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Youcun Qi and Jian Zhang

Abstract

The melting of aggregated snow/crystals often results in an enhancement of the reflectivity observed by weather radars, and this is commonly referenced as the bright band (BB). The locally high reflectivity often causes overestimation in radar quantitative precipitation estimates (QPE) if no appropriate correction is applied. When the melting layer is high, a complete BB layer profile (including top, peak, and bottom) can be observed by the ground radar, and a vertical profile of reflectivity (VPR) correction can be made to reduce the BB impact. When a melting layer is near the ground and the bottom part of the bright band cannot be observed by the ground radar, a VPR correction cannot be made directly from the Weather Surveillance Radar-1988 Doppler (WSR-88D) radar observations. This paper presents a new VPR correction method under this situation. From high-resolution precipitation profiler data, an empirical relationship between BB peak and BB bottom is developed. The empirical relationship is combined with the apparent BB peak observed by volume scan radars and the BB bottom is found. Radar QPEs are then corrected based on the estimated BB bottom. The new method was tested on 13 radars during seven low brightband events over different areas in the United States. It is shown to be effective in reducing the radar QPE overestimation under low brightband situations.

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Jian Ling and Chidong Zhang

Abstract

Diabatic and latent heating profiles from four global reanalyses and three Tropical Rainfall Measuring Mission (TRMM) algorithms were compared: first generally for the tropics and then in the context of the Madden–Julian oscillation (MJO). Most of them exhibit three heating maxima corresponding to known convection centers over South America, Africa, and the Indian–western Pacific warm pool, but they still differ substantially in many ways. Most noticeably, a double-peak vertical structure with one peak in the upper and the other in the lower troposphere and relative weak heating over the Maritime Continent in comparison to heating over the Indian and western Pacific Oceans are clearly seen in some heating data but absent in others. Heating profiles associated with the MJO were diagnosed through composites and case studies. The composites were constructed as functions of MJO phases at three longitudes representing the Indian Ocean, Maritime Continent, and western Pacific, respectively. Four MJO events were chosen for the case studies, two over the Indian Ocean and two over the western Pacific. No consistent structural evolution of heating profiles through the life cycle of the MJO could be found either among different datasets in their composites at a given longitude and their case studies for a given individual MJO event or among different longitudes and MJO events within a given dataset. Nonetheless, the previously reported westward tilt in the heating field of the MJO, composed of low-level heating preceding deep heating in an active phase of the MJO and upper-level heating immediately following the active phase, is more likely to be observed over the western Pacific than other locations. The discrepancies among the datasets illustrate the infancy of estimating diabatic heating profiles from satellite observations and the need to improve the quality of the data assimilation products.

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Jian Zhang and Youcun Qi

Abstract

The bright band (BB) is a layer of enhanced reflectivity due to melting of aggregated snow and ice crystals. The locally high reflectivity causes significant overestimation in radar precipitation estimates if an appropriate correction is not applied. The main objective of the current study is to develop a method that automatically corrects for large errors due to BB effects in a real-time national radar quantitative precipitation estimation (QPE) product. An approach that combines the mean apparent vertical profile of reflectivity (VPR) computed from a volume scan of radar reflectivity observations and an idealized linear VPR model was used for computational efficiency. The methodology was tested for eight events from different regions and seasons in the United States. The VPR correction was found to be effective and robust in reducing overestimation errors in radar-derived QPE, and the corrected radar precipitation fields showed physically continuous distributions. The correction worked consistently well for radars in flat land regions because of the relatively uniform spatial distributions of the BB in those areas. For radars in mountainous regions, the performance of the correction is mixed because of limited radar visibility in addition to large spatial variations of the vertical precipitation structure due to underlying topography.

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Chidong Zhang and Jian Ling

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This study explores the extent to which the dynamical structure of the Madden–Julian oscillation (MJO), its evolution, and its connection to diabatic heating can be described in terms of potential vorticity (PV). The signature PV structure of the MJO is an equatorial quadrupole of cyclonic and anticyclonic PV that tilts westward and poleward. This PV quadrupole is closely related to positive and negative anomalies in precipitation that are in a swallowtail pattern extending eastward along the equator and splitting into off-equatorial branches westward. Two processes dominate the generation of MJO PV. One is linear, involving MJO diabatic heating alone. The other is nonlinear, involving diabatic heating and relative vorticity of perturbations spectrally outside the MJO domain but spatially constrained to the MJO convective envelope. The MJO is thus partially a self-sustaining system and partially a consequence of scale interaction of MJO-constrained stochastic processes. Convective initiation of the MJO over the Indian Ocean features a swallowtail pattern of negative anomalous precipitation and associated anticyclonic PV anomalies at the early stage, and increasing cyclonic PV generation straddling the equator in the midtroposphere due to increasing positive anomalies in precipitation. These lead to the swallowtail pattern in positive anomalous precipitation and the associated PV quadrupole that signifies the fully developed MJO. The equatorial Kelvin and Rossby waves bear PV structures distinct from that of the MJO. They contribute insignificantly to the structure and generation of MJO PV. Solely based on the PV analysis, a hypothesis is proposed that the fundamental dynamics of the MJO depends on neither Kelvin nor Rossby waves.

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Youcun Qi and Jian Zhang

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The U.S. Weather Surveillance Radar-1988 Doppler (WSR-88D) network has provided meteorologists and hydrologists with quantitative precipitation observations at an unprecedented high spatial–temporal resolution since its deployment in the mid-1990s. Since each single radar can only cover a maximum range of 460 km, a mosaic of multiple-radar observations is needed to generate any national-scale products. The Multi-Radar Multi-Sensor (MRMS) system utilizes a physically based two-dimensional mosaicking algorithm of the WSR-88D data to generate seamless national quantitative precipitation estimation (QPE) products. For areas covered by multiple radars, the mosaicking scheme first determines if precipitation is present by checking the lowest-altitude observation. If the lowest observed radar data indicate no precipitation, then the mosaicked value is set to no precipitation. Otherwise, a weighted mean of multiple-radar observations is taken as the mosaicked value. The weighting function is based on multiple factors, including the distance from the radar and the height of the observation with respect to the melting layer. The mosaic algorithm uses the physically lowest radar observations with no/little blockage while maintaining a spatial continuity in the mosaicked field. The performance of the MRMS seamless radar mosaic algorithm was examined for various precipitation events of different characteristics. The results of these case evaluations are presented in this paper.

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Chidong Zhang and Jian Ling

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Explanations for the barrier effect of the Indo-Pacific Maritime Continent (MC) on the MJO should satisfy two criteria. First, they should include specific features of the MC, namely, its intricate land–sea distributions and elevated terrains. Second, they should include mechanisms for both the barrier effect and its overcoming by some MJO events. Guided by these two criteria, a precipitation-tracking method is applied to identify MJO events that propagate across the MC (MJO-C) and those that are blocked by the MC (MJO-B). About a half of MJO events that form over the Indian Ocean propagate through the MC. Most of them (>75%) become weakened over the MC. The barrier effect cannot be explained in terms of the strength, horizontal scale, or spatial distribution of MJO convection when it approaches the MC from the west. A distinction between MJO-B and MJO-C is their precipitation over the sea versus land in the MC region. MJO-C events rain much more over the sea than over land, whereas rainfall over the sea never becomes dominant for MJO-B. This suggests that inhibiting convective development over the sea could be a possible mechanism for the barrier effect of the MC. Preceding conditions for MJO-C include stronger low-level zonal moisture flux convergence and higher SST in the MC region. Possible connections between these large-scale conditions and the land versus sea distributions of MJO rainfall through the diurnal cycle are discussed.

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Jian Ling and Chidong Zhang

Abstract

Diabatic heating profiles are extremely important to the atmospheric circulation in the tropics and therefore to the earth’s energy and hydrological cycles. However, their global structures are poorly known because of limited information from in situ observations. Some modern global reanalyses provide the temperature tendency from the physical processes. Their proper applications require an assessment of their accuracy and uncertainties. In this study, diabatic heating profiles from three recent global reanalyses [ECMWF Interim Re-Analysis (ERA-Interim), Climate Forecast System Reanalysis (CFSR), and Modern Era Retrospective Analysis for Research and Applications (MERRA)] are compared to those derived from currently available sounding observations in the tropics and to each other in the absence of the observations. Diabatic heating profiles produced by the reanalyses match well with those based on sounding observations only at some locations. The three reanalyses agree with each other better in the extratropics, where large-scale condensation dominates the precipitation process in data assimilation models, than in the tropics, where cumulus parameterization dominates. In the tropics, they only agree with each other in gross features, such as the contrast between the ITCZs over different oceans. Their largest disagreement is the number and level of heating peaks in the tropics. They may produce a single, double, or triple heating peak at a given location. It is argued that cumulus parameterization cannot be the sole source of the disagreement. Implications of such disagreement are discussed.

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Jian Zhang and Shunxin Wang

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An automated 2D multipass velocity dealiasing scheme has been developed to correct velocity fields when wind velocities are very large compared to the Nyquist velocity of the weather Doppler radars. The new velocity dealiasing algorithm is based on the horizontal continuity of velocity fields. The algorithm first determines a set of reference radials and gates by finding the weakest wind region. Then from these reference radials and gates, the scheme checks continuities among adjacent gates and corrects for the velocity values with large differences that are close to 2 × (Nyquist velocity). Multiple passes of unfolding are performed and velocities identified as “folded” with low confidence in an earlier pass are not unfolded until a discontinuity is detected with high confidence at a subsequent pass. The new velocity dealiasing scheme does not need external reference velocity data as do many existing algorithms, thus making it more easily applicable. Over 1000 radar volume scans that include tornadoes, hurricanes, and typhoons are selected to test and to evaluate the new algorithm. The results show that the new algorithm is very robust and very computationally efficient. In cases with many data voids, the new algorithm shows improvements over the current WSR-88D operational velocity dealiasing scheme. The new dealiasing algorithm is a simple and stand-alone program that can be a very useful tool to various Doppler radar data users.

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Jian Zhang and Tzvi Gal-Chen

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A single Doppler radar analysis scheme is developed, and three-dimensional wind fields am retrieved from single Doppler radar reflectivity and radial velocity fields. The retrieval is based on two assumptions: 1) the Lagrangian conservation of the radar reflectivity and 2) the steadiness of the eddy structures in the wind field. To least violate these assumptions, a moving frame of reference is found where (in the least square sense) the observations are as stationary as possible. Multiple time levels of observations are used to avoid ill-conditioned computations. The retrieval equations, that is, the conservation equation of reflectivity and the relationship between the total wind and its radial component for several time levels, form a simple linear system. This linear system is overdetermined with respect to the three unknown Cartesian components u, v, and w of the vector wind. Thus u, v, and w are solved in the least square sense. Dual Doppler radar analyses are performed to provide verifications for the single-Doppler retrievals. The results show very good agreement between the wind fields from single-Doppler retrievals and the dual-Doppler analyses. The important findings from various experiments include that 1) the weighting of each term in the cost function is crucial to the retrieval accuracy; 2) performing the retrievals in the moving frame improves the results significantly; and 3) proper filtering in space and time can reduce errors in retrieved wind fields. Two independent cases are studied to test the robustness of the scheme.

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Jian Zhang and S. Trivikrama Rao

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Aircraft measurements taken during the North American Research Strategy for Tropospheric Ozone-Northeast field study reveal the presence of ozone concentration levels in excess of 80 ppb on a regional scale in the nocturnal residual layer during ozone episodes. The air mass containing increased concentrations of ozone commonly is found on a horizontal spatial scale of about 600 km over the eastern United States. The diurnal variation in ozone concentrations at different altitudes, ozone flux measurements, and vertical profiles of ozone suggest that ozone and its precursors trapped aloft in the nocturnal residual layer can influence the ground-level ozone concentrations on the following day as the surface-based inversion starts to break up. A simple one-dimensional model, treating both meteorological and chemical processes, has been applied to investigate the relative contributions of vertical mixing and photochemical reactions to the temporal evolution of the ground-level ozone concentration during the daytime. The results demonstrate that the vertical mixing process contributes significantly to the ozone buildup at ground level in the morning as the mixing layer starts to grow rapidly. When the top of the mixing layer reaches the ozone-rich layer aloft, high ozone concentrations are brought down into the mixing layer, rapidly increasing the ground-level ozone concentration because of fumigation. As the mixing layer grows further, it contributes to dilution while the chemical processes continue to contribute to ozone production. Model simulations also were performed for an urban site with different amounts of reduction in the ground-level emissions as well as a 50% reduction in the concentration levels of ozone and its precursors aloft. The results reveal that a greater reduction in the ground-level ozone concentration can be achieved by decreasing the concentrations of ozone and precursors aloft than can be achieved from a reduction of local emissions. Given the regional extent of the polluted dome aloft during a typical ozone episode in the northeastern United States, these results demonstrate the necessity and importance of implementing emission reduction strategies on the regional scale; such regionwide emission controls would reduce effectively the long-range transport of pollutants in the Northeast.

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