An Automated Climatology of Cool Season Cutoff Lows over South-Eastern Australia, and Relationships with the Remote Climate Drivers

Cutoff low pressure systems have been found to be the synoptic system responsible for the majority of rainfall in South-Eastern Australia during the cool season (April to October inclusive). Meanwhile, rainfall in South-Eastern Australia at the seasonal and interannual scale is known to be related to remote climate drivers, such as the El Niño Southern Oscillation, the Indian Ocean Dipole, and the Southern Annular Mode. In this study, a new automated tracking scheme to identify synoptic scale cutoff lows is developed, then applied to 500 hPa geopotential height data from the NCEP1 and ERA-Interim reanalyses, to create two databases of cool season cutoff lows for South Eastern Australia for the years 1979 to 2018 inclusive. Climatological characteristics of cutoff lows identified in both reanalyses are presented and compared, highlighting differences between the NCEP1 and ERA-Interim reanalyses over the Australian region. Finally, cool-season and monthly characteristics of cutoff low frequency, duration and location are plotted against cool-season and monthly values of climate driver indices (Oceanic Nino Index, Dipole Mean Index, and Antarctic Oscillation), to identify any evidence of linear correlation. Correlations between these aspects of cutoff low occurrence and the remote drivers were found to be statistically significant at the 95% level for only a single isolated month at a time, in contrast to results predicted by previous works. It is concluded that future studies of cutoff low variability over SEA should employ identification criteria that capture systems of only upper-level origin, and differentiate between cold-cored and cold-trough systems.

Seamless Detection of Cutoff Lows and Preexisting Troughs

We propose a new scheme based on geopotential height fields to detect cutoff lows starting in the preexisting trough stage. The intensity and scale derived from the proposed scheme will allow for a better understanding of the cutoff low life cycle. These cutoff lows often accompany mesoscale disturbances, causing adverse weather-related events, such as intense torrential rainfall and/or tornadoes. The proposed scheme quantifies the geometric features of a depression from its horizontal height profile. The height slope of a line intersecting the depression bottom and the nearest tangential point (optimal slope) locally indicates the intensity and scale of an isolated depression.The strength of the proposed scheme is that, by removing a local background height slope from a geopotential height field, the cutoff low and its preexisting trough are seamlessly detected as an identical depression. The distribution maps for the detected cutoff lows and preexisting troughs are illustrated along with their intensities, sizes, and local background flows estimated from snapshot height fields. We conducted climatological comparisons of cutoff lows to determine the utility of the proposed scheme.

Seamless Detection of Cutoff Low and Preexisting Trough

<p><strong>Cutoff lows are cyclones existing in the upper troposphere developing from precursory preexisting troughs. We introduce a new method to seamlessly detect cutoff lows and even preexisting troughs aiming to improve lead time of meso scale disturbances like tornadoes. The method is based on a geometric character; in this method, a slope defined as the tangential line from a minimum point of each height depression is measured on an isobaric surface. This slope evaluates an intensity and horizontal extension (radius) of each depression. Adopting a mathematical assumption, we successfully achieved to make an algorithm to separate the depression and the local background flow. To remove the background flow enables us to detect both cutoff lows and preexisting troughs seamlessly in reanalysis height fields. So, our method would allow the life cycle to be illustrated continuously from the birth of the cutoff low, that is, from the precursory preexisting trough, and is expected to contribute to the improvement of the lead time for predicting severe weathers. Some further application examples, including tornado accompanying cases, and even for blocking highs, would be shown.</strong></p>

Atmospheric mechanisms controlling extreme winter precipitation in the Altiplano

<p>During the austral winter (June-August, JJA), precipitation events in the Altiplano (20°S-15°S, > 3000 m.a.s.l.) are uncommon. These events are responsible for damaging road infrastructure and devastating entire crop fields, loss of cattle, and even for the loss of human lives. Thus, an analysis of these events and the understanding of their precursory atmospheric mechanisms are of high importance to diminish their negative impacts. In this study, using 90 rain-gauge stations in the northern Altiplano, we identified days with a precipitation value above the percentile 90 (P90) for the 1979-2014 period. These days were considered as extreme precipitation events. If consecutive single events are separated by a gap of 5 days, we decided to consider those as a new single event. Thus, it was cataloged 129 extreme precipitation events over the northern Altiplano. Moreover, we found that 56 events lasted only for one day (EV1), 28 events for 2 days (EV2), and 45 events for at least 3 days and a maximum of 12 days (EV3). In order to understand the atmospheric mechanisms associated with these extreme events, we used the K-means cluster analysis in the geopotential height at 500 hPa (ERA-Interim) for days inside EV1, EV2 and EV3, respectively. Then, composite analyses of atmospheric circulation at 850, 500 and 200 hPa were done for each cluster group. We observe that two cluster groups in EV1, EV2, and EV3, respectively (98 events in total), are characterized by anomalies of winds, temperature and geopotential height resembling a cutoff low system over the eastern Pacific between 30°S-10°S at 200 and 500 hPa. Over South America, we observed that these events are also associated with southerly cold air intrusions arriving at 20°S and a moistened lower troposphere over the western Amazon. Indeed, the lower troposphere moistening over the western Amazon in previous days seems to be necessary to sustain long-lasting events. One cluster group in EV1 (8 events) and EV2 (6 events), respectively, is associated with southerly cold air intrusions to the east of the Andes originating at high latitudes, and arriving in equatorial regions. In addition, 17 events belonging to EV3 are associated with an anomalous South American Low-Level Jet at 850 hPa and atmospheric anomalies at 200, 500 and 850 hPa, resembling the cutoff low system over the eastern Pacific between 30°S and 10°S.</p>

A Climatological Analysis of Tropical Cyclone Rapid Intensification in Environments of Upper-Tropospheric Troughs

Tropical cyclone (TC)–trough interactions are a common occurrence in the North Atlantic basin and lead to a variety of TC intensity changes, from rapid intensification (RI) to rapid weakening. To test whether certain TC–trough configurations are more favorable for RI than others, the upper-tropospheric troughs involved in such interactions were objectively classified into one of three clusters through the implementation of a machine-learning, dimensionality-reduction technique in conjunction with a k-means clustering algorithm. Through composite analyses, the upper-tropospheric potential vorticity structure, the TC convective structure, and the TC environment were examined for both rapidly intensifying TCs and nonrapidly intensifying (non-RI) TCs. As a whole, RI episodes were associated with upper-tropospheric troughs of shorter zonal wavelengths and greater upstream TC–trough displacements than non-RI episodes. RI was found to occur most frequently when an upper-tropospheric cutoff low was located approximately 500–1000 km southwest of the TC location. RI occurred preferentially in environments associated with less ventilation of the TC warm core with low-entropy environmental air. An examination of potential trough-induced forcing for convection revealed little relationship between RI and eddy flux convergence of angular momentum. Nonetheless, RI episodes were associated with anomalously vigorous convective activity within the TC inner core, as diagnosed by infrared and passive microwave satellite imagery.

Precipitation Enhancement via the Interplay between Atmospheric Rivers and Cutoff Lows

A significant enhancement of precipitation can result from the interplay between two independent, large-scale phenomena: an atmospheric river (AR) and a cutoff low. An AR is a long, narrow region with a deep moist layer. A cutoff low is an upper-level cyclonic eddy isolated from the meandering upper-level westerly jet. Herein, we construct composites of cutoff lows both close to an AR (AR-close category) and distant from an AR (AR-distant category) over a 14-yr period across the western North Pacific region. A comparison between the two categories shows an enhanced precipitation area to the northwest of the cutoff low and to the south of the AR axis in the AR-close category. The horizontal formation among the AR, cutoff low, and enhanced precipitation area in the composite coincides with that in a disastrous flood event that occurred in Hiroshima, Japan, in 2014. The deep moist layer associated with the AR, and the destabilization and isentropic up-gliding effect associated with the cutoff low are also observed in both the composite and the Hiroshima cases. We further evaluate the distribution of quasigeostrophic forcing (Q vector) for vertical motion. This shows that warm air advection associated with the AR overcomes the descending forcing inherent in the northwest of the cutoff low and makes the instability and up-gliding effect in that region more effective. These results indicate that the interplay between ARs and cutoff lows is a common mechanism in the enhancement of precipitation and the Hiroshima case is an extreme precipitation event caused by this interplay.

Validation of a Rapid Attribution of the May/June 2016 Flood-Inducing Precipitation in France to Climate Change

The extreme precipitation that resulted in historic flooding in central-northern France began 26 May 2016 and was linked to a large cutoff low. The floods caused some casualties and over a billion euros in damage. To objectively answer the question of whether anthropogenic climate change played a role, a near-real-time “rapid” attribution analysis was performed, using well-established event attribution methods, best available observational data, and as many climate simulations as possible within that time frame. This study confirms the results of the rapid attribution study. We estimate how anthropogenic climate change has affected the likelihood of exceedance of the observed amount of 3-day precipitation in April–June for the Seine and Loire basins. We find that the observed precipitation in the Seine basin was very rare, with a return period of hundreds of years. It was less rare on the Loire—roughly 1 in 20 years. We evaluated five climate model ensembles for 3-day basin-averaged precipitation extremes in April–June. The four ensembles that simulated the statistics agree well. Combining the results reduces the uncertainty and indicates that the probability of such rainfall has increased over the last century by about a factor of 2.2 (>1.4) on the Seine and 1.9 (>1.5) on the Loire due to anthropogenic emissions. These numbers are virtually the same as those in the near-real-time attribution study by van Oldenborgh et al. Together with the evaluation of the attribution of Storm Desmond by Otto et al., this shows that, for these types of events, near-real-time attribution studies are now possible.

Strong downdrafts preceding rapid tropopause ascent and their potential to identify cross-tropopause stratospheric intrusions

The capability of measuring three-dimensional wind and tropopause structure with relatively high time and vertical resolution makes very-high-frequency (VHF) radars a potentially important tool for studying various processes of the atmosphere. However, at present several unanswered questions remain regarding the use of VHF radars to identify possible stratospheric intrusions. Here the potential detection of stratospheric intrusion events is discussed using the Beijing MST (mesosphere–stratosphere–troposphere) radar located at Xianghe (39.75∘ N, 116.96∘ E). During the passage of a cutoff low in late November 2014, a deep V-shaped tropopause structure and strong downdrafts (> 0.8 m s−1) immediately preceding the rapid tropopause ascent (> 0.2 km h−1) were observed. Within the height region of the downdrafts, the stability of the radar tropopause seems to be weakened. Analysis results from global reanalysis and the satellite data, as well as the trajectory model, have shown clear evidence of downward stratospheric intrusions (dry ozone-rich and depleted methane air) associated with the strong downdrafts. A total of 20 typical cases of such strong downdrafts, occurring during various synoptic processes in different seasons, have been presented, and 15 of them are exactly associated with some form of stratospheric intrusions. Four years (2012–2015) of such downdrafts are further discussed. The observations reveal that the strong downdrafts preceding the rapid tropopause ascent can be a valuable diagnostic for monitoring intrusion events, which helps us to gain a better understanding of stratospheric intrusions in VHF radar observations.

Synoptic Environments and Characteristics of Convection Reaching the Tropopause over Northeast China

Overshooting convection that penetrates the lapse rate tropopause is defined globally using 3 years of Global Precipitation Measurement (GPM) observations and ERA-Interim data. Overshooting convection in the subtropics is mainly found over a few hot spot regions, including central North America and Argentina. A relatively high density of events with overshooting convection is also found over northeast China in the summer months, where 203 events are identified during 2014–16. These convective events extending above the tropopause occur under various synoptic conditions. The synoptic conditions during these events are categorized into three different types, namely, trough, cutoff low, and ridge types, with a subjective analysis based on the wind and pressure fields at 500 hPa. The precipitation systems with overshooting convection ahead of a deep trough have larger sizes than other types. Those in the cutoff low environment are mostly embedded within a large precipitation system. The ridge-type systems have a stable midtroposphere and a high moist instability at low levels and are mostly isolated convective systems, characterized by smaller sizes, higher radar echo top, and larger convective area and precipitation fraction than the other two types.

Meteorological Interpretation of Orographic Precipitation Gradients along an Andes West Slope Basin at 30°S (Elqui Valley, Chile)

To better forecast streamflow and water resource availability, it is important to have an understanding of the meteorological drivers of the orographic precipitation gradient (OPG), especially critical in semiarid mountainous areas. Although forced ascent over topography typically results in precipitation increasing with altitude (positive OPGs), mean annual OPGs and especially OPGs associated with individual storms can change widely in magnitude and even sign. Precipitation measurements from the Elqui Valley in the semiarid Andes of Chile (30°S) reveal a mean annual OPG of 6.3 mm km−1 (millimeters of precipitation over kilometers in elevation) ranging from −42 to 52 mm km−1 for individual storms over the last 35 years (1979–2013). Reanalysis data and precipitation measurements are used to characterize the observed OPG in this region in relation with their synoptic-scale flow. It is found that the Froude number correlates positively with the OPG, reflecting stronger zonal winds and less static stability during storms that have positive OPGs. Altitude of the Andes barrier jet shows only a weak relationship with the OPG. Significant storms with positive OPGs are typically linked with an austral blocking of the westerlies and an equatorward migration of the midlatitude storm track. For negative OPGs, either a cutoff low or the northern edge of a surface migratory cyclone reaches the Elqui Valley in such a way that significant rainfall only occurs in the near-coastal region without major snowfall accumulation over the Andes.