A case study of an extreme flooding episode in Charikar, Eastern Afghanistan

Since the early 21st century, Afghanistan's communities have experienced extreme vulnerability hazards such as drought and flash flooding (Web-Ref. Climate risk country profile: Afghanistan, 2021).

The risk of flooding is very common in Afghanistan, regardless of the normally arid, low-precipitation situation. Also, in spite of the high limited data, there is enough indication to reveal that flooding triggers at least 100 deaths per year, which shows that Afghanistan is a significant disaster hotspot area (Elalem & pal 2015) and flash flooding is of specific concern. Besides the heavy precipitation occurrences, Afghanistan's mountainous areas are also subjected to the danger of melted glacier surge floods (Mergili et al. 2013). Finally, Afghanistan's communities are also faced with river flooding, and based on the World Resources Institute's AQUEDUCT Global Flood Analyzer by 2010, the annual Afghanistan population faced flooding is estimated at 561,500 people impacting $619.8 million in AMU DARYA BASIN (WRI 2023). It is noted that there is limited research on climate change and flooding trends in Afghanistan. Together these processes are inadequately examined in Afghanistan, but warning to extreme situations needs to prevent losses and damage and health effects.

This study examines the synoptic dynamical analysis to understand the weather pattern causing the deadly recent flooding of Afghanistan. The flooding, affected by torrential rain showers, mostly struck the city of Charikar in Parvan province early in the morning, while most people were asleep. This caused 150 deaths, and nearly 500 houses were destroyed. Also, this study investigates the anomalies in different mereological parameters in comparison with the normal mean climatological values and aims to realize the possibility of the climate change role in the stated brutal Charikar flash floods during late August 2020. This work aims to show how an active frontal weather system has been stronger due to climate change and is considered an extreme event.

This study employs different surface and upper meteorological variables such as temperature, geopotential height, mean sea level (MSL) pressure, wind humidity, and winds from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis and MERRA database. NCEP/NCAR is an atmospheric reanalysis produced by the NCEP and NCAR.

NCEP data can provide a globally gridded climate data assimilation. The model with a resolution of T62 (209 km) has 28 vertical sigma levels, and the results are available at 6-h intervals. The local ingestion process brought only the 0Z, 6Z, 12Z, and 18Z forecasted values, and therefore, only those were used to make the monthly mean and daily time series. Also, there are more than 80 different variables in several different coordinate systems, for instance, 17 pressure levels at 2.5 by 2.5-degree grids, 11 isentropic levels on 2.5 by 2.5-degree grids, and 28 sigma levels on 192 by 94 Gaussian grids. The precipitation model datasets are formed based on the monthly global precipitation project. The NCEP datasets (Kalnay et al. 1996) are obtainable from 1948 to the present. In this work, first, temporal variations of surface temperature, relative humidity, and precipitation rate during the last two decades for the month of August over the study area have been investigated. Besides, the daily mean composites were considered as the variable average over the study period including daily mean composite maps for 26, 27, and 28 August 2020, and anomalies were calculated on a daily basis as the average's departure from the climate normal mean (1991–2020). The composite mean charts and anomaly maps were developed with the NOAA/ERSL Physical Sciences Division (www.ersl.noaa.gov/psd) support. Here, the daily and monthly mean composites were studied as the variable average over the study periodic time, and the anomalies were calculated as the average periodic time departure from the climate normal (1991–2020) as a standard reference period for the long-term climate change calculations as a typical reference period recommended by World Meteorological Organization. Also, the synoptic and dynamic analyses have been done for both composite mean and anomaly for the resulting surface and upper-level weather charts.

Here, daily weather maps have been analysed and explained for the understanding of the specific weather structure during the severe recent flooding case in Afghanistan. This work has assessed both horizontal and vertical structures in different atmospheric parameters such as temperature, humidity, and wind vectors. Also, a three-hourly time average of the Ertel potential vorticity at 300 hPa during the flooding case over the study area during the flooding case has been obtained using the NASÁs Modern-Era Retrospective Analysis for Research and Applications (MERRA) Model (Rienecker et al. 2011). Also, the historical total precipitation observational dataset over the study area was obtained from the Afghan Meteorological Department and analysed in the form of a time series. An overall methodology adopted is provided in Figure 1.

For the climatic aspect of this specific weather system for 26 August 2020, the anomaly maps for MSL and 500 hPa (Figures 5(d) and 6(d)) display an intensification of both northern dynamical low-pressure tongue (around more than 8 hPa over east of China) linked with a deepening of the mid-tropospheric cyclonic trough (with a maximum of 70 gpm over Uzbekistan). This is along with the deepening of the southern heat low (around 2 hPa in the south of Pakistan) along with the subtropical ridge intensification (around 40 gpm in the northeast of India) rather than climate normal respectively. So, a comparison of the mean long-term climatic characteristics of the low- and mid-level tropospheric shows rather more possible baroclinicity than climate normal values caused by a relatively more intense active weather system over the study area. The general pattern of the daily MSL pressure for 27 August 2020 (Figure 5(b)) is nearly like Figure 5(a). However, the pressure curvature over eastern Afghanistan around the Charikar area has decreased in comparison with the previous day. This is accompanied by being more tilted and movement of the upper north-westerly trough to the east (shown with a blue dash line) extending from Uzbekistan to the northeast of Afghanistan (Figure 6(b)). During this time, the study area has been influenced by an active short-wave formation (depicted by a short straight blue dashed line). So, by developing the upper-level trough, the surface low-pressure system has been deepening along with the active frontal system.

A comparison of the MSL (Figure 5(e)) and 500 hpa (Figure 6(e)) map for long-term analysis with this day shows the passage of a very short wave in the eastern area of Afghanistan with respect to 26 August 2020. The anomaly maps for MSL and 500 hPa (Figures 5(e) and 6(e)) display an intensification of both northern dynamical low-pressure tongue (with the maximum value around 7 hPa over the north of Turkmenistan) associated with the deepening of the mid-tropospheric cyclonic trough (with a maximum of 45 gpm over Uzbekistan, Tajikistan, and Kyrgyzstan).

The general pattern of the daily MSL pressure for the day of 28 August 2020 (Figure 5(c)) shows nearly like Figure 5(a) and 5(b). However, the pressure gradient over the eastern Afghanistan around the Charikar area has been slightly decreased in comparison with 27 August 2020. However, the upper trough has changed into an elongated form during this time, and the eastern part of Afghanistan has been influenced by nearly flat westerly flows, which can cause the possible minor troughs passage (Figure 6(c)).

The atmospheric processes producing heavy precipitation on 26 August 2020 and causing severe floods affecting the northern districts of Parwan province, in Charikar, were analysed and explained in this study.

The temporal variations of surface temperature, relative humidity, and precipitation rate over Charikar during the last two decades for the month of August have been analysed. This has revealed a positive temperature trend, a rather drier atmosphere, and less precipitation mostly during the recent decade in comparison with the last two decades over the study area with a dramatic variation for all mentioned meteorological parameters during the year 2020 averaged for the month of August. Also, the calculation of the convective available potential energy and Showalter index obtained from the Skew-T log-pressure diagram for a grid point very close to the study region on the flooding day at 06Z show a high value of CAPE around 2,632 J/kg and −6.6 for Showalter index showing a very extreme instability in the study area during the study time. The daily MSL pressure for 26 August 2020 showed the extension of the surface heat low-pressure system to the study area in the form of a dynamical low-pressure trough along with the existence of a deep mid-tropospheric north-westerly trough cited over Uzbekistan and Tajikistan affecting the study region. Consideration of these patterns clearly showed the existence of an active classical frontal weather system on 26 August 2020 over the study region. A comparison of the mean long-term climatic characteristics of the low- and mid-level tropospheric shows more possible baroclinicity than climate normal values caused by a relatively more intense active weather system over the study area. Ertel's potential vorticity at 300 hPa over the Charikar area on 26 August 2020 shows at around 0.5 PVU, which is not a considerable potential streamer. So, it seems that this system should be triggered mostly by other factors such as thermodynamical and orographic aspects rather than the PV streamer. Low-level air temperatures and horizontal wind vectors have shown that the warm air flows from southwest encounter into northeast cold air flows in the study area along with strong vertical motion, which has led to form a convectional system over the study area. A comparison of the climate normal and composite mean displays an intensified jet stream core with a wavy pattern at ∼30 m/s shifted southward from ∼40°N to 35 °N over the study area. However, the pattern for the climate normal shows a broken and straight jet stream over Turkmenistan. However, a southward shift would result in cooling with a wavier pattern accompanied by stronger frontal activities with a rather long life as we can see in the case study. This work showed a rather stronger active frontal weather system over the study area during the flooding case. Also, it should be considered that local topography has a strong influence on the atmosphere, which can alter the airflow. The interactions between topography and the atmosphere can create different precipitation patterns in different spatial scales, which may differ in the size and the structure of the valley and ridges.

Here, in this work, the atmospheric processes have been strongly influenced by the specific regional topographical characteristics of the Charikar located in the Koh Daman Valley, which is surrounded by mountains. So, the westerly air flowing toward the mountains became warm and dry, holding low moisture and encountering rather colder, more moisture in the valley area along with stronger omega (strengthening the low-level convergence) rather than normal climate. This resulted in rather stronger baroclinicity forming a relatively stronger frontal system and forming severe precipitation in the study area.

But an extra systematic understanding of variations in the short-timescale extreme weather events, mainly in climate and weather quantities like temperature, winds, humidity, and precipitation, requires long-term observational datasets of higher temporal and even spatial resolution, such as daily or sub-daily basis. This has some limitations in many regions like Afghanistan. Also due to the importance of flood modelling for preventing and controlling, future works can be done in this direction.

Thanks are given to NOAA/ESRL PSD, Physical Science Division, Boulder Colorado web page through http://www.esrl.noaa.gov/psd/ and Giovanni online data system, developed and maintained by the NASA GES DISC and NASA LP DAAC at the USGS EROS Center. Also, thanks to the web page through https://www.globalsecurity.org/index.html for the Afghanistan map. Thanks to the Afghan Meteorological Department (AMD) for providing the required precipitation data. The authors would like to thank the anonymous referees for their valuable comments.

F.F.R. worked on conceptualization, data curation, formal analysis, and writing – original draft. V.S. worked on project administration, resources, validation, visualization, writing – review and editing.

The authors declare that the article is not funded by any scientific institution.

All relevant data are included in the paper or its Supplementary information.

The authors declare there is no conflict.