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2022 Vol. 39, No. 5

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Editorial Notes
Preface to the Special Issue: Predictability, Data Assimilation, and Dynamics of High Impact Weather—In Memory of Dr. Fuqing ZHANG
Zhiyong MENG
2022, 39(5): 673-675. doi: 10.1007/s00376-022-2002-9
In Memoriam
Contributions of Fuqing ZHANG to Predictability, Data Assimilation, and Dynamics of High Impact Weather: A Tribute
Zhiyong MENG, Eugene E. CLOTHIAUX
2022, 39(5): 676-683. doi: 10.1007/s00376-021-1362-x
This article reviews Fuqing ZHANG’s contributions to mesoscale atmospheric science, from research to mentoring to academic service, over his 20-year career. His fundamental scientific contributions on predictability, data assimilation, and dynamics of high impact weather, especially gravity waves and tropical cyclones, are highlighted. His extremely generous efforts to efficiently transmit to the community new scientific knowledge and ideas through mentoring, interacting, workshop organizing, and reviewing are summarized. Special appreciation is given to his tremendous contributions to the development of mesoscale meteorology in China and the education of Chinese graduate students and young scientists.
Original Paper
The Impact of Moist Physics on the Sensitive Area Identification for Heavy Rainfall Associated Weather Systems
Huizhen YU, Zhiyong MENG
2022, 39(5): 684-696. doi: 10.1007/s00376-021-0278-9
The impact of moist physics on the sensitive areas identified by conditional nonlinear optimal perturbation (CNOP) is examined based on four typical heavy rainfall cases in northern China through performing numerical experiments with and without moist physics. Results show that the CNOP with moist physics identifies sensitive areas corresponding to both the lower- (850−700 hPa) and upper-level (300−100 hPa) weather systems, while the CNOP without moist physics fails to capture the sensitive areas at lower levels. The reasons for the CNOP peaking at different levels can be explained in both algorithm and physics aspects. Firstly, the gradient of the cost function with respect to initial perturbations peaks at the upper level without moist physics which results in the upper-level peak of the CNOP, while it peaks at both the upper and lower levels with moist physics which results in both the upper- and lower-level peaks of the CNOP. Secondly, the upper-level sensitive area is associated with high baroclinicity, and these dynamic features can be captured by both CNOPs with and without moist physics. The lower-level sensitive area is associated with moist processes, and this thermodynamic feature can be captured only by the CNOP with moist physics. This result demonstrates the important contribution of the initial error of lower-level systems that are related to water vapor transportation to the forecast error of heavy rainfall associated weather systems, which could be an important reference for heavy rainfall observation targeting.
Numerical Simulations of a Florida Sea Breeze and Its Interactions with Associated Convection: Effects of Geophysical Representation and Model Resolution
Nessa HOCK, Feimin ZHANG, Zhaoxia PU
2022, 39(5): 697-713. doi: 10.1007/s00376-021-1216-6
The Florida peninsula in the USA has a frequent occurrence of sea breeze (SB) thunderstorms. In this study, the numerical simulation of a Florida SB and its associated convective initiation (CI) is simulated using the mesoscale community Weather Research and Forecasting (WRF) model in one-way nested domains at different horizontal resolutions. Results are compared with observations to examine the accuracy of model-simulated SB convection and factors that influence SB CI within the simulation. It is found that the WRF model can realistically reproduce the observed SB CI. Differences are found in the timing, location, and intensity of the convective cells at different domains with various spatial resolutions. With increasing spatial resolution, the simulation improvements are manifested mainly in the timing of CI and the orientation of the convection after the sea breeze front (SBF) merger into the squall line over the peninsula. Diagnoses indicate that accurate representation of geophysical variables (e.g., coastline and bay shape, small lakes measuring 10–30 km2), better resolved by the high resolution, play a significant role in improving the simulations. The geophysical variables, together with the high resolution, impact the location and timing of SB CI due to changes in low-level atmospheric convergence and surface sensible heating. More importantly, they enable Florida lakes (30 km2 and larger) to produce noticeable lake breezes (LBs) that collide with the SBFs to produce CI. Furthermore, they also help the model reproduce a stronger convective squall line caused by merging SBs, leading to more accurate locations of postfrontal convective systems.
Correlation Structures between Satellite All-Sky Infrared Brightness Temperatures and the Atmospheric State at Storm Scales
2022, 39(5): 714-732. doi: 10.1007/s00376-021-0352-3
This study explores the structures of the correlations between infrared (IR) brightness temperatures (BTs) from the three water vapor channels of the Advanced Baseline Imager (ABI) onboard the GOES-16 satellite and the atmospheric state. Ensemble-based data assimilation techniques such as the ensemble Kalman filter (EnKF) rely on correlations to propagate innovations of BTs to increments of model state variables. Because the three water vapor channels are sensitive to moisture in different layers of the troposphere, the heights of the strongest correlations between these channels and moisture in clear-sky regions are closely related to the peaks of their respective weighting functions. In cloudy regions, the strongest correlations appear at the cloud tops of deep clouds, and ice hydrometeors generally have stronger correlations with BT than liquid hydrometeors. The magnitudes of the correlations decrease from the peak value in a column with both vertical and horizontal distance. Just how the correlations decrease depend on both the cloud scenes and the cloud structures, as well as the model variables. Horizontal correlations between BTs and moisture, as well as hydrometeors, in fully cloudy regions decrease to almost 0 at about 30 km. The horizontal correlations with atmospheric state variables in clear-sky regions are broader, maintaining non-zero values out to ~100 km. The results in this study provide information on the proper choice of cut-off radii in horizontal and vertical localization schemes for the assimilation of BTs. They also provide insights on the most efficient and effective use of the different water vapor channels.
Improving the Analyses and Forecasts of a Tropical Squall Line Using Upper Tropospheric Infrared Satellite Observations
Man-Yau CHAN, Xingchao CHEN
2022, 39(5): 733-746. doi: 10.1007/s00376-021-0449-8
The advent of modern geostationary satellite infrared radiance observations has noticeably improved numerical weather forecasts and analyses. However, compared to midlatitude weather systems and tropical cyclones, research into using infrared radiance observations for numerically predicting and analyzing tropical mesoscale convective systems remain mostly fallow. Since tropical mesoscale convective systems play a crucial role in regional and global weather, this deficit should be addressed. This study is the first of its kind to examine the potential impacts of assimilating all-sky upper tropospheric infrared radiance observations on the prediction of a tropical squall line. Even though these all-sky infrared radiance observations are not directly affected by lower-tropospheric winds, the high-frequency assimilation of these all-sky infrared radiance observations improved the analyses of the tropical squall line’s outflow position. Aside from that, the assimilation of all-sky infrared radiance observations improved the analyses and prediction of the squall line’s cloud field. Finally, reducing the frequency of assimilating these all-sky infrared radiance observations weakened these improvements to the analyzed outflow position, as well as the analyses and predictions of cloud fields.
A Survey of Statistical Relationships between Tropical Cyclone Genesis and Convectively Coupled Equatorial Rossby Waves
Shuguang WANG, Juan FANG, Xiaodong TANG, Zhe-Min TAN
2022, 39(5): 747-762. doi: 10.1007/s00376-021-1089-8
Convectively coupled equatorial Rossby waves (ERW) modulate tropical cyclone activities over tropical oceans. This study presents a survey of the statistical relationship between intraseasonal ERWs and tropical cyclone genesis (TCG) over major global TC basins using four-decade-long outgoing longwave radiation (OLR) and TC best-track datasets. Intraseasonal ERWs are identified from the OLR anomalies using an empirical orthogonal function (EOF) analysis method without imposing equatorial symmetry. We find that westward-propagating ERWs are most significant in four tropical ocean basins over the summer hemisphere and that ERWs exhibit similar northeast-southwest (southeast-northwest) tilted phase lines in the northern (southern) hemisphere, with an appreciable poleward advance of wave energy in most TC basins. The EOF-based ERW indices quantitatively show that ERWs significantly modulate TC genesis. The convectively active (suppressed) phases of ERWs coincide with increased (reduced) TCG occurrences. The TCG modulation by ERWs achieves the maximum where the ERWs propagate through the climatological TCG hotspots. As a result, the total number of TCG occurrences in the TC basins varies significantly according to the ERW phase. The ERW-TCG relationship is significant over the northwestern Pacific Ocean, northeastern Pacific Ocean, and the northern Indian Ocean during the northern summer seasons. In the southern summer season, the ERW-TCG relationship is significant over the southern Indian Ocean, Indonesian-Australia basin, and the southwestern Pacific Ocean. However, ERW activities are weak in the main TC development region of the Atlantic Ocean; and the impact on Atlantic TCG appears to be insignificant.
Initiation and Evolution of Long-Lived Eastward-Propagating Mesoscale Convective Systems over the Second-Step Terrain along Yangtze-Huaihe River Valley
Yuanchun ZHANG, Jianhua SUN, Ruyi YANG, Ruoyun MA
2022, 39(5): 763-781. doi: 10.1007/s00376-022-1303-3
Based on the previous statistical analysis of mesoscale convective systems (MCSs) over the second-step terrain along Yangtze-Huaihe River Valley, eight representative long-lived eastward-propagating MCSs are selected for model-based sensitivity testing to investigate the initiation and evolution of these types of MCSs as well as their impact on downstream areas. We subject each MCS to a semi-idealized (CNTL) simulation and a sensitivity (NOLH) simulation that neglects condensational heating in the formation region. The CNTL experiment reveals convection forms in the region downstream of a shortwave trough typified by persistent southwesterly winds in the low-to midtroposphere. Upon merging with other convective systems, moist convection develops into an MCS, which propagates eastward under the influence of mid-tropospheric westerlies, and moves out of the second-step terrain. The MCS then merges with pre-existing local convection over the plains; the merged convection reinforces the cyclonic wind perturbation into a mesoscale vortex at 850 hPa. While this vortex moves eastward to regions with local vortex at 850 hPa, another vortex at 925 hPa is also intensified. Finally, the vortices at 850 and 925 hPa merge together and develop into a mesoscale convective vortex (MCV). In contrast, MCSs fail to form and move eastward in the NOLH experiment. In the absence of eastward-propagating MCSs, moist convection and mesoscale vortices still appear in the plains, but the vortex strength and precipitation intensity are significantly weakened. It is suggested the eastward-propagating MCSs over the second-step terrain significantly impact the development and enhancement of moist convection and vortices in the downstream areas.
Influence of Coastal Marine Boundary Layer Jets on Rainfall in South China
Yu DU, Yian SHEN, Guixing CHEN
2022, 39(5): 782-801. doi: 10.1007/s00376-021-1195-7
Coastal marine boundary layer jets (CMBLJs) play an important role in coastal and inland rainfall in South China. Using 21 years of ERA5 and CMORPH rainfall data, two main CMBLJs are found, one on each side of Hainan Island (named BLJ-WEST and BLJ-EAST), which are always strengthened jointly. Both CMBLJs often occur in the pre-summer rainy season and exhibit an evident diurnal cycle with a maximum at night. With the emergence of the CMBLJs, rainfall is significantly enhanced in South China, particularly downstream of each CMBLJ. The response of rainfall to the CMBLJs is mainly attributed to convergence at the terminus of each CMBLJ, terrain-induced lifting, and relevant atmospheric stratification. Coastal rainfall downstream of the BLJ-WEST is much weaker than that downstream of the BLJ-EAST because of higher CIN over the Beibu Gulf, which is caused by lower temperature lapse rates and adiabatic heating in the lee of the Annamite Range. The inland rainfall increases along with CMBLJ intensity, whereas coastal rainfall reaches a maximum in the presence of moderate CMBLJs rather than stronger CMBLJs. Stronger CMBLJs induce stronger dynamic lifting but higher CIN near the coastal area. Additionally, CAPE near the coast does not become highest with strongest CMBLJs, because the CAPE generation contributed by coastal dynamic lifting can be offset by the negative contribution caused by the horizontal advection of cold and dry air from the Indochina Peninsula. Therefore, anomalous dynamic lifting, moisture flux convergence, and CAPE/CIN associated with CMBLJ intensity jointly result in anomalous rainfall.
Radar-based Characteristics and Formation Environment of Supercells in the Landfalling Typhoon Mujigae in 2015
Lanqiang BAI, Zhiyong MENG, Ruilin ZHOU, Guixing CHEN, Naigeng WU, Wai-Kin WONG
2022, 39(5): 802-818. doi: 10.1007/s00376-021-1013-2
This study presents the radar-based characteristics and formation environment of supercells spawned by the tornadic landfalling Typhoon Mujigae (2015) in October 2015. More than 100 supercells were identified within a 24-hour period around the time of the typhoon’s landfall, of which three were tornadic with a rotational intensity clearly stronger than those of non-tornadic supercells. The identified supercells were concentrated within a relatively small area in the northeast quadrant beyond 140 km from the typhoon center. These supercells were found more likely to form over flat topography and were difficult to maintain in mountainous regions. During the study period, more supercells formed offshore than onshore. The mesocyclones of the identified supercells were characterized by a small diameter generally less than 5 km and a shallow depth generally less than 4 km above ground level. An environmental analysis revealed that the northeast quadrant had the most favorable conditions for the genesis of supercell in this typhoon case. The nondimensional supercell composite parameter (SCP) and entraining-SCP (E-SCP) were effective in separating supercell from non-supercell environment. Even though the atmosphere in the typhoon’s northeast quadrant was characterized by an E-SCP/SCP value supportive of supercell organization, orography was an impeditive factor for the supercell development. These findings support the use of traditional parameters obtained from midlatitude supercells to assess the supercell potential in a tropical cyclone envelope.