Midlatitude Events Research Articles

The Characteristics of Tropical and Midlatitude Mesoscale Convective Systems as Revealed by Radar Wind Profilers

AbstractThis study contrasts characteristics of mature squall‐line mesoscale convective systems (MCSs) observed by extended ground‐based radar wind profiler (RWP) deployments from the U.S. Department of Energy Atmospheric Radiation Measurement program. This analysis compares the dynamical structure, precipitation, and cold pool properties associated with MCS events over RWP sites in Oklahoma, USA, (midlatitude) to those observed during a 2‐year RWP deployment to Manaus, Brazil, during GoAmazon2014/5 campaign (tropical). The MCSs indicate similar convective line rainfall rates and total rainfall accumulations. However, midlatitude events suggest a larger fractional stratiform contribution to total precipitation. For both regions, convective line cold pools are associated with sharp decreases (approximately 10 K) in the surface equivalent potential temperature (θe) near the time of line passage. Surface θe properties for both regions suggest a modest relationship between rainfall rate and the probability of observing measurable surface rainfall. The probability of observing convective updrafts in both tropical and midlatitude MCS events is found to be similar as a function of low‐level radar reflectivity. However, midlatitude MCSs are associated with more intense convective updrafts, with upward air motions (mean, maximum) peaking at higher altitude. The most pronounced contrast is the propensity for deeper and more intense downdrafts in midlatitude MCSs. An analysis based on observed downdraft properties is performed using simple mixing assumptions. For these events, the vertical gradient of θe in the lower troposphere is relatively consistent between the Amazon and Oklahoma, suggesting similar mixing rates for downdrafts originating below 3 km (0.1 km−1). However, if downdrafts originate nearer to the level of minimum θe at SGP, mixing may be occurring at rates comparable to 0.3 km−1.

Possible role of auroral oval-related currents in two intense magnetic storms recorded by old mid-latitude observatories Clementinum and Greenwich

Some recent studies point out that currents related to the auroral oval, electrojets and field aligned currents (FACs), are serious candidates for the mechanism of the intense mid-latitude magnetic storms. It is interesting to re-analyse historical data under the light of this modern knowledge. In this aim, we analysed two intense magnetic storms that were recorded by observatories Clementinum (Prague) and Greenwich on 17 November 1848 and 4 February 1872, respectively. The latter has been marked as an extraordinary event by several authors, in particular in connection with auroras. The former, however, has been little known in the space weather community. Both these events possessed swift and extensive variations of the horizontal (H) component (>400 nT and >500 nT, respectively) and were accompanied by auroras sighted at very low magnetic latitudes. This implies that the auroral oval on the north hemisphere was vastly extended southward. The variations of the magnetic declination also indicate that during these events the auroral oval was situated at magnetic latitudes lower than those of the observatories. The storms studied in this paper occurred at different magnetic local times (MLTs), ~23 MLT and ~19 MLT. Therefore, they might represent mid-latitude events related to different parts of the auroral oval. In this paper, theH-variation recorded at Clementinum in 1848 is interpreted to be a substorm due to the ionospheric substorm electrojet. The Greenwich event registered in 1872 then seems to be a combination of the ring-current storm with a positive variation of theH-component caused by the eastward electrojet. Both the events of 1848 and 1872 appear to exemplify phenomena that are common in high magnetic latitudes but which may occasionally happen also at mid-latitudes.

Mesospheric Temperature During the Extreme Midlatitude Noctilucent Cloud Event on 18/19 July 2016

AbstractA rare noctilucent cloud (NLC) event was observed at 48.8∘ N, 13.7∘ E above GERman Experimental Seismic System station in southern Germany on 18/19 July 2016 using the Compact Rayleigh Autonomous Lidar. Strong southward winds due to a quasi 2‐day planetary wave allowed for the influx of mesospheric polar air to midlatitudes on this day. The NLC observed by lidar was preceded by strong NLC displays in Cloud Imaging and Particle Size (CIPS) Experiment satellite images above the North Sea and by strong mesospheric summer radar echoes 800 km north of the lidar site and was also observed visually in central Europe. The NLC occurred at low altitude and was bright and thin with strong oscillations in altitude and brightness. Darkness allowed for high‐resolution temperature measurements at NLC altitudes. The ice particles were embedded in the upper part of a cold region with temperatures below 150 K. Significantly higher temperatures were found directly above the cloud with large vertical temperature gradients of 25 K/km at the top boundary. Spectral analysis reveals that NLC particles existed within cold phases of gravity waves within a region of high static stability. In order to study the evolution of NLC brightness in this environment, we drive the microphysical model Community Aerosol and Radiation Model for Atmospheres with lidar temperature soundings. We find that NLC particles can survive and grow in the conditions defining this midlatitude event. We conclude that the ice particles did not nucleate at the site of observation but were meridionally transported and vertically confined to a thin layer due to a large vertical temperature gradient, wind reversal, and low levels of mesospheric turbulence.

Attribution of extreme weather events in Africa: a preliminary exploration of the science and policy implications

Extreme weather events are a significant cause of loss of life and livelihoods, particularly in vulnerable countries and communities in Africa. Such events or their probability of occurring may be, or are, changing due to climate change with consequent changes in the associated risks. To adapt to, or to address loss and damage from, this changing risk we need to understand the effects of climate change on extreme weather events and their impacts. The emerging science of probabilistic event attribution can provide scientific evidence about the contribution of anthropogenic climate change to changes in risk of extreme events. This research has the potential to be useful for climate change adaptation, but there is a need to explore its application in vulnerable developing countries, particularly those in Africa, since the majority of existing event attribution studies have focused on mid-latitude events. Here we explain the methods of, and implications of, different approaches to attributing extreme weather events in an African context. The analysis demonstrates that different ways of framing attribution questions can lead to very different assessments of change in risk. Crucially, defining the most appropriate attribution question to ask is not a science decision but one that needs to be made in dialogue with those stakeholders who will use the answers. This is true of all attribution studies but may be particularly relevant in a tropical context, suggesting that collaboration between scientists and policy-makers is a priority for Africa.

Long-Lived Response of the Midlatitude Circulation and Storm Tracks to Pulses of Tropical Heating

Abstract The midlatitude response to localized equatorial heating events that last 2 days is examined through experimentation with an atmospheric general circulation model. Such responses are argued to be important because many tropical rainfall events only last a short time and because the responses to such pulses serve as building blocks with which to study the impacts of more general heating fluctuations. The experiments indicate that short-lived heating produces responses in midlatitudes at locations far removed from the source and these responses persist much longer than the pulses themselves. Indeed pulse forcing, which is essentially white in time, produces upper-tropospheric responses that have an e-damping time of almost a week and that are detectable for more than two weeks in the experiments. Moreover the upper-tropospheric structure of the reaction to short pulses is remarkably similar to the reaction to steady tropical heating, including having a preference for occurring at special geographical locations and being composed of recurring patterns that resemble the leading patterns of responses to steady heating. This similarity is argued to be a consequence of the responses to pulses having little or no phase propagation in the extratropics. The impact of short-lived tropical heating also produces a persistent response in midlatitude surface fields and in the statistics of synoptic eddies. The implications these results have for subseasonal variability are discussed. These include 1) the potential for improving subseasonal prediction through improved assimilation and short-range forecasts of tropical precipitation and 2) the difficulties involved in attributing subseasonal midlatitude events to tropical heating.

A FAST study of quasi‐static structure (“Inverted‐V”) potential drops and their latitudinal dependence in the premidnight sector and ramifications for the current‐voltage relationship

Utilizing FAST satellite electron measurements, we present the first reported investigation of the dependency on latitude of quasi‐static structure (“inverted‐V”) potential drop magnitude (Φ). A trend of lower Φ at lower latitudes in the premidnight sector on field lines with dark foot points was observed. This trend is supported both statistically and in individual satellite crossings. The existence of two distinct peaks in occurrence probability for Φ was also observed: one between ~2 kV and 10 kV and the other at somewhat less than 1 kV. The relative occurrence of structures with Φ in the higher (>2 kV) peak is significantly reduced with decreasing latitude. This partially accounts for the statistical trend of lower potential drop magnitudes at lower latitudes. The two Φ occurrence frequency peaks correspond to two different regimes (one with eΦ/kTe ~ or > 1 and one with eΦ/kTe < 1). In the lower Φ (<1 kV, eΦ/kTe < ~1) regime which constitutes the vast majority of midlatitude events, these results are consistent with a voltage source configuration or more complex current‐voltage relation where source electron density rather than Φ is most directly controlled by the field‐aligned current density. These observations and their ramifications represent a significant step forward in the understanding of field‐aligned currents, auroral acceleration, and magnetospheric‐ionospheric coupling.

Remote weather associated with North Pacific subtropical sea level high properties

AbstractRemote events influencing North Pacific (NP) subtropical high properties in monthly and daily data are identified. Variability in the NP during summer is far more strongly dominated by midlatitude events than in South Pacific (SP); low‐pass filtering is required to see tropical associations.The dominant pattern in composites, correlations, and regressions is a midlatitude wave train. A stronger NP high was led by higher sea‐level pressure (SLP) just east of Japan and lower SLP over central Canada and to a lesser extent over western tropical Pacific.Various mechanisms have been proposed to force the NP high: Heating over southwestern North America (with cooling off the west coast). However, higher temperatures over North America follow stronger SLP over the NP high and occur much further east than postulated. Higher SLP occurs where temperatures are lower over western North America and adjacent ocean. Thermal pattern is consistent with temperature advection between NP high and Canadian low. Precipitation over and near Central America. However, SLP increase on the SE side of the high is led by higher SLP (and higher outgoing longwave radiation (OLR)) along the west coast of Mexico and Central America. Normalized regressions find a very weak lower OLR in North American monsoon preceding stronger NP high, but the region is much smaller in size and magnitude than other significant areas. Precipitation over Indonesia and southeast Asia. Statistics provide some support for lower SLP and OLR over Indonesia preceding higher SLP in the center, west, and northwest sides of NP high. The lower SLP and OLR appear to migrate into southeast Asia, perhaps independently, perhaps from stronger NP high. The NP high has a strong connection to El Niño during winter, but no significant link during summer. Only the south side of NP high appears (weakly) linked to the Madden Julian oscillation (MJO). Copyright © 2006 Royal Meteorological Society

The development of tropical cyclones in the north-west of Australia

Case studies of developing and non-developing tropical depressions in the north-west Australian region during the 1979/80 and 1980/81 cyclone seasons have been constructed using conventional and satellite observations. These show that enhanced low-level winds can occur during the lifetime of non-developing disturbances and are not exclusively associated with cyclogenesis. Both types of system possessed lower-level cyclonic circulations and there was an overlapping such that non-developing storms could have stronger circulations than some developers. At upper levels, development was associated with the subtropical ridge being situated over the surface position of the disturbance. In contrast, the ridge tended to be located southwards of non-developers. These findings are supported by calculations of area-averaged vorticity at 850 and 200 mb. Cross-sections of zonal winds show that non-developing systems could exhibit weaker vertical wind shears than some developing storms. Developers had only moderate shears at worst and maintained a well organized lower cyclonic circulation. Such features are consistent with both classes of disturbance existing within the monsoon shearline. Non-development was caused by interactions with large-scale flows in the southern hemisphere that prevented the necessary conditions for cyclogenesis from being all fulfilled. This suggests a need for greater understanding of the influence of mid-latitude events on depressions within the monsoon trough.

Phenomenological study of LHR hiss

The 1963 and 1964 Alouette 1 records from the Ottawa station were studied to determine the statistical characteristics of VLF hiss exhibiting a sharp lower-cutoff frequency associated with the lower-hybrid resonance (LHR). A particular class of midlatitude hiss events were emphasized and designated ‘S-type hiss.’ These events have lower-cutoff frequencies that vary smoothly with time, have limited bandwidths, and do not appear to be associated with other VLF phenomena. Their diurnal time of peak occurrence provides evidence that their sources may be connected with the type of magnetospheric activity usually associated with auroral-zone phenomena; yet, they are definitely midlatitude events. A likely means for transporting the electromagnetic energy is a horizontal duct that occurs as a result of the presence of more than one ion species in the topside ionosphere. Nonducted whistler-mode signals may be trapped in the duct, and satellites within the region observe signals with characteristics determined by this guiding structure. Other forms of LHR hiss, such as whistler-triggered events, may also be associated with these ducts. The characteristics of noise at altitudes above such ducts, or in the absence of ducts, may be different from that of S-type hiss. Some of the observed Alouette 1 events may be such nontrapped noises.