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The relative importance of various potential vorticity (PV) perturbations and their mutual interactions associated with the superstorm of 12-14 March 1993 are investigated by applying a piecewise PV inversion diagnostic system to a 36-h simulation of the storm. It is shown that the contributions from all PV anomalies to the surface development increase with time, although their relative significance varies during the rapid deepening stage. In general, the upper-level dry PV anomalies contribute the most to the rapid deepening of the storm, followed, in order, by the lower-level thermal anomaly and latent heat release. Comparing the PV anomalies and their inverted circulations reveals that there exists a favorable phase tilt between the upper- and lower-level anomalies that allows lower- and upper-level mutual interactions, in which the circulations associated with the upper-level PV anomalies enhance the lower-level anomalies and vice versa. In addition to the vertical interactions, lateral interactions are also present among the upper-level PV anomalies and the background flow. It is also found that the background flow advection dominates the vortex-vortex and vortex-background flow interactions in the deepening of the storm. The vortex-vortex interactions of the two upper-level positive PV anomalies cause the negative tilt of the main upper-level trough during the rapid deepening period. [ABSTRACT FROM AUTHOR]

In Part I of this series of papers, the static potential vorticity (PV) inversion diagnostics are performed to examine the lateral and vertical interactions between two upper-level (northern and southern) troughs and their possible influences on the development of the March 1993 superstorm. This study continues the investigation of the relative importance of two PV anomalies associated with the short-wave troughs and their interaction in the surface cyclogenesis by considering their individual roles as an initial-value problem. The piecewise PV inversion technique is first used to isolate the upper-level disturbances with a balanced vortex for each. The vortex is then either removed or its intensity doubled by subtracting the balanced vortex from or adding to the control initial conditions. With the modified initial conditions, a mesoscale model is integrated for 36 h to examine how each of the upper-level disturbances contributes to the surface development. It is found that the northern and southern troughs play different roles in the surface cyclogenesis. Without the northern trough, the eastward propagation of the southern trough slows down and the surface development is severely impeded. More rapid cyclogenesis occurs when the northern trough's intensity is doubled. On the other hand, the deepening rate increases considerably when the southern trough is removed. Doubling the southern trough intensity slows the eastward progression of the northern trough and leads to a reduced deepening rate. In conclusion, the northern trough is crucial for the rapid development of the superstorm, whereas the southern trough is important only during the incipient stage of the cyclone. In particular, a much stronger surface cyclone could be spawned in the absence of the southern trough. This finding appears to contradict the previous work on the roles of trough mergers in extratropical cyclogenesis. [ABSTRACT FROM AUTHOR]

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Temperature and water vapor profiles are essential to climate change studies and weather forecasting. Hyperspectral instruments are of great value for retrieving temperature and water vapor profiles, enabling accurate monitoring of their changes. Successful retrievals of temperature and water vapor profiles require accuracy of hyperspectral radiometer measurements. In this study, the radiometric accuracy of an airborne hyperspectral microwave radiometer, the High Spectral Resolution Airborne Microwave Sounder (HiSRAMS), and a ground-based hyperspectral infrared radiometer, the Atmospheric Emitted Radiance Interferometer (AERI), is simultaneously assessed by performing radiative closure tests under clear-sky conditions in Ottawa, Canada. As an airborne instrument, HiSRAMS has two radiometers measuring radiance in the oxygen band (49.6–58.3 GHz) and water vapor band (175.9–184.6 GHz) for zenith-pointing and nadir-pointing observations. AERI provides ground-based, zenith-pointing radiance measurements between 520 and 1800 cm -1. A systematic warm radiance bias is present in AERI observations in the window band. Upon removal of this bias, improved radiative closure was attained in the window band. The brightness temperature (BT) bias in nadir-pointing HiSRAMS observations is smaller than at the zenith. A novel but straightforward method is developed to diagnose the radiometric accuracy of the two instruments in comparison based on the relationship between radiometric bias and optical depth. Compared to AERI, HiSRAMS demonstrates similar radiometric accuracy for nadir-pointing measurements but exhibits relatively poor accuracy for zenith-pointing measurements, which requires further characterization. Future work on temperature and water vapor concentration retrievals using HiSRAMS and AERI is warranted. [ABSTRACT FROM AUTHOR]

This paper surveys the current state of knowledge regarding large-scale meteorological patterns (LSMPs) associated with short-duration (less than 1 week) extreme precipitation events over North America. In contrast to teleconnections, which are typically defined based on the characteristic spatial variations of a meteorological field or on the remote circulation response to a known forcing, LSMPs are defined relative to the occurrence of a specific phenomenon—here, extreme precipitation—and with an emphasis on the synoptic scales that have a primary influence in individual events, have medium-range weather predictability, and are well-resolved in both weather and climate models. For the LSMP relationship with extreme precipitation, we consider the previous literature with respect to definitions and data, dynamical mechanisms, model representation, and climate change trends. There is considerable uncertainty in identifying extremes based on existing observational precipitation data and some limitations in analyzing the associated LSMPs in reanalysis data. Many different definitions of "extreme" are in use, making it difficult to directly compare different studies. Dynamically, several types of meteorological systems—extratropical cyclones, tropical cyclones, mesoscale convective systems, and mesohighs—and several mechanisms—fronts, atmospheric rivers, and orographic ascent—have been shown to be important aspects of extreme precipitation LSMPs. The extreme precipitation is often realized through mesoscale processes organized, enhanced, or triggered by the LSMP. Understanding of model representation, trends, and projections for LSMPs is at an early stage, although some promising analysis techniques have been identified and the LSMP perspective is useful for evaluating the model dynamics associated with extremes. [ABSTRACT FROM AUTHOR]

Temperature and water vapor profiles are essential to climate change studies and weather forecasting. Hyperspectral instruments are of great value for retrieving temperature and water vapor profiles, enabling accurate monitoring of their changes. Successful retrievals of temperature and water vapor profiles require hyperspectral radiometer measurement accuracy. In this study, the radiometric accuracy of an airborne hyperspectral microwave radiometer, High Spectral Resolution Airborne Microwave Sounder (HiSRAMS), and a ground-based hyperspectral infrared radiometer, Atmospheric Emitted Radiance Interferometer (AERI), is simultaneously assessed by performing radiative closure tests under clear-sky conditions. As an airborne instrument, HiSRAMS has two radiometers measuring radiance in the oxygen band (49.6-58.3 GHz) and water vapor band (175.9-184.6 GHz) for zenith-pointing and nadir-pointing observations. AERI provides ground-based, zenithpointing radiance measurements between 520 and 1800 cm-1 . A systematic warm radiance bias is present in the temperaturesensitive channels in AERI observations in the window band. Upon removal of this bias, improved radiative closure was attained in the window band. The brightness temperature (BT) bias in nadir-pointing HiSRAMS observations is smaller than at the zenith. A novel but straightforward method is developed to diagnose the radiometric accuracy of the two instruments in comparison based on the relationship between radiometric bias and optical depth. Compared to AERI, HiSRAMS demonstrates similar radiometric accuracy for nadir-pointing measurements but exhibits relatively poor accuracy for zenith-pointing measurements, which requires further characterization. Future work on temperature and water vapor concentration retrievals using HiSRAMS and AERI is warranted. [ABSTRACT FROM AUTHOR]

Future changes of extratropical cyclones (ETCs) over East Asia are investigated using the models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). To quantify ETC frequency, intensity, and genesis changes in a warming climate, the objective tracking algorithm is applied to nine CMIP5 models. These models provide 6-hourly wind data with no missing values in the high-terrain regions. The historical simulations reasonably well capture the spatial distribution of ETC properties, except for nonnegligible biases in and downstream of the high-terrain regions. Such biases are particularly pronounced in the models with a coarse spatial resolution and smooth topography, which weaken lee cyclogenesis. The best five models, which show better performance for historical simulations than other models, are used to evaluate the possible changes of East Asian ETCs under the RCP8.5 scenario. These models project a reduced cyclogenesis on the leeward side of the Tibetan Plateau, and over the East China Sea and western North Pacific in the late twenty-first century, resulting in a reduced ETC frequency from the east coast of China to the western North Pacific. The ETC intensity also shows a hint of weakening over the North Pacific. These ETC property changes are largely consistent with an enhanced static stability and a reduced vertical wind shear in a warming climate. This result indicates that the local baroclinicity, instead of increased moisture content, may play a critical role in determining the future changes of East Asian ETCs. [ABSTRACT FROM AUTHOR]

An important aspect of understanding the impacts of climate change on society is determining how the distribution of weather regimes will change. Arctic amplification results in greater warming over the Arctic compared to the midlatitudes, and this study examines how patterns of Arctic air masses will be affected. The authors employ the Community Earth System Model Large Ensemble (CESM-LE) RCP 8.5, consisting of 30 ensemble members run through the twenty-first century. Self-organizing maps are used to define archetypes of 850-hPa equivalent potential temperature anomalies with respect to a changing climate and assess changes in their frequency of occurrence. In the model, a pattern with negative anomalies over the central Arctic becomes less frequent in the future. There is also an increase in the frequency of patterns associated with an amplified ridge (trough) with positive (negative) anomalies over western (eastern) North America. It is hypothesized that the increase in frequency of such patterns is the result of enhanced forcing of baroclinic waves owing to reduced sea ice over the western Arctic. There is also a decline in patterns that have anomalously high over the North Atlantic, a pattern that is associated with intense ridging in the 500-hPa flow over the North Atlantic and colder over Europe. The authors relate the decrease of these patterns to an enhancement of the North Atlantic jet induced by a warming deficit in the North Atlantic Ocean. [ABSTRACT FROM AUTHOR]

Extratropical cyclones (ETCs) in East Asia are automatically detected and tracked by applying a Lagrangian tracking algorithm to the 850-hPa relative vorticity field. The ETC statistics, which are derived from ERA-Interim reanalysis dataset from 1979 to 2017, show that East Asian ETCs primarily form over Mongolia, East China, and the Kuroshio Current region with a maximum frequency of six to seven cyclones per month. Both Mongolia and East China ETCs are initiated on the leeward side of the mountains. While Mongolia ETCs downstream of the Altai–Sayan Mountains develop slowly, East China ETCs downstream of the Tibetan plateau develop rapidly as they travel across the warm ocean. Both of them show a maximum frequency and intensity in spring rather than in winter. In contrast, oceanic ETCs across the Kuroshio Current and the Kuroshio–Oyashio Extension, where sea surface temperature gradient is sharp, reach a maximum frequency in winter although their intensity is still maximum in spring. On the decadal timescale, both ETC frequency and intensity exhibit insignificant trends. Exceptions are springtime East China and summertime Mongolia ETCs whose frequencies have slightly decreased since 1979. This declining trend is consistent with the enhanced static stability in the region. [ABSTRACT FROM AUTHOR]