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Hiroyuki Yamada and Hiroshi Uyeda

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 of vegetation. These results suggest a significant impact of the plateau surface upon the modification of the rainfall characteristics. This impact is the largest under the condition with a near-surface heat low forming due to strong solar heating.

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Hiroshi Uyeda and Dusan S. Zrnić

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.

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Masaki Katsumata, Hiroshi Uyeda, Koyuru Iwanami, and Guosheng Liu

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 depend on some assumptions are also examined.

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Satoshi Endo, Taro Shinoda, Tetsuya Hiyama, Hiroshi Uyeda, Kenji Nakamura, Hiroki Tanaka, and Kazuhisa Tsuboki

Abstract

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.

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Shingo Shimizu, Hiroshi Uyeda, Qoosaku Moteki, Takeshi Maesaka, Yoshimasa Takaya, Kenji Akaeda, Teruyuki Kato, and Masanori Yoshizaki

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-like storm differed from those of a typical supercell. For example, a weak downdraft, a weak outflow, a weak inflow, and a short time maintenance of a single cyclonically rotating updraft (about 30 min) were noted. Dual-Doppler radar analysis revealed that the convergence between the weak outflow and the weak inflow kept its location just under the updraft for about 30 min; in other words, the strength of the outflow balanced the strength of the inflow. The observed features were simulated well using CReSS, and the thermodynamical features of the formation mechanism were revealed. The weak downdraft with a small evaporative cooling rate was simulated in a moist layer below the melting layer at 3 km in height. The small evaporation cooling was a major cause of the weak downdraft and the weak outflow. Because the outflow was weak and did not cut off the initial updraft, the weak inflow was able to keep supplying warm air to the initial updraft for about 30 min. Therefore, the present supercell-like storm could form as a result of the balance of the strengths of the weak inflow and the weak outflow in a moist environment.

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Takeharu Kouketsu, Hiroshi Uyeda, Tadayasu Ohigashi, Mariko Oue, Hiroto Takeuchi, Taro Shinoda, Kazuhisa Tsuboki, Mamoru Kubo, and Ken-ichiro Muramoto

Abstract

A 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 Z h, differential reflectivity Z dr, specific differential phase K dp, and correlation coefficient ρ hv 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 imaging system are used to validate the HC method for dry graupel and dry snow aggregates. For dry snow aggregates and ice crystals, the HC method is validated using simultaneous observations from a balloonborne instrument [hydrometeor videosonde (HYVIS)] and an X-pol range–height indicator directed toward the HYVIS. The HC method distinguishes effectively between dry graupel, dry snow aggregates, and ice crystals, and is therefore valid for HC under moist environments.

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