the simulated reflectivity profiles have shown some degree of consistency with observations.
　The results based on statistical analysis of the simulated effective reflectivity factors (Zes) at 13.6 (Ze₁₄) and 35 (Ze₃₅) indicate that: a) Rain and dry snow signatures are separable, if Ze₁₄is above ~10 dBZ, b) Separation of rain and wet snow signatures is feasible, only if Ze₁₄ is between ~25 and ~45 dBZ, while above ~45 dBZ (Ze₁₄) rain and wet snow signatures are poorly separable, and c) Separation of Ze signatures for dry and wet snow is rarely feasible.
　Moreover, from the analysis of the maximum allowable fluctuation in the standard deviations in Zes for rain and snow (dry and wet), this study proposes “rain/snow discrimination thresholds”. The rain/snow separation aspect is also studied with different distributions of snow density. Also, this study proposes conventional type Ze₁₄-R >relationships for snowfall rate retrieval for dry and wet snow regions.

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