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Pengfei Zhang, Shun Liu, and Qin Xu

Abstract

Radar echoes from migrating birds can severely contaminate Doppler velocity measurements. For meteorological applications, especially quantitative applications in radar data assimilation, it is necessary to remove bird-contaminated velocity scans by using an automated identification technique. Such a technique should be also useful for ornithologists in selecting bird echoes automatically from radar scans. This technique can be developed in two steps: (i) extract the main features of migrating-bird echoes from reflectivity and Doppler velocity images and find proper parameters to quantify these features; (ii) utilize these parameters to develop an automated quality control procedure to identify and flag migrating-bird-contaminated Doppler velocity scans (sweeps). The first step is accomplished in this study (Part I) by identifying possible migrating-bird echoes in the level II data collected from the Oklahoma KTLX radar during the 2003 spring migrating season. The identifications are further verified by polarimetric radar measurements from the National Severe Storms Laboratory (NSSL) KOUN radar, Geostationary Operational Environmental Satellite (GOES) IR images, and rawinsonde measurements. Three proper parameters are found, and their histograms are prepared for the second step of development (reported in Part II).

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Shun Liu, Qin Xu, and Pengfei Zhang

Abstract

Based on the Bayesian statistical decision theory, a probabilistic quality control (QC) technique is developed to identify and flag migrating-bird-contaminated sweeps of level II velocity scans at the lowest elevation angle using the QC parameters presented in Part I. The QC technique can use either each single QC parameter or all three in combination. The single-parameter QC technique is shown to be useful for evaluating the effectiveness of each QC parameter based on the smallness of the tested percentages of wrong decision by using the ground truth information (if available) or based on the smallness of the estimated probabilities of wrong decision (if there is no ground truth information). The multiparameter QC technique is demonstrated to be much better than any of the three single-parameter QC techniques, as indicated by the very small value of the tested percentages of wrong decision for no-flag decisions (not contaminated by migrating birds). Since the averages of the estimated probabilities of wrong decision are quite close to the tested percentages of wrong decision, they can provide useful information about the probability of wrong decision when the multiparameter QC technique is used for real applications (with no ground truth information).

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Amanda M. Murphy, Alexander Ryzhkov, and Pengfei Zhang

Abstract

A novel way to process polarimetric radar data collected via plan position indicator (PPI) scans and display those data in a time–height format is introduced. The columnar vertical profile (CVP) methodology uses radar data collected via multiple elevation scans, limited to data within a set region in range and azimuth relative to the radar, to create vertical profiles of polarimetric radar data representative of that limited region in space. This technique is compared to others existing in the literature, and various applications are discussed. Polarimetric ice microphysical retrievals are performed on CVPs created within the stratiform rain region of two mesoscale convective systems sampled during two field campaigns, where CVPs follow the track of research aircraft. Aircraft in situ data are collocated to microphysical retrieval data, and the accuracy of these retrievals is tested against other retrieval techniques in the literature.

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Pengfei Zhang, Gang Chen, and Yi Ming

Abstract

While there is substantial evidence for tropospheric jet shift and Hadley cell expansion in response to greenhouse gas increases, quantitative assessments of individual mechanisms and feedback for atmospheric circulation changes remain lacking. We present a new forcing–feedback analysis on circulation response to increasing CO2 concentration in an aquaplanet atmospheric model. This forcing–feedback framework explicitly identifies a direct zonal wind response by holding the zonal mean zonal wind exerting on the zonal advection of eddies unchanged, in comparison with the additional feedback induced by the direct response in zonal mean zonal wind. It is shown that the zonal advection feedback accounts for nearly half of the changes to the eddy-driven jet shift and Hadley cell expansion, largely contributing to the subtropical precipitation decline, when the CO2 concentration varies over a range of climates. The direct response in temperature displays the well-known tropospheric warming pattern to CO2 increases, but the feedback exhibits negative signals. The direct response in eddies is characterized by a reduction in upward wave propagation and a poleward shift of midlatitude eddy momentum flux (EMF) convergence, likely due to an increase in static stability from moist thermodynamic adjustment. In contrast, the feedback features a dipole pattern in EMF that further shifts and strengthens midlatitude EMF convergence, resulting from the upper-level zonal wind increase seen in the direct response. Interestingly, the direct response produces an increase in eddy kinetic energy (EKE), but the feedback weakens EKE. Thus, the forcing–feedback framework highlights the distinct effect of zonal mean advecting wind from direct thermodynamic effects in atmospheric response to greenhouse gas increases.

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Pengfei Zhang, Yimin Liu, and Bian He

Abstract

Occupying the upper troposphere over subtropical Eurasia during boreal summer, the South Asian high (SAH) is thought to be a regulator of the East Asian summer monsoon (EASM), which is particularly important for regional climate over Asia. However, there is feedback of the condensational heating associated with EASM precipitation to SAH variability. In this study, interannual variation of SAH intensity and the mechanisms are investigated. For strong SAH cases, the high pressure system intensifies and expands. Significant positive anomalies of the geopotential height and upper-tropospheric temperature were found over the Middle East and to the east of the Tibetan Plateau (TP), namely, the western and the eastern flanks of the SAH. The dynamical diagnosis and the numerical experiments consistently show that the interannual variation of SAH intensity is strongly affected by EASM precipitation over the eastern TP–Yangtze River valley. The feedback of the condensational heating anomaly to the SAH is summarized as follows: Excessive EASM heating excites a local anticyclone in the upper troposphere and warms the upper troposphere, leading to the eastward extension of the SAH’s eastern edge and reinforcing geopotential height anomalies over East Asia. Furthermore, the monsoonal heating excites a westward-propagating Rossby wave that increases the upper-tropospheric geopotential height and warms the upper troposphere over the Middle East. In conclusion, this study suggests a mechanistic paradigm in which the EASM may also be a modulator of SAH variation rather than just a passive result of the latter as traditionally thought. The results suggest that the EASM and the SAH are a tightly interactive system.

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Pengfei Zhang, Gang Chen, and Yi Ming

Abstract

While there is substantial evidence for tropospheric jet shift and Hadley cell expansion in response to greenhouse gas increases, quantitative assessments of individual mechanisms and feedback for atmospheric circulation changes remain lacking. We present a new forcing-feedback analysis on circulation response to increasing CO2 concentration in an aquaplanet atmospheric model. This forcing-feedback framework explicitly identifies a direct zonal wind response by holding the zonal mean zonal wind exerting on the zonal advection of eddies unchanged, in comparison with the additional feedback induced by the direct response in zonal mean zonal wind. It is shown that the zonal advection feedback accounts for nearly half of the changes to the eddy-driven jet shift and Hadley cell expansion, largely contributing to the subtropical precipitation decline, when the CO2 concentration varies over a range of climates. The direct response in temperature displays the well-known tropospheric warming pattern to CO2 increases, but the feedback exhibits negative signals. The direct response in eddies is characterized by a reduction in upward wave propagation and a poleward shift of midlatitude eddy momentum flux (EMF) convergence, likely due to an increase in static stability from moist thermodynamic adjustment. In contrast, the feedback features a dipole pattern in EMF that further shifts and strengthens midlatitude EMF convergence, resulting from the upper-level zonal wind increase seen in the direct response. Interestingly, the direct response produces an increase in eddy kinetic energy (EKE), but the feedback weakens EKE. Thus, the forcing-feedback framework highlights the distinct effect of zonal mean advecting wind from direct thermodynamic effects in atmospheric response to greenhouse gas increases.

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Pengfei Zhang, Guoping Li, Xiouhua Fu, Yimin Liu, and Laifang Li

Abstract

Tibetan Plateau (TP) vortices and the related 10–30-day intraseasonal oscillation in May–September 1998 are analyzed using the twice-daily 500-hPa synoptic weather maps, multiple reanalysis datasets, and satellite-retrieved brightness temperature. During the analysis period, distinctively active and suppressed periods of TP vortices genesis are noticed. In 1998, nine active periods of TP vortices occurred, which were largely clustered by the cyclonic circulations associated with the intraseasonal oscillation of 500-hPa relative vorticity. In addition to the well-recognized 30–60-day oscillation, the clustering of TP vorticity in the 1998 summer are more likely modulated by the 10–30-day oscillation, because all active periods of TP vortices fall into the positive phase of the 10–30-day oscillation in 1998. Even in the negative (i.e., anticyclonic) phases of the 30–60-day oscillation, the positive (i.e., cyclonic) 500-hPa 10–30-day oscillation can excite the clustering of TP vortices. This result indicates that the 10–30-day oscillation more directly modulates the activities of TP vortices by providing a favorable (unfavorable) cyclonic (anticyclonic) environment. The analysis of the 10–30-day atmospheric oscillation suggests that the westerly trough disturbances, in conjunction with convective instability due to low-level warm advection from the Indian monsoon region, are important in the clustering of TP vortex activities. In particular, the moisture flux from the southwest boundary of TP is essential to the accumulation of convective energy. Thus, a better understanding and prediction of the 10–30-day intraseasonal oscillation is needed to advance the extended-range forecasting of TP vortices and their downstream impacts on the weather and climate over East Asia.

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Malte Diederich, Alexander Ryzhkov, Clemens Simmer, Pengfei Zhang, and Silke Trömel

Abstract

In a series of two papers, rain-rate retrievals based on specific attenuation A at radar X-band wavelength using the R(A) method presented by Ryzhkov et al. are thoroughly investigated. Continuous time series of overlapping measurements from two polarimetric X-band weather radars in Germany during the summers of 2011–13 are used to analyze various aspects of the method, like miscalibration correction, ground clutter contamination, partial beam blockage (PBB), sensitivity to precipitation characteristics, and sensitivity to temperature assumptions in the retrievals. In Part I of the series, the relations inherent to the R(A) method were used to calculate radar reflectivity Z from specific attenuation and it was compared with measured reflectivity to estimate PBB and calibration errors for both radars. In this paper, R(A) rain estimates are compared to R(Z) and R(K DP) retrievals using specific phase shift K DP. PBB and calibration corrections derived in Part I made the R(Z) rainfall estimates almost perfectly consistent. Accumulated over five summer months, rainfall maps showed strong effects of clutter contamination if R(K DP) is used and weaker impact on R(A). These effects could be reduced by processing the phase shift measurements with more resilience toward ground clutter contamination and by substituting problematic R(K DP) or R(A) estimates with R(Z). Hourly and daily accumulations from rain estimators are compared with rain gauge measurements; the results show that R(A) complemented by R(Z) in segments with low total differential phase shift correlates best with gauges and has the lowest bias and RMSE, followed by R(K DP) substituted with R(Z) at rain rates below 8 mm h−1.

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Malte Diederich, Alexander Ryzhkov, Clemens Simmer, Pengfei Zhang, and Silke Trömel

Abstract

In a two-part paper, radar rain-rate retrievals using specific attenuation A suggested by Ryzhkov et al. are thoroughly investigated. Continuous time series of overlapping measurements from two twin polarimetric X-band weather radars in Germany during the summers of 2011–13 are used to analyze various aspects of rain-rate retrieval, including miscalibration correction, mitigation of ground clutter contamination and partial beam blockage (PBB), sensitivity to precipitation characteristics, and the temperature assumptions of the R(A) technique. In this paper, the relations inherent to the R(A) method are used to estimate radar reflectivity Z from A and compare it to the measured Z in order to estimate PBB and calibration offsets for both radars. The fields of Z estimated from A for both radars are consistent, and the differences between Z(A) and measured Z are in good agreement with the ones calculated using either consistency relations between reflectivity at horizontal polarization Z H, differential reflectivity Z DR, and specific differential phase K DP in rain or a digital elevation model in the presence of PBB. In the analysis, the dependence of A on temperature appears to have minimal effects on the overall performance of the method. As expected, the difference between Z(A) and attenuation-corrected measured Z observations varies with rain type and exhibits a weak systematic dependency on rainfall intensity; thus, averaging over several rain events is required to obtain reliable estimates of the Z biases caused by radar miscalibration and PBB.

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Jiaxi Hu, Daniel Rosenfeld, Alexander Ryzhkov, and Pengfei Zhang

Abstract

This study analyzes the microphysics and precipitation pattern of Hurricanes Harvey (2017) and Florence (2018) in both the eyewall and outer rainband regions. From the retrievals by a satellite red–green–blue scheme, the outer rainbands show a strong convective structure while the inner eyewall has less convective vigor (i.e., weaker upper-level reflectivities and electrification), which may be related to stronger vertical wind shear that hinders fast vertical motions. The WSR-88D column-vertical profiles further confirm that the outer rainband clouds have strong vertical motion and large ice-phase hydrometeor formation aloft, which correlates well with 3D Lightning Mapping Array source counts in height and time. From the results from this study, it is determined that the inner eyewall region is dominated by warm rain, whereas the external rainband region contains intense mixed-phase precipitation. External rainbands are defined here as those that reside outside of the main hurricane circulation, associated with surface tropical storm wind speeds. The synergy of satellite and radar dual-polarization parameters is instrumental in distinguishing between the key microphysical features of intense convective rainbands and the warm-rain-dominated eyewall regions within the hurricanes. Substantial amounts of ice aloft and intense updrafts in the external rainbands are indicative of heavy surface precipitation, which can have important implications for severe weather warnings and quantitative precipitation forecasts. The novel part of this study is to combine ground-based radar measurement with satellite observations to study hurricane microphysical structure from surface to cloud top so as to fill in the gaps between the two observational techniques.

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