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Kenji Nakamura

Abstract

The biases of various rain retrieval algorithms for a spaceborne rain radar due to nonuniformity of rain are studied using simple models and an actual time sequence of rainfall rate. A conventional rain retrieval algorithm in which measured radar reflectivity factors are simply converted to rainfall rates always overestimates the rainfall rate, but the bias is moderate. The surface reference method, which is one of the algorithms using rain attenuation, always underestimates the rainfall rate. The bias is larger for larger rain depth and has a possibility to give a significant error in estimating mean rainfall rate. The dual-frequency method, which is another algorithm using rain attenuation, has a bias that depends on the rain depth and the distance between two range gates where rain attenuation is estimated and shows large variations from overestimation to underestimation. The biases are also studied using actual rainfall rates.

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Haruya Minda and Kenji Nakamura

Abstract

Rain radar measures instantaneous spatial-average rainfall, while conventional rain gauges directly measure point rainfall with low temporal resolution. Thus differences in the resolution of the sensors create difficulties for rain radar validation, especially for spaceborne rain radar. Accordingly, rainfall measurement by microwave link has been proposed for several decades, as it estimates instantaneous path-average rainfall. Thus it is expected that the microwave link rain gauge will overcome, at least partly, the problems in the rain radar validation, toward which a 50-GHz band microwave link [the path-averaged rain gauge (PRG)] was developed that has been in operation since September 2000. In this paper, the authors show the potential of the PRG system by a simple model and rainfall comparison with a disdrometer and a tipping-bucket rain gauge. Differences observed by the instruments were within 15% (within 10% in half of the cases) during actual rain events in 2003. This confirmed that the PRG system displayed good performance as a rain gauge. Finally, the possibility of the PRG system being applied for spaceborne rain radar validation is considered.

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Junji Ikai and Kenji Nakamura

Abstract

Surface rain rates over the ocean derived from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and precipitation radar (PR) are compared and systematic differences between TMI-derived rain rates and PR-derived rain rates are shown. Three plausible reasons for these differences were found. One is a problem in the freezing-level assumption in the TMI algorithm for midlatitude regions in the winter, which results in underestimation of TMI-derived rain rates. Second is inadequate ZR or kR relationships for convective and stratiform types in the PR algorithm. Third is the incorrect interpretation of the rain layer when the freezing level is low and the rain type is convective. The strong brightband echo seems to be interpreted as rain and a too strong rain attenuation correction is applied. This results in a too strong rain rate by the PR algorithm.

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Masafumi Hirose and Kenji Nakamura

Abstract

The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) has made it possible for the first time to evaluate global characteristics of vertical structures of rainfall over monsoon Asia. This study is intended to depict features of seasonal variations of the vertical gradient of rainfall rate using TRMM PR data from 1998 to 2000.

The features of downward decreasing (DD) or downward increasing (DI) rainfall rate in the lower part of the vertical profile are focused on. Horizontal maps were made showing decreasing/increasing characteristics in the vertical gradient of rainfall rate over Asia. The pattern showed a clear land–ocean contrast and had monthly variation as the monsoon progressed in Asia. The DD pattern migrated northward around the monsoon onset and withdrew southward in the retrogressing period. The seasonal march of the DD pattern was clear especially over the India subcontinent. The DD seemed to be bordering monsoon rainfall over India. Seasonal changes in characteristics of the vertical profile were observed and are reflected in the horizontal extent of rain and the storm height. It is inferred that large storm systems tend to have high storm height and DD profiles. Most isolated rain systems with low storm height show a DI profile. The DD profile often appears around the border between dry and wet regions and occurs over the wet land.

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Kenji Nakamura and Hideyuki Inomata

Abstract

The effect of non-Rayleigh scattering appearing in a rain observation experiment using a dual-wavelength rain radar, is investigated. The non-Rayleigh scattering effect causes a small difference between the effective radar reflectivity factor and the actual radar reflectivity factor that is defined as the summation of the sixth power of raindrop diameter in a unit volume. This difference causes errors in the rain retrieval algorithm based on attenuation. A simultaneous rain observation by a dual-wavelength radar and a C-band Doppler radar was performed to confirm the existence of the non-Rayleigh scattering effect.

Raindrop-size distributions are estimated from Doppler spectra at C band measured by the vertically pointing Doppler radar. The difference between effective radar reflectivity factors at X and Ka band was determined in two different ways. One way was to estimate the difference in the effective radar reflectivity factors at these wavelengths based upon raindrop-size spectra determined from the Doppler spectra of the C-band radar. The second was to use the measured values of the effective radar reflectivity factors obtained directly from the X- and Ka-band radar. It is found that both differences vary according to the raindrop-size distribution variations and have a good correlation in a time scale of less than a few minutes. This fact confirms that the effective radar reflectivity factors are affected by the non-Rayleigh scattering.

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Ji Li and Kenji Nakamura

Abstract

The vertical distribution of Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR)-observed precipitation reflectivity and their mirror image (MI) reflectivity are outlined in this paper. The purpose of this study is to investigate the possibility and the limitation of the MI method, which can be used to estimate rain attenuation. Because the MI returns are attenuated much more greatly than the direct image returns, and also because the MI return is affected by the surface reflection and surface scattering, the MI returns are much smaller and more complex than the direct image (DI) returns. However, because the MI returns might be contaminated by the surface or contributed by bistatic scattering near the surface, there are many strong mirror returns between the surface and below surface at 1 km. The ratio of detectable MI return pixels to detected DI return pixels depends on rain rate, target height, and storm height. In addition, differences also exist between the convective and stratiform rain. The reason for this might be because the difference of the surface cross section and the difference of the storm height between the two types of rainfall. Furthermore, the direct and the mirror returns for a 35-GHz radar are also estimated. The virtue of the MI method of the Ka-band radar may reside in expanding the dynamic range of the MI method from 4–30 to 0.6–30 mm h−1.

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Ji Li and Kenji Nakamura

Abstract

A mirror image is a virtual image of precipitation from “below” the ocean surface when an airborne or a spaceborne radar is used to view rainfall over the ocean. It is due to a reflection of energy from the sea surface to the precipitation and back to the radar via a second reflection at the sea surface. The mirror image characteristics were investigated using Tropical Rainfall Measuring Mission (TRMM) precipitation radar data and the following was found. 1) The radar can detect the mirror image clearly over the ocean. 2) The mirror image echo corresponds well to the direct rain echo at nadir and near-nadir incidence angles. 3) In a weak rain region, the mirror echo intensity is nearly proportional to the direct echo power except near the radar noise level. 4) In the strong rain region, rain attenuation effects clearly appear. 5) The ratio of the mirror echo power to the direct echo power is affected by the rain attenuation, which varies with the brightband height and the range of the target rain from surface. Further, a simple simulation was performed in order to confirm the above characteristics. The signal fluctuation, noise contamination, rain attenuation, and surface cross section are taken into account in the simulation. The results of simulation confirmed the observation results.

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Masafumi Hirose and Kenji Nakamura

Abstract

Seasonal and spatial variation of the vertical gradient of rainfall rate was investigated using global precipitation data observed by the Precipitation Radar (PR) on the Tropical Rainfall Measuring Mission (TRMM) satellite. The vertical gradient was rendered by features of downward decreasing (DD) or downward increasing (DI) rainfall rate in the lower part of the profile. The DD profiles dominated tropical interior landmasses such as Africa and the Brazilian Plateau in summer. The DI profiles were observed over land in winter and over ocean except for regions with very little rainfall. In addition, DI profiles appeared during the height of the wet season even over the tropical landmasses, such as the mature monsoon period over inland India and over the Amazon River basin. Individual precipitation systems were also investigated in terms of their areally averaged DD and DI characteristics mainly over India. Deep (shallow) profiles tended to be DD (DI) for all seasons except the premonsoon season. As the rain area increased, the vertical gradient of rainfall rate decreased (DD tendency). Embedded in the dominant DD signature for deep storms, deep but significant DI profiles were observed in every month. They characterized the precipitation in the premonsoon season. More than half of the mesoscale/ synoptic-scale systems (rain areas >104 km2) having the significant DD or DI regions had both of them as part of their slant cores. The vertical gradients for these systems had a similar trend for both their stratiform and convective parts. During the mature period of the southwest monsoon, the number of small systems that were DI and widespread systems with moderate vertical gradient increased.

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Toshiaki Kozu and Kenji Nakamura

Abstract

Rain rate estimation properties of multiparameter radar measurements combining radar reflectivity and microwave attenuations are studied through simulations using a two-year disdrometer dataset. In the first simulation, properties of “complete” multiparameter measurements, where error-free measurements are assumed for each radar resolution cell, are investigated. The result indicates that dual-parameter (DP) measurements provide an excellent accuracy in rain rate estimation. Most of the DP measurements possible from space are “semi-” dual-parameter (SDP) measurements, defined as DP measurements in which spatial resolution for the attenuation measurement is coarser than the resolution required for rain profiling. Considering this fact, an SDP measurement simulation is also made. The SDP measurement is an extension of the concept of the DP measurement in terms of the spatial resolution, and can provide information on raindrop size distribution (DSD) by employing a “two-scale” DSD model. It is shown that the SDP measurement provides an improved rain rate estimation depending upon the resolution of the attenuation measurement. The simulation result allows an evaluation to be made of the effect of DSD variations on rain rate profiling from space.

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Yuji Ohsaki and Kenji Nakamura

Abstract

Spaceborne rain radars often operate with a fairly low SNR (signal-to-noise ratio) because of several hardware limitations and the large distance between the radar and target. Rain radar do not directly measure the rain echo power but instead the total received power, which is the sum of the rain echo power and the receiver noise power. The rain echo power is obtained by taking the difference between the total received power and the receiver noise power. The experimentally obtained rain echo power is sometimes a negative value when the SNR is low. The rainfall rate cannot be directly estimated from this negative data. This paper proposes three data processings (truncation, zero rain, and negative rain methods) for mean rainfall rate estimations from rain echo data that includes data with a negative value. The bias error analysis for the three methods is done by computer simulation. The bias error from truncation is fairly large compared with that caused by the other two methods. When the receiver noise power is small, the zero rain method, which gives relatively small bias error, will be useful. When the receiver noise power is large, the negative rain method, which effectively reduces the bias error, will be useful. Another feature of the negative rain method is that it automatically cancels the effect of misidentification of rain or no-rain.

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