Antti Mäkelä

The Daily Cloud-to-Ground Lightning Flash Density in the Contiguous United States and Finland

AbstractA method is developed to quantify thunderstorm intensity according to cloud-to-ground lightning flashes (hereafter ground flashes) determined by a lightning-location sensor network. The method is based on the ground flash density ND per thunderstorm day (ground flashes per square kilometer per thunderstorm day) calculated on 20 km × 20 km fixed squares. Because the square size roughly corresponds to the area covered by a typical thunderstorm, the flash density for one square defines a unit thunderstorm for the purposes of this study. This method is tested with ground flash data obtained from two nationwide lightning-location systems: the National Lightning Detection Network (NLDN) in the contiguous United States and the portion of the Nordic Lightning Information System (NORDLIS) in Finland. The distribution of daily ground flash density ND is computed for all of Finland and four 800 000 km2 regions in the United States (identified as western, central, eastern, and Florida). Although Finland and a...

Attachment of Natural Lightning Flashes to Trees: Preliminary Statistical Characteristics

Lightning attachment to trees was studied based on 37 trees that were struck in Finland in summer 2007 and 2008. The type and severity of lightning damage was correlated with multiple parameters related to the flash, the meteorological characteristics of the strike time, and the surroundings of the tree. Damage was classified into three categories: bark-loss (minor), wood-loss (extensive), end explosive (complete loss of tree material). Four statistically significant parameters were found. The absolute value of the peak flash current is strongly correlated with damage; also, an indirect argument suggests that positive flashes cause more damage than negative flashes. The amount of damage is inversely correlated with the rainfall in the previous three hours, indicating that a wet ground and tree surface protect trees against damage by providing a conducting path to the ground. The ground type also has a weaker statistically significant effect, with poorly conducting ground leading to more extensive damage. Old and rotten trees are statistically most likely to experience explosive damage. Other parameters are inconclusive. The distribution of tree heights points to the possibility that the electrogeometric method does not necessarily predict the strike probability to a given tree. In many cases, the struck trees were clearly within the protective radius of a higher structure, although this is often difficult to determine from photographs. It is therefore suggested that models of lightning flashes to trees should include both the conductivity and height of the tree. The results may be significant for remote prediction of lightning damage, as well as for understanding lightning protection of structures which include trees.

Real-Time Hazard Approximation of Long-Lasting Convective Storms Using Emergency Data

AbstractConvective storms cause several types of damage, including economic and ecological losses, every year. This paper focuses on an automatic hazard-level determination of convective storms based on a largely unused information source: real-time emergency report data. In addition to the location of the report, the emergency response centers classify cases into different emergency types and deliver a free-form verbal description of the incident for online use. This study uses archived weather-related emergency reports to determine hazard levels for convective storms detected by the weather radar. To develop an algorithm for estimating the hazard level of convective storms, a weather radar–databased convective storm-tracking algorithm was applied with a method that links reported emergency events to individually tracked convective storms. Based on the relationship between each convective storm track and an emergency report, the algorithm determines the hazard level of the storms automatically. Moreover,...

Flash Cells in Thunderstorms

When convection develops into a thundercloud stage of at least moderate intensity, its cellular appearance may be monitored not only with weather radars but also with lightning location systems. For intense thunderstorms, the ground-flash rate alone is sufficient for making apparent the underlying convection-cell structure. if flash cells can be properly identified, they show longitudinal development complementary to the usual view of advancing transverse front. A method is described to analyse the flash-cell structure of thunderstorms and is illustrated with an example from Finland. With proper choice of parameters, the largest, most interesting flash cells generally have realistic sizes, lifetimes and motion.

Lightning location system accuracy determined from strikes to trees

Lightning strikes to 41 trees are used to estimate the accuracy of the Finnish Lightning Location System (LLS). With some limitations, the data set is large enough to estimate the validity of the uncertainty ellipse which is provided by the LLS. Such an estimation does not appear to have been done before for natural lightning. The method may give slightly overoptimistic results because extreme outliers have been removed. However, using a two-part quality control system, the flash causing the damage could be identified with high reliability. It is shown that the uncertainty ellipse given by the LLS is statistically consistent with the distribution of lightning location errors inferred from tree strikes.

Surface air relative humidities spuriously exceeding 100% in CMIP5 model output and their impact on future projections

In 17 out of the 29 Phase 5 of Coupled Model Intercomparison Project (CMIP5) climate models examined in this work, near-surface air relative humidity (RH) frequently exceeded 100% with respect to ice in polar areas in winter. The degree of supersaturation varied considerably across the models, and the same evidently applies to the causes of the phenomenon. Consultations with the modeling groups revealed three categories of explanations for supersaturation occurrence: specification of RH with respect to ice rather than liquid water; inconsistencies in the determination of specific humidity and air temperature for the near-surface level; and the nonlinearity of saturated specific humidity as a function of temperature. Modeled global warming tended to reduce the artificial supersaturations, inducing a spurious negative trend in the future RH change. For example, over East Antarctica under Representative Concentration Pathway 8.5, the multimodel mean RH would decrease by about 10% by the end of the ongoing century. Truncation of overly high RHs to a maximum value of 100% cut the RH response close to zero. In Siberia and northern North America, truncation even reversed the sign of the response. The institutes responsible for the CMIP6 model experiments should be aware of the supersaturation issue, and the algorithms used to produce near-surface RH should be developed to eliminate the problem before publishing the RH output data.

Atmospheric electricity and aerosol-cloud interactions in earth’s atmosphere

Firstly, atmospheric ions play an important role in the fair weather electricity in Earth’s atmosphere. Small ions, or charged molecular clusters, carry electric currents in the atmosphere. These small ions are continuously present, and their lifetime in lower atmosphere is about one minute. It’s essential to find out a connection between the production rate of cluster ions, ion-ion recombination, and ion-aerosol attachment, and their ambient concentrations, in order to understand electrical properties of air. Secondly, atmospheric ions are important for Earth’s climate, due to their potential role in secondary aerosol formation, which can lead to increased number of cloud condensation nuclei (CCN), which in turn can change the cloud properties. Our aim is to quantify the connections between these two important roles of air ions based on field observations.

Estimation of lightning hazard of an approaching thunderstorm

Lightning location data from Finland has been analysed to provide information about the usability of the 30/30 rule, widely used in the personal lightning safety. The criteria are formalized so that a flash within 10 km of a given location launches the alarm. The storm is defined to produce a critical danger situation when a flash hits within 1 km of the location. With these definitions, the probability of false alarms is relatively high, up to 97%. On the other hand, in about 10% of the cases the 30–30 rule does not provide any pre-warning (i.e. the very first strike in the storm is 1 km away). In the cases where a prewarning is given, average lead times from first warning to danger are as much as 20 minutes. Although these results are preliminary, they suggest that the 30–30 rule may not be optimal in the case of a country with very low flash rates and densities, such as Finland.