Narrow, tropopause-level wind speed maxima known as jet streams or jets are among the most ubiquitous structural characteristics of Earth’s atmosphere. Two species, the polar and subtropical jets, can be observed on any given day. The polar jet is tied, via eddy momentum flux convergence associated with extratropical wave development, to the troposphere-deep baroclinicity of the midlatitudes, while the subtropical jet is tied, by angular momentum constraints, to the poleward edge of the tropical Hadley cell. As a consequence of their different origins, the polar and subtropical jets are separated by both latitude and elevation. However, there are times when these two usually separate features become vertically superposed to form a single, intense jet core designated as a jet superposition or superposed jet. An objective method for identifying tropopause-level jets is employed in the construction of 50-yr cold season (November–March) synoptic climatologies of the Northern Hemisphere polar jet, subtropical jet, and jet superpositions. The analysis demonstrates that while superposition events are relatively rare, there are clear geographical maxima. Superpositions are most frequent in the western Pacific from December through February, with a secondary peak in southern North America and along its eastern seaboard. Consistent with expectations, the spatiotemporal maxima in jet superpositions appear to be coincident with maxima in the polar and subtropical jets.

A method for representing geographic variability in vertical motion profile top-heaviness in reanalysis data is introduced. The results from this method are compared to a satellite-based method for estimating top-heaviness of vertical motion profiles over the oceans. The satellite-based method utilizes basis functions, idealized or from reanalysis, along with scatterometer wind convergence data and rainfall to estimate the top-heaviness of the vertical motion profile. Results from the two methods of estimating top-heaviness are significantly correlated. Both estimates of top-heaviness are compared to stratiform, shallow, and convective rain fraction. Findings show geographic variability in stratiform rain fraction is not well correlated with estimated profile top-heaviness. Shallow rain fraction is not variable enough to explain this finding. The results may be due to geographic variations in the shape of convective or stratiform heating profiles. An example is given of how variations in convective heating profiles could lead to a region with more stratiform rain having a more bottom-heavy profile.

Research suggests non-experts associate different content with the terms “global warming” and “climate change.” We test this claim with Twitter content using supervised learning software to categorize tweets by topic and explore differences between content using “global warming” and “climate change” between 1 January 2012 and 31 March 2014. Twitter data were combined with temperature records to observe the extent to which temperature was associated with Twitter discussions. We then used two case studies to examine the relationship between extreme temperature events and Twitter content. Our findings underscore the importance of considering climate change communication on social media.

The vertical structure of tropical deep convection strongly influences interactions with larger-scale circulations and climate. This paper focuses on investigating this vertical structure and its relationship with mesoscale tropical weather states. The authors test the hypothesis that latent heating plus turbulent flux convergence varies (in space and time) in association with weather state type.The authors estimate mean-state vertical motion profile shape and latent heating plus turbulent flux convergence for six weather states defined using cloud-top pressure and optical depth properties from the International Satellite Cloud Climatology Project (ISCCP) dataset. Assuming two modes of vertical motion profile variability, these modes are statistically extracted from reanalysis data using a principal component analysis. Using these modes and the relationship between vertical motion, the dry static energy budget, and mass continuity, the authors estimate vertical motion profile shape. In these estimates, the authors use Global Precipitation Climatology Project (GPCP) [and Tropical Rainfall Measuring Mission (TRMM) 3B42] precipitation and Quick Scatterometer (QuikSCAT) surface convergence data in the ITCZ region from 2001 to 2006. Finally, these profile shapes are categorized by weather state type and spatiotemporally averaged to generate mean-state vertical motion profiles and latent heating plus turbulent flux convergence.The authors find that vertical motion profile shape varies by weather state. The isolated systems convective regime exhibits more ''bottom heaviness'' than the other convectively active regimes, with maximum upward vertical motion occurring in the lower troposphere rather than the middle to upper troposphere. The variability observed does not coincide with the conventional profile variability based on stratiform rain fraction.

Although considerable research attention has been devoted to examination of the Northern Hemisphere polar and subtropical jet streams, relatively little has been directed toward understanding the circumstances that conspire to produce the relatively rare vertical superposition of these usually separate features. This study investigates the structure and evolution of large-scale environments associated with jet superposition events in the northwest Pacific. An objective identification scheme, using NCEP–NCAR Reanalysis 1 data, is employed to identify all jet superpositions in the western Pacific (30°–40°N, 135°–175°E) for boreal winters (DJF) between 1979/80 and 2009/10. The analysis reveals that environments conducive to western Pacific jet superposition share several large-scale features usually associated with East Asian winter monsoon (EAWM) northerly cold surges, including the presence of an enhanced Hadley cell–like circulation within the jet entrance region. It is further demonstrated that several EAWM indices are statistically significantly correlated with jet superposition frequency in the western Pacific. The life cycle of EAWM cold surges promotes interaction between tropical convection and internal jet dynamics. Low–potential vorticity (PV), high- air, appearing to be associated with anomalous convection in the western Pacific lower latitudes, is advected poleward toward the equatorward side of the jet in upper-tropospheric isentropic layers, resulting in anomalous anticyclonic wind shear that accelerates the jet. This, along with geostrophic cold air advection in the left jet entrance region that drives the polar tropopause downward through the jet core, promotes the development of the deep, vertical PV wall characteristic of superposed jets. A conceptual model synthesizing the results of this analysis is introduced.