Satellite weather is a great way to see the weather at a distance. New satellite observations become available every 5 to 15 minutes, depending on the location.
In the southern Indian state of Kerala, rainfall was exceptionally high during August 2018, causing widespread flooding. Prediction and early warning of severe weather events in vulnerable areas is vital for disaster management.
Monsoon
The monsoon is one of the most important climate events in India. Over the course of four months, it brings more than 70 per cent of the country’s annual rainfall.
As the monsoon moves across southern India, it often comes in with heavy rain and thunderstorms. These can create landslides, and if not dealt with quickly enough, can cause widespread flooding.
Satellite weather data can provide an early warning of the onset of these storms. It can show whether there is an increase in deep cells or congestus cells in areas that are likely to experience these thunderstorms.
Doppler radar data can also be used to show whether there is an increase in convective activity around a particular date. It can also show how much precipitation is expected over a region.
During the monsoon,’mesoscale’ systems of thunderstorms can grow rapidly and erupt over large regions, making it difficult to forecast their development. This is especially true in Kerala, where lightning and rainfall can be particularly devastating.
At 5-day resolution, there is a strong correlation between the area covered by both deep and congestus type cells and the amount of precipitation that is expected for any given day. However, some sites, such as Bhopal and Nagpur, do not experience a significant increase in both types of cells, but are still expected to receive a substantial amount of rainfall (Figure 3(i) and (j)).
In addition, the areas covered by both deep and congestus cells can show an anti-correlation between the onset of monsoon and the advancing moist tropical air in some locations. This is because drier air can begin to push north into the country, but monsoon onset can be associated with the arrival of westerly winds bringing the moist tropical air and resulting in an increase in precipitation over regions that are likely to receive these storms.
Assam has been hit hard by this year’s monsoon, with more than 15 people killed in a series of flash floods and landslides. The state has been devastated for the past two weeks, with many communities living in a state of fear and uncertainty. The worst flooding has affected Dima Hasao, a district in the northern part of the state. The area is now largely buried under a layer of mud and debris, with the Meenachil river overflowing.
Rainfall
Rainfall is one of the most important weather parameters for the sliver of land between the Arabian Sea and the towering Western Ghats that are home to Kerala. It is essential for the livelihood of millions, and navigation demands an accurate forecast.
In August 2018, Kerala received an unprecedented influx of heavy rainfall that led to widespread flooding. These extreme rainfall events were caused by large-scale emergence of moist convective clouds that grew quickly in the tropics and are difficult to predict.
These clouds triggered severe storms that dropped 50 millimetres of rain within two hours and resulted in flash floods across the region. Scientists haven’t figured out how these’mesoscale’ systems of thunderstorms grow, but they can wreak havoc when they do.
Satellite-based precipitation data indicates the first spell occurred over northern and central Kerala (see Figure 1a), while the second spell lasted for four days in the south of the state (Figure 1b). The observations of station and gauge-based rainfall together with water vapor images from INSAT-3DR suggest that these extreme rainfall episodes were associated with an offshore trough and a deep depression over the Bay of Bengal (BOB).
High-resolution global and regional reanalysis data reveals that the anomalous atmospheric conditions that led to the occurrence of these heavy rainfall events in Kerala were associated with a monsoon low-pressure trough, an intense offshore trough over the west coast of India, and a deep depression over the BOB. These three climatic factors combined to cause the sustained occurrence of heavy rainfall in Kerala between August 1 and 19.
Moreover, the observational trends manifested from high-resolution cloud resolving Weather Research and Forecasting (WRF) model simulations also indicate that the strong low-level westerly jet on hilly topography, mechanical uplifting of air and moist convection, and the transport of moisture from the BOB were responsible for the prolonged occurrence of these rainfall episodes.
A detailed investigation of the underlying mechanisms that lead to these extreme rainfall episodes revealed that these events were characterized by a westward-propagating barotropic Rossby wave, which was accompanied by a large-scale convergence of moisture over Kerala. This study shows that ensemble forecasts of extreme rainfall by state-of-the-art prediction systems are skillful for longer lead times than deterministic models, and these forecasts can be used to provide early warnings.
Floods
Kerala, one of the most arid and densely populated states in India, is also home to one of the country’s most vulnerable areas for flooding. The state is surrounded by mountains, the Western Ghats in particular, which are prone to heavy rainfall during the monsoon.
In 2018, more than 100 people died and at least a million were displaced in the worst floods in Kerala’s history, according to an assessment from India’s central government. A warming climate could make these events more common, according to climate researchers.
The Indian government’s national disaster response agency is deploying military helicopters to bring relief supplies and rescue personnel to flooded areas. The state’s chief minister, Pinarayi Vijayan, urged residents to remain calm and take precautions.
While flooding is a normal part of the monsoon season in India, it is a particularly serious problem this year due to an exceptional rainfall. Thousands of people have been evacuated, and 184 relief camps have been established across the state.
Scientists are investigating why flooding occurred in this particular year, and whether it is related to changes in climate patterns. Using data from satellites, researchers will study cloud formation and precipitation over the region, as well as examine how the weather system affected the coastal area.
A major flood hit southern India’s Kerala state in August 2018, killing more than 200 people and leaving at least 800,000 homeless. It was the worst in a century. Despite the damage, residents are still recovering.
As a result of the flooding, some villages are still without electricity and clean drinking water, the Associated Press reports. The government has also announced financial aid for those who lost their homes and crops.
But it’s difficult to predict when these extreme events will occur, says Emmanuel, who lives along the Meenachil River and is helping local schoolchildren and teachers set up a network of improvised rain gauges that measure daily rain in the surrounding hills. The data gathered by Emmanuel’s team help predict when the Meenachil and other rivers will overflow, as well as how quickly the flooding will spread.
Winds
Weather satellites provide new observations every 5 to 15 minutes, allowing meteorologists to observe changes in the weather. This helps to better understand the development of weather systems and track the movement of clouds, which is important for forecasting.
Winds are an important component of the weather system and they affect the cloud formation and cloud movement in different regions. They also affect the intensity of the rain and flooding, which can be devastating for people living in areas prone to these conditions.
In the southern Indian state of Kerala, strong monsoon winds and a low-pressure area in the Bay of Bengal were responsible for heavy rainfall events during August 2018. These winds caused floods in many areas of the state and led to widespread damage.
However, these events are not always predictable and predicting them requires models of high resolution. For this purpose, NCMRWF (National Centre for Medium Range Weather Forecasting) has developed operational numerical weather prediction (NWP) models that can be used to predict the rainfall over a specific area.
The NCMRWF Unified Model (NCUM) and the NCMRWF Ensemble Prediction System (NEPS) were used to assess forecasts over the recent heavy rainfall event in Kerala and coastal Karnataka. During this event, the global and regional model predictions showed excessive rainfall over the Arabian Sea compared to observed data.
Nevertheless, these global and regional model forecasts successfully captured the rain over parts of Kerala and coastal Karnataka. They show enhanced convection and a realistic amount of rainfall over the sea during this event.
Although these models have very high grid spacing, they can never be completely accurate because of uncertainty in the forecasts at smaller geographical scales such as a district or a city. The EPS-grams and the PQPFs generated from these models can be useful for forecasting and early warning of severe weather events.
The EPS-grams (top panels) and the PQPFs in the Day 3 and Day 4 forecasts (bottom panels) consistently predict heavy rainfall over northern Kerala (Wayanad, Kozhikode, Palakkad and Malappuram) as well as Idukki in the south. The probability of heavy rainfall (> 16 cm*day-1) is very high and the probability of very heavy rainfall (> 11.5 cm*day-1) is much lower.