The Predictability of Northern Hemispheric Blocking Using an Ensemble Mean Forecast System

Abstract

Some weather extremes can be the result of atmospheric blocking, which can be responsible for the stagnation of weather patterns. These large-scale quasi-stationary mid-latitude flow regimes can result in significant temperature and precipitation anomalies in the regions that the blocking event impacts. The ability to predict periods of anomalous weather conditions due to atmospheric blocking is a major problem for medium-range forecasting. Analyzing the NCEP Ensemble 500-mb pressure heights (240 h) ten-day forecasts, and using the University of Missouri blocking archive to identify blocking event, the forecasted duration and intensity of model blocking events are compared to observed blocks. Comparing these differences using four case studies occurring over a one-year period across the Northern Hemisphere has shown the continued need for improvement of the duration and intensity of blocking events. Additionally, a comparison of the block intensity to a diagnostic known as the Integrated Regional Enstrophy (IRE) was performed in order to determine if there is a correlation between these quantities. Having a better understanding of knowing how long each block will last and their associated anomalies can help society prepare for the damage they can cause. Simulating and identifying blocks correctly is important in improving forecast issues.

Integrated Regional Enstrophy and Block Intensity as a Measure of Kolmogorov Entropy

“…The calculation for the correlation coefficient and the significance can be found in any introductory statistics text (e.g., [37]). However, if BI is lagged by 24 h, testing the assertion of [35,36], the correlation is 0.72, which is significant at the 99% confidence level supporting the possible lag relationship suggested by [35,36]. For the winter 2017 blocking case (Table 2), BI was relatively high for both the onset and intensification period, however, IRE did not increase markedly for 72 h following onset.…”

“…They also showed that the central height value and, thus, BI continues to increase for a short period following upstream cyclogenesis. Then, the pilot study of [35] suggested that first cyclogenesis occurs and then IRE increases, followed by BI for four case studies, including the winter 2017 case here. Additionally, in order to attempt to explain the difference in the lag times for each case, the deepening rates of the upstream cyclones were calculated.…”

“…Unlike the 2014 event described above, this event persisted in a negative PNA environment (mean PNA index = −0.669) and underwent decay as the PNA transitioned to the positive phase. This event was studied initially in order to test the ability of an ensemble model to forecast the onset of the event up to seven days in advance [35]. This pilot study [35] showed that the event was forecast only up to four days in advance by an ensemble model, but the intensity and duration of the event were under forecast as most previous blocking studies suggest (e.g., [11]).…”

“…This event was studied initially in order to test the ability of an ensemble model to forecast the onset of the event up to seven days in advance [35]. This pilot study [35] showed that the event was forecast only up to four days in advance by an ensemble model, but the intensity and duration of the event were under forecast as most previous blocking studies suggest (e.g., [11]). For this event, there was a mid-lifecycle peak in intensity that was associated with a strong upstream cyclone development, and the onset of the Atlantic event described above.…”

“…1 Due to the algorithm used to calculate KSE, the value obtained was the mean from 14 to 24 January 2014, while for the IRE the range of dates was 20-24 January 2014, and BI could only be 23-24 January 2014, since a block must be present for BI to be calculated. This lag between IRE and BI was studied in [35]. The work of [36] (and references therein) suggested that upstream cyclogenesis is a necessary condition for block onset or block intensification during the mid-block period.…”