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T. Tajima and T. Nakamura

Abstract

Measurements of the flow field were made of the axisymmetric flow in a differentially heated rotating fluid annulus by using a long-term tracking of a tracer particle. Its meridional flow profile is composed of a flow circulating in a large direct (Hadley) cell, which consists of thin boundary and top horizontal layers, and another one in the interior of that cell. Our measurements show that little mixing of the flow between the Hadley cell and the interior occurs and two secondary cells exist in the interior. These cells cause a trajectory of the tracer particle to be chaotic, where the particle follows the main cyclic route but is occasionally trapped in the secondary cells for indefinite times. Very close to the transition to the wave regime, a mixed structure appears consisting of both the axisymmetric and the wave flows in the meridional flow field: an unsteady wave flow occurs in the interior of the Hadley cell.

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WINTERS T. NAKAMURA

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WINTERS T. NAKAMURA

SYNOPSIS

By a method explained in detail the probabilities of the occurences of extreme rainfalls were computed for a number of stations on Oahu. These values are given in table 3. The coefficients of variation were determined for 42 stations and plotted in figure 10. The significance of this map is discussed in the latter part of the paper.

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T. Tajima, T. Nakamura, and T. Sakata

Abstract

The basic structures of steady baroclinic waves observed in a differentially heated rotating-fluid annulus are well known to be composed of high and low pressure vortices, upper (eastward) and lower (westward) jet streams meandering through the vortices, and boundary layers. In order to find the structure of the vortices, the authors have conducted a series of experiments on a rotating-fluid annulus by injecting a few drops of red ink (and/or uranine solution) into a vortex or a jet and observing the results in the corotating reference frame of the wave that drifted eastward (counterclockwise) relative to the rotating annulus. The observed 3D ink patterns appearing in the fluid revealed the inner regions of the vortices. Their structures are composed of a core and a transition zone (separatrix layer). The core is a rather well-isolated region around the axis of the vortex and is split into separate upper and lower layers. The transition zone is a thin layer next to the core and the fluid is frequently transported to and from its outside, but rarely to the core. Then a weak upwelling caused by the Ekman pumping was observed in the lower layer of the core in the low pressure vortex and similarly a weak downwelling in the upper layer of the core in the high pressure vortex. In addition, the present experiments were found to yield some results that are similar to those determined in the numerical simulations of which followed the Lagrangian motion of a fluid particle in the annulus.

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H. Nakamura, G. Lin, and T. Yamagata

Decadal wintertime variability in the North Pacific climate system observed over the last few decades is documented. The decadal sea surface temperature (SST) variability is found to be concentrated around two major oceanic fronts. The variability around the subtropical front, accompanied by the anomalous subtropical high, exhibits strong negative simultaneous correlation with the tropical SST variability, but that around the subarctic front does not. In fact, cooling around the subarctic front in the mid-1970s cannot be attributed to the influence through the atmosphere of tropical warming that occurred about two years later. During the coolest period around the subarctic front in the mid-1980s, the enhanced surface westerlies associated with the intensified Aleutian low seemed to reinforce the underlying SST anomalies. The westerlies tended to be substantially weaker during the warmest period around 1970. These findings are suggestive of self-maintaining mechanisms inherent to the northern North Pacific climate system for the decadal variability.

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H. Luce, T. Takai, T. Nakamura, M. Yamamoto, and S. Fukao

Abstract

Humidity is, among other things, a key parameter in the evolution of atmospheric dynamics and in the formation of clouds and precipitation through latent heat release. The continuous observation of its vertical distribution is thus important in meteorology. In the absence of convection, humidity in the lower troposphere is distributed into nearly horizontally stratified layers. The thin humidity gradients at the edges of these layers are known to be the main cause of very high-frequency (VHF) stratosphere–troposphere (ST) radar backscatter in the lower troposphere. This property has been experimentally demonstrated many times in the literature from comparisons between balloon measurements and low-resolution radar observations. In the present work, original results of comparisons between Raman lidar measurements of water vapor and middle- and upper-atmosphere (MU) radar measurements of echo power using a range-imaging technique are shown at high spatial and temporal resolutions (∼50 m, ∼20 s). Other tremendous advantages of such comparisons are the simultaneity, time continuity, and colocalization of the lidar and radar measurements. The results show that the radar can be used for continuously monitoring the thin positive and negative gradients of humidity when operated in range-imaging mode. With additional information from balloon measurements, it would be possible to retrieve humidity profiles in the lower troposphere at an unprecedented vertical and time resolution.

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T. Yabe, R. Tanaka, T. Nakamura, and F. Xiao

Abstract

Two semi-Lagrangian schemes that guarantee exactly mass conservation are proposed. Although they are in a nonconservative form, the interpolation functions are constructed under the constraint of conservation of cell-integrated value (mass) that is advanced by remapping the Lagrangian volume. Consequently, the resulting schemes conserve the mass for each computational grid cell. One of them (CIP–CSL4) is the direct extension of the original cubic-interpolated propagation (CIP) method in which a cubic polynomial is used as the interpolation function and the gradient is calculated according to the differentiated advection equation. A fourth-order polynomial is employed as the interpolation function in the CIP–CSL4 method and mass conservation is incorporated as an additional constraint on the reconstruction of the interpolation profile. In another scheme (CIP–CSL2), the CIP principle is applied to integrated mass and the interpolation function becomes quadratic. The latter one can be readily extended to multidimensions. Besides the linear advection transportation equation, these schemes are also applied to the nonlinear advection problem with a large Courant–Freidrichs–Lewy number.

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Stephen T. Garner, Noboru Nakamura, and Isaac M. Held

Abstract

The equilibration of two-dimensional baroclinic waves differs fundamentally from equilibration in three dimensions because two-dimensional eddies cannot develop meridional temperature or velocity structure. It was shown in an earlier paper that frontogenesis together with diffusive mixing in a two-dimensional Eady wave brings positive potential vorticity (PV) anomalies deep into the atmosphere from both boundaries and allows the disturbance to settle into a steady state without meridional gradients. Here we depart from the earlier explanation of this equilibration and associate the PV intrusions with essentially the same kind of vortex “roll-up” that characterizes the evolution of barotropic shear layers.

To avoid subgrid turbulence parameterizations and computational diffusion, the analogy is developed using Eady's generalized baroclinic instability problem. Eady's generalized model has two semi-infinite regions of large PV surrounding a layer of relatively small PV. Without boundaries, frontal collapse, or strong diffusion the model still produces equilibrated states, with structure similar to the vortex streets that emerge from unstable barotropic shear layers. The similarity is greatest when the baroclinic development is viewed in isentropic coordinates. The contrast between the present equilibrated solutions, which exhibit no vertical tilt, and Blumen's diffusive frontogenesis model, which allows the wave to retain its phase tilt, is briefly discussed.

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T. Narayana Rao, N. V. P. Kirankumar, B. Radhakrishna, D. Narayana Rao, and K. Nakamura

Abstract

An automated precipitation algorithm to classify tropical precipitating systems has been described in a companion paper (Part I). In this paper, the algorithm has been applied to 18 months of lower atmospheric wind profiler measurements to study the vertical structure and statistical features of different types of tropical precipitating systems over Gadanki, India. The shallow precipitation seems to be an important component of tropical precipitation, because it is prevalent for about 23% of the observations, with a rainfall fraction of 16%. As expected, the deep convective systems contribute maximum (60%) to the total rainfall, followed by transition and stratiform precipitation. Nonprecipitating clouds (clouds associated with no surface rainfall) are predominant in transition category, indicating that evaporation of precipitation is significant in this region. The quantitative rainfall statistics in different precipitation regimes are compared and contrasted between themselves and also with those reported at different geographical locations obtained with a wide spectrum of instruments, from rain gauges to profilers and scanning radars. The results herein agree with the reports based on scanning radar measurements but differ from profiler-based statistics. The discrepancies are discussed in light of differences in classification schemes, variation in geographical conditions, etc. The sensitivity of the algorithm on the choice of thresholds for identifying different types of precipitating systems is also examined.

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T. Narayana Rao, N. V. P. Kirankumar, B. Radhakrishna, D. Narayana Rao, and K. Nakamura

Abstract

The lower atmospheric wind profiler (LAWP) measurements made at Gadanki, India, have been used to develop an objective algorithm to classify the tropical precipitating systems. A detailed investigation on the existing classification scheme reveals major shortcomings in the scheme. In the present study, it is shown with examples that the Doppler velocity gradient (DVG) criterion is a necessary but certainly not a sufficient condition to identify the radar bright band. Such gradients in Doppler velocity can exist in other types of rain systems, for example, in convection, due to the modulation of Doppler velocity of hydrometeors by vertical air motion. The approach of the new classification scheme deviates considerably from that of existing algorithms. For instance, the new algorithm, in contrast to identifying the stratiform rain and assuming the remaining rain as convection, identifies first convection and later stratiform precipitation based on their specific characteristics. The other interesting feature in this algorithm is that it was built on the strengths of other potential classification schemes and theoretically accepted thresholds for classification of the precipitation. The performance of the new algorithm has been verified with the help of time–height maps of profiler moments and corresponding surface rainfall patterns.

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