Bright Band Region Research Articles

A Real-Time Algorithm to Identify Convective Precipitation Adjacent to or within the Bright Band in the Radar Scan Domain

AbstractHydrological hazards usually occur after heavy precipitation, especially during strong convection. Therefore, accurately identifying convective precipitation is very helpful for hydrological warning and forecasting. However, separating the convective, bright band (BB), and stratiform precipitation is found to be challenging when the convection is adjacent to or within the BB region. A new convection/BB/stratiform precipitation segregation algorithm is proposed in this study to resolve this challenging issue. This algorithm is applicable for a single radar volume scan data in native (polar) coordinates and consists of four processes: 1) check the freezing (0°C) level to roughly assess whether convection is occurring or not; 2) identify the convective cores through analyzing composite reflectivity (maximum reflectivity for a given range gate among all the sweeps), vertically integrated liquid water (VIL), VIL horizontal gradient, and reflectivity at the levels of 0°, −10°, and above −10°C; 3) delineate the whole convective region through the seeded region growing method by taking account of the microphysical differences between the BB and convective regions; and 4) delineate BB features in the stratiform region. The proposed algorithm utilizes the physical characteristics of different precipitation types for precisely segregating the convective, BB, and stratiform precipitation. This algorithm has been tested with radar data of different precipitation events and evaluated with three months of rain gauge data. The results show that the proposed algorithm performs consistently well for challenging precipitation events with the convection adjacent to or within a strong BB. Furthermore, the proposed algorithm could be used to improve the vertical profile of reflectivity (VPR) correction and reduce the overestimation of rainfall in the BB precipitation region.

Identification and Separation of Turbulence Echo From the Multipeaked VHF Radar Spectra During Precipitation

This paper describes an automated data processing algorithm to identify the turbulence echo in the multipeaked very high-frequency radar spectra, generally observed during the precipitation. Although a few multipeak-picking algorithms exist, they identify the turbulence echo from the clutter and noise, but they do not distinguish turbulence and precipitation echoes. The present algorithm identifies the turbulence echo from multiple peaks based on several criteria, such as the significance of the echoes, separation between the echoes within a sliding window, and a reference turbulence echo. A new method [symmetric method (SM)] is introduced to estimate the moments after proper identification of the turbulence echo, in fact, half portion of the echo. Comparison of the moments estimated by the SM and dual-frequency method (another popular method for the extraction of moments from multipeaked spectra) with those obtained by the routine single-peak detection algorithm reveals large discrepancies, particularly in convective cores and in and above the radar bright band region. In these regions, the error in moments by the traditional single-peak-picking algorithm is found to be substantial. Performance evaluation of these two methods has also been carried out against simulated spectra. The analysis reveals that the error in obtaining moments by the SM and dual-frequency method is less than 15%, except for a few cases. Also, the error in moments by the SM is, in general, smaller than by the dual-frequency method.

A case study on bright band transition from very light to heavy rain using simultaneous observations of collocated X- and Ka-band radars

The observations of bright band carried out simultaneously with X- and Ka-band radars for the first time over the Indian region have been examined to reveal various contrasting characteristics of bright band at the two wavelengths. The study reports the bright band observations on September 12–13, 2015 at millimeter and centimeter wavelengths and brings out a comparative analysis of the bright band features (e.g., intensity, thickness, height, etc.) under three different rain conditions ranging from very light (<0.1 mm/hr) to light (0.1–3 mm/hr) to heavy (3–5 mm/hr). It is seen that the bright band region at Ka-band is always narrower and situated at a higher altitude than at X-band frequency. Our analysis shows that at Ka-band frequency, the polarimetric fields like LDR can be utilized to detect and determine the bright band features using an appropriate selection of a threshold value of LDR, which is found to be −22 dB in this study and could be associated reasonably with the top and bottom heights of the bright band. This study explores the potential of both radars, particularly the Ka-band radar for probing the bright band effect and estimating its features which would be helpful to improve the quantitative estimates of precipitation.

Wind Profiler Observation on Vertical Structure of a Mei-yu Front Cloud Bands

An L-band wind profiler deployed at an industrial park (120.38°E, 22.6°N) close to Kao-Hsiung city in southern Taiwan is utilized to diagnose the vertical structure of Mei-yu frontal precipitating cloud systems observed during 9-11 June 2006. Observational results show several mesoscale convection systems associated with the Mei-yu frontal cloud produced heavy rainfall over Taiwan island. Mei-yu precipitating clouds are classified into three types: convective, mixed convective-stratiform and stratiform based on vertical profile of Signal-to-noise ratio (SNR)/Reflectivity and Doppler velocity during approaching, passing, and leaving stages of the cloud systems. In approaching and pre-passing stages of Mei-yu frontal clouds, a pre-dominant convective precipitation without a significant bright band is observed. In contrast, a pronounced bright band around 4 km was observed during post-passing and leaving stages of the frontal clouds with stratiform precipitation. Doppler velocity profiles show hydrometeors (ice/snow) at 5 km and rain below 4 km for stratiform precipitation. For Mei-yu frontal stratiform precipitation the melting particles accelerations of 3.3 ms-1 km-1 are observed for the bright band region.

Role of mixed precipitating cloud systems on the typhoon rainfall

Abstract. L-band wind profiler data are utilized to diagnose the vertical structure of the typhoon precipitating cloud systems in Taiwan. For several typhoons, a pronounced bright band (BB) around 5 km is commonly observed from the observation. Since strong convection within typhoon circulation may disturb and/or disrupt the melting layer, the BB shall not appear persistently. Hence, an understanding of the vertical structure of the BB region is important because it holds extensive hydrometeors information on the type of precipitation and its variability. Wind profiler observational results suggest that the mixture of convective and stratiform (embedded type) clouds are mostly associated with typhoons. In the case of one typhoon, BB is appeared around 5.5 km with embedded precipitation and also BB height of 1 km higher than ordinary showery precipitation. This is evident from the long-term observations of wind profiler and Tropical Rainfall Measuring Mission. The Doppler velocity profiles show hydrometers (ice/snow) at 6 km but liquid below 5 km for typhoons and 4 km for showery precipitation. In the BB region the melting particles accelerations of 5.8 ms−1 km−1 and 3.2 ms−1 km−1 are observed for typhoon and showery precipitation, respectively.

Effect of Ag under-layer on patterning of periodic magnetic structure using femtosecond laser-induced crystallization

Femtosecond laser-interference crystallization (FLIC) was used to form a spatially periodic magnetic structure by selectively crystallizing a paramagnetic amorphous Co 2MnSi thin film, which was covered with an Ag thin film. Depending on the incident beam direction, presence of the Ag capping layer improved FLIC of a-Co 2MnSi by accentuating the structural difference between dark and bright band regions. Although the crystalline region remained unchanged with or without insertion of the Ag film, crystallization of the dark band region was reduced by the Ag film. The dark band region was in a microcrystalline state, containing a non-equilibrium solid solution of Co, Mn and Si. During FLIC, the Ag film acted as a heat sink, efficiently absorbing the energy released by the crystallization of amorphous Co 2MnSi thin film, thus sharpening lateral thermal gradient producing FLIC and help confining the laser energy to the bright band region.

Large rain drops in the bright band

It has recently been suggested1–3, based on dual-polarized radar measurements, that in the bright-band region, melting snow exhibits the characteristics of rain drops having very large effective drop diameters. It is postulated that snow flakes falling below the 0 °C isotherm melt slowly with small water droplets first forming on the outer edges of the flakes; as the snow continues to melt water forms on the surface of the flake, thus it takes on the behaviour of a large rain/ice drop; in the final stages of melting the particles start to collapse into smaller drops. Here another technique for inferring rain-drop sizes4 is described which produces results which, being consistent with those above, support the bright-band model.