Hiroshi Uyeda

Automatic Detection of Gust Fronts

Abstract We have developed a procedure that detects and tracks gust fronts automatically. It does not rely on a single method but requires simultaneous operation of two related algorithms. The convergence algorithm measures radial convergence and hence only gusts propagating along radials can be readily detected. The mesocyclone-shear algorithm measures azimuthal shear and is suitable for detecting gusts parallel with radials as well as low-level vortices. Long shear lines that these algorithms detect are classified as gusts whereas symmetric shear features are rejected if their shear and flux or “momentum” are insignificant; otherwise they are classified as low-level vortices. To locate gusts we use second-order polynomials in the range-azimuth plane. It is shown that predicted gust locations from simple linear projections of the least square fitted curves agree very well with actual gust locations.

Transition of the Rainfall Characteristics Related to the Moistening of the Land Surface over the Central Tibetan Plateau during the Summer of 1998

Abstract This paper describes a transition of rainfall characteristics related to the moistening of the land surface over the central Tibetan Plateau. This transition was observed three weeks after the onset of the summer rainy season of 1998. The objective is to clarify the potential of the plateau surface to modify the characteristics of monsoon rainfall. Summer rain events were first separated according to large-scale conditions into three types: one with a near-surface heat low and a Tibetan upper high, one with a near-surface low associated with a midlatitude trough, and one without a near-surface low. The first type was studied in further detail because of its intraseasonal variability in the rainfall amount (from 2.8 mm day−1 in June to 5.7 mm day−1 in August). The smaller amounts of the diurnal rain in June than July are related to the evaporation of precipitation within a drier and deeper subcloud layer. The moistening of this layer was related to the increase in the soil moisture and activation ...

The Response of 36- and 89-GHz Microwave Channels to Convective Snow Clouds over Ocean: Observation and Modeling

Abstract The first study in both observing and modeling radiative properties of snow clouds in the microwave frequencies is described in this paper. Snow clouds over ocean were observed simultaneously using an airborne microwave radiometer and an X-band Doppler radar. Results show that brightness temperatures at 36- and 89-GHz microwave channels responded well to the horizontal variations of precipitation particles and to the cloud dynamic structures determined by the Doppler radar, which reflect the development stages of convective cells. For the quantitative validation, physical retrievals of liquid water and snow water amounts were performed using a radiative transfer model. The retrieved snow water amount agrees well with the observed snow water amount that was converted from observed radar reflectivity. In the retrieval method, the model-simulated brightness temperatures were able to match the observed values within 3 K per channel for the most part. The ambiguities of the retrieved parameters that d...

Calibration of TRMM Derived Rainfall Over Nepal During 1998-2007

In this study rainfall is calculated from Tropical Rainfall Measuring Mission (TRMM) Version 6 (V6) 3B42 datasets and calibrated with reference to the observed daily rainfall by rain-gauge collected at 15 locations over Nepal during 1998-2007. In monthly, seasonal and annual scales TRMM estimated rainfalls follow the similar distribution of historical patterns obtained from the rain-gauge data. Rainfall is large in the Southern parts of the country, especially in the Central Nepal. Day-to-day rainfall comparison shows that TRMM derived trend is very similar to the observed data but TRMM usually underestimates rainfall on many days with some exceptions of overestimation on some days. The correlation coefficient of rainfalls between TRMM and rain-gauge data is obtained about 0.71. TRMM can measure about 65.39% of surface rainfall in Nepal. After using calibration factors obtained through regression expression the TRMM estimated rainfall over Nepal becomes about 99.91% of observed data. TRMM detection of rainy days is poor over Nepal; it can approximately detect, under-detect and over-detect by 19%, 72% and 9% of stations respectively. False alarm rate, probability of detection, threat score and skill score are calculated as 0.30, 0.68, 0.53 and 0.55 respectively. Finally, TRMM data can be utilized in measuring mountainous rainfall over Nepal but exact amount of rainfall has to be calculated with the help of adjustment factors obtained through calibration procedure. This preliminary work is the preparation of utilization of Global Precipitation Measurement (GPM) data to be commencing in 2013.

Structure and Formation Mechanism on the 24 May 2000 Supercell-Like Storm Developing in a Moist Environment over the Kanto Plain, Japan

Abstract The structure and formation mechanism of a supercell-like storm in a moist environment below a melting layer were investigated using dual-Doppler radar analysis and a cloud-resolving storm simulator (CReSS). The supercell-like storm developed over the Kanto Plain, Japan, on 24 May 2000. The environment of the supercell-like storm possessed large convective available potential energy (1000 J kg−1), strong vertical wind shear (4.2 × 10−3 s−1 between the surface and 5 km above sea level), and a moist layer (the relative humidity was 60%–90% below a melting layer at 3 km in height). The dual-Doppler radar analysis with a variational method revealed that the supercell-like storm had similar structures to those of a typical supercell in a dry environment below a melting layer, such as that in the Great Plains in the United States. The structures included a hook echo, an overhanging echo structure, and a strong updraft with strong vertical vorticity. However, some of the characteristics of the supercell...

Comparison of TRMM 3B42 products with surface rainfall over bangladesh

AbstractFor better understanding and monitoring of rainfalls over Bangladesh, where ground-based rain measurement network is not enough, TRMM-PR data is useful. An attempt was made to compare TRMM 3B42 products rainfalls with surface-based rain-gauge (RNG) rainfall obtained at 31 stations over Bangladesh for the years 1998-2002. Day-by-day rainfall amounts determined by TRMM and RNG are compared for continues 274 days (from 1 March to 30 November) in each year and at every station. Out of 274 days, averaged for 5 years rainfall over 31 stations, 97.08% and 98.91% days are detected as rainy day by TRMM and RNG respectively. Rainy days detected by TRMM matched 95.99% of the same detected by RNG. On an average, TRMM can determine about 98.24% of the RNG rainfall. The TRMM overestimates rainfall during pre-monsoon and underestimates during monsoon while alike during postmonsoon period. Overall, TRMM underestimates rainfall in the heavy-rainfall regions of Bangladesh.

Two types of precipitation particle distribution in convective cells accompanying a Baiu frontal rainband around Okinawa Island, Japan

[1] Polarimetric radar variables and ground-based raindrop size distributions (DSDs) of two convective cells in a Baiu frontal rainband over the Okinawa region were analyzed to clarify the precipitation particle distributions in convective cells with low echo top heights. One cell existed in the stratiform rain zone, and the other was in the convective rain zone of the rainband. Both cells had echo top (30 dBZ) heights of approximately 5.5 km above sea level, with large radar reflectivity (Z h ) greater than 50 dBZ in each core. For the cell in the stratiform rain zone, polarimetric variables indicated that small raindrops predominated; differential reflectivity (Z DR ) was smaller than 1.5 dB, and the correlation coefficient (ρ hv ) was greater than 0.98 with large Z h (>40 dBZ). The DSD showed high number densities of small raindrops with diameters of 1-2 mm. For the other cell, polarimetric variables indicated the presence of large raindrops; Z DR greater than 1.5 dB and ρ hv smaller than 0.98 predominated in large Z h (>40 dBZ). The DSD for this cell showed lower number densities of raindrops with diameters of 1-2 mm and higher number densities of raindrops exceeding 3 mm. The significance of these distributions was confirmed by the Z DR and ρ hv for 25 cells in the stratiform rain zone and for 28 cells in the convective rain zone. It is notable that different precipitation particle distributions in convective cells were found in the stratiform and convective rain zones of a Baiu frontal rainband with common characteristics of low echo top heights and large Z h .

Characteristics of Vertical Circulation in the Convective Boundary Layer over the Huaihe River Basin in China in the Early Summer of 2004

The purpose of this study is to clarify the characteristics of the convective boundary layer (CBL) over a humid terrestrial area, the Huaihe River basin in China, which is covered by a large, nearly flat plain with uniform farmland. Data were collected in early summer 2004 using a 32-m flux tower and a 1290-MHz wind profiler radar. When mature wheat fields or bare fields dominated (the first period), the sensible heat flux (SHF) from the land surface was nearly equal to the latent heat flux (LHF). After vegetation changed to paddy fields (the second period), the LHF was much larger than the SHF. Two clear days from the first and second periods were selected and are referred to as the dry case and wet case, respectively. For the dry case, a deep CBL developed rapidly from the early morning, and thermal updrafts in the CBL were vigorous. For the wet case, a shallow CBL developed slowly from late morning, and thermals were weak. To study the thermodynamic process in the CBL, a large-eddy simulation (LES) was conducted. The simulation adequately reproduced the surface heat flux and the CBL development for both the dry case and the wet case. For the dry case, sensible heat contributed to nearly all of the buoyancy flux. In contrast, for the wet case, heat and moisture made equal contributions. The large contribution of moisture to the buoyancy is one of the main characteristics of the CBL over humid terrestrial areas.

A Hydrometeor Classification Method for X-Band Polarimetric Radar: Construction and Validation Focusing on Solid Hydrometeors under Moist Environments

AbstractA fuzzy-logic-based hydrometeor classification (HC) method for X-band polarimetric radar (X-pol), which is suitable for observation of solid hydrometeors under moist environments producing little or no hail, is constructed and validated. This HC method identifies the most likely hydrometeor at each radar sampling volume from eight categories: 1) drizzle, 2) rain, 3) wet snow aggregates, 4) dry snow aggregates, 5) ice crystals, 6) dry graupel, 7) wet graupel, and 8) rain–hail mixture. Membership functions are defined on the basis of previous studies. The HC method uses radar reflectivity Zh, differential reflectivity Zdr, specific differential phase Kdp, and correlation coefficient ρhv as its main inputs, and temperature with some consideration of relative humidity as supplemental information. The method is validated against ground and in situ observations of solid hydrometeors (dry graupel, dry snow aggregates, and ice crystals) under a moist environment. Observational data from a ground-based ima...

TRMM Observed Vertical Structure and Diurnal Variation of Precipitation in South Asia

This paper represents the vertical structure (using TRMM-2A25 data of 2000-2003) and diurnal variation (using TRMM- 3B42RT data of 2002-2004) of precipitation from pre-monsoon to post-monsoon periods in and around Bangladesh. TRMM-2A25 data analysis reveals that pre-monsoon, monsoon and post-monsoon precipitations are strong, moderate and less intensified, respectively. Strong rain rates are found at higher altitudes for pre-monsoon and relatively at lower altitudes for later periods. Averages from 4 years data, it is found that maximum rain rate in April, July and October is 114.19, 73.88 and 49.28 mm/h, respectively. In general, pre-monsoon echoes are tall compared to monsoon and post-monsoon periods. However, the maximum echo top height of about 18.25, 18.8 and 18.25 km is found during the pre-monsoon, monsoon and post-monsoon periods, respectively. Analyzing TRMM-3B42RT data it is found that the maximum rainfall over Bangladesh and northeast of the Bay of Bengal are appeared at 06 LT (local time) during monsoon period. In the same period the maximum rainfall over India is found at 18 LT. For the entire rainy season (March-November) the maximum rainfall over Bangladesh is occurred at 06 LT with a secondary maximum peak at 15 LT. The morning maximum rainfall at 06 LT over Bangladesh is confirmed after compared with the same obtained from ground-based rain- gauge data. This analysis reveals that in Bangladesh the overestimation and underestimation of rainfall by TRMM in pre-monsoon and monsoon respectively depends on different vertical structures of precipitation fields in corresponding periods.