Meteorological observations clearly demonstrate that global climate change has occurred since the beginning of the Industrial Revolution. That change has been particularly pronounced since about 1950, and includes changes in weather and extreme climate events. Changes in weather and climate extremes can significantly increase the impacts on society, leading to a greater number of disaster worldwide. One of the world's most disaster-prone regions is the Asia-Pacific. Since 1970, disasters have killed two million people in the Asia and Pacific region — 59% of the global death toll. The most frequent natural hazards in the region are hydro-meteorological events [1]. There is a pressing need to develop and implement new tools for global monitoring of these increasingly frequent and severe hazardous phenomena, including using modern satellite remote sensing techniques..
Recognizing the importance of this issue, WMO launched a two-year (2018–2019) Demonstration Project on space-based weather and climate extremes monitoring. The project is focused on drought and heavy precipitation over the South-East Asia region and the Pacific Ocean. In June 2019, the Eighteenth World Meteorological Congress (Cg-18) reviewed the outcomes and recommended the expansion of the project to other WMO regions and adopted an implementation plan to transition it into an operational phase. This article highlights the outcomes of the Demonstration Project in two Australian case studies: during the Millennium Drought and extreme precipitation events during the 2010/2011 La Niña event.
Knowledge transfer essential
The Demonstration Project was set up following the recommendations from a February 2017 WMO workshop on Operational Space-based Weather and Climate Extremes Monitoring. The workshop gathered representatives from satellite operators, research and development agencies, WMO Regional Climate Centres (RCCs) and National Meteorological and Hydrological Services (NMHSs) to stimulate a dialog about enhancing utilization of space-based observation data and products for monitoring weather and climate extremes.
The workshop recognized that many developing and least developed countries were not benefiting from the significant progress made in space-based observations in most geophysical domains – several high-resolution satellite products are available on a quasi-real time basis, enabling enhanced use for monitoring weather and climate extremes from space. Participants recommended the strengthening of human and technological capacity through knowledge transfer in order for all countries to benefit fully from the advantages of modern space-based data and derived products. The RCCs could enable such a knowledge transfer. Following the workshop's recommendations, WMO established the Demonstration Project to make a case for the use of space-based observations and products for monitoring weather and climate extremes.
Situation in Asia-Pacific region
Most NMHSs in South-East Asian and Pacific countries use conventional surface-based rain gauge observations for extreme precipitation monitoring. Rain gauge observations provide accurate point-based measurements of precipitation; however, data are restricted to the locations of meteorological observation stations. Over Australia, for example, the spatial distribution of rain gauges is not uniform: while the most densely populated regions are well covered, spatial coverage in other regions – like western Tasmania and the interior of the country – is very poor. This issue of non-uniform spatial coverage is typical in the Asia-Pacific region, where the rain gauge density in many areas is inadequate. Therefore, complementary rainfall estimates derived from space-based observations would better address various users' needs for precipitation information.
Current operational climate products for drought monitoring – derived from surface-based observations – focus typically on identifying rainfall deficits over extended periods (months to years) using percentile and/or decile analysis. And heavy precipitation is diagnosed typically on a monthly time scale (although higher resolution time information is available from gauges). Using space-based observations, it is possible to monitor extreme precipitation events over shorter time periods – pentad (5 days), week and longer periods of up to a month, in order to respond to current and future users' requirements. Thus, space-based observations can address users' needs for information about precipitation extremes on short timescales. Both RCCs and NMHSs consider the monitoring of weather and climate extremes on shorter time scales as a valuable extension of their operational products to enhance climate services for users in Asia-Pacific.
The Standardized Precipitation (SPI) is also widely used for meteorological drought detection and monitoring. Positive values of the SPI correspond to precipitation above median and negative values to precipitation below median. Drought conditions are classified when the SPI values are equal to or below -1.0. For example, an SPI of -1.0 or lower is classified as "moderately dry", -1.5 or lower – as "severely dry", and -2.0 or lower as "extremely dry".
Precipitation Products
The Demonstration Project aimed to demonstrate the benefits of using space-based extreme precipitation observations to the operational services of RCCs and NMHSs. It was implemented in WMO Regions II (Asia) and V (South-West Pacific), covering the geographical area of the South-East Asia region and the Pacific Ocean – from 40°N to 45°S; 50°E to 160°W. Two agencies – Japan Aerospace Exploration Agency (JAXA) and the Climate Prediction Center, National Oceanic and Atmospheric Administration (CPC/NOAA) – provide satellite data and products for the region.
The Demonstration Project based its definition of drought and heavy rainfall events on the Intergovernmental Panel on Climate Change (IPCC) Assessment Report 5 Working Group I definition of extreme events: “An extreme weather event is an event that is rare at a particular place and time of year. Definitions of rare vary, but an extreme weather event would normally be as rare as or rarer than the 10th or 90th percentile of a probability density function estimated from observations. By definition, the characteristics of what is called extreme weather may vary from place to place in an absolute sense. When a pattern of extreme weather persists for some time, such as a season, it may be classed as an extreme climate event, especially if it yields an average or total that is itself extreme (e.g., drought or heavy rainfall over a season).”
WMO recommendations and extensive consultations with the satellite data providers and users (RCCs and NMHSs in South-East Asia and the Pacific) determined that the Demonstration Project should aim to satisfy users' requirements for monitoring precipitation extremes on short timescales. Using JAXA and CPC/NOAA satellite-based near-real-time products for monitoring “heavy precipitation” and “drought” events operationally for climate analysis, the Demonstration Project developed pentad (5-day) to weekly and up to monthly climate information services for users.
JAXA precipitation products are based on the Global Satellite Mapping of Precipitation (GSMaP) [3]. For the Demonstration Project’s users in Asia-Pacific, JAXA provided mean precipitation estimates derived from GSMaP version 6 for hourly, daily (00-23UTC), pentad (5 days), weekly (Monday – Sunday), 10 days and monthly precipitation with spatial resolution of 0.1° latitude/longitude grid box. In addition, statistics for daily, pentad and weekly extreme precipitation (90th to 99th percentiles) and percentage of rainy (>=1mm/day) days in a month was provided. For drought monitoring, the SPI (1-month, 2-month and 3-month) for grid boxes over land with spatial resolution of 0.25°lat/long grid box was provided. These data are available within a few hours of their observation.
CPC/NOAA provided the Demonstration Project users with a similar set of products based on the Climate Prediction Center’s morphing technique (CMORPH) satellite precipitation estimates (see [4] for detail). The climatology of the CMORPH products are defined for a 20-year period from 1998 to the 2017. In addition to the SPI, weekly normalized differential vegetation index (NDVI) and the vegetation health index (VHI) were also made available.
Case study: Drought Monitoring in Australia
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Figure 1. Three-months SPI for June-July-August 2007 derived from the JAXA GSMaP (left) and the CPC/NOAA CMORPH bias-corrected precipitation data (right). |
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Figure 2. Rainfall deciles for Australia in June-July-August 2007 derived from the Australian Bureau of Meteorology rain gauge observations. |
The Demonstration Project examined the usefulness of space-based observations for drought monitoring over Australia in 2007, a critical year of the Millennium Drought, using three-month SPI values derived from JAXA GSMaP data.
Australia is the driest inhabited continent on Earth. Some 70% of Australia receives less than 500 millimetres of rain annually, classifying them as arid or semi-arid areas. In Australia, drought monitoring is, therefore, vital for informed decision-making in agriculture, disaster risk management, water management and other sectors. Australia’s Bureau of Meteorology (BOM) reports drought when rainfall over a three-month period remains in the lowest decile recorded for that region in the past [5]. BOM operational records go back over a century in Australia, show that on average drought occurs once every 18 years, however, the severity and duration of droughts vary.
One of the severest Australian droughts – the Millennium Drought – occurred in the 2000s and affected vast parts of the country. The Murray-Darling basin, Australia’s largest agricultural region, was severely affected as were the supply of water resources of many cities and towns, including Melbourne, Sydney, Brisbane and Adelaide. The drought commenced with a rainfall deficit in 1996/1997 and continued with very dry years through to 2001/2002. During 2006, south-eastern parts of Australia had their second driest year on record. By 2007, the Murray-Darling basin was experiencing its seventh consecutive year of below average rainfall. The dry, hot conditions continued to affect Australia through to early 2010. The 2010/2011 La Niña, one of the strongest on record, brought the Millennium Drought to the end. It resulted in record-breaking rainfall in the Murray-Darling basin and above average rainfall over the south-east of the country. The continuing above average rainfall significantly increased surface water storage and soil moisture, ending the drought.
The examination of three-month SPI for June-July-August 2007 (Figure 1) in the Murray-Darling basin derived from both the JAXA GSMaP and the CPC/NOAA CMORPH indicated SPI values below -1.5 (“severely dry”) in areas defined as “very much below average” on rainfall deciles map derived from BOM’s rain gauge observations (Figure 2). Space-based and in situ observations were, therefore, in agreement over the Murray-Darling basin in south-eastern Australia where the density of surface-based observations is high. However, there were noticeable discrepancies between SPI values and rainfall decile maps over the central parts of the country where the density of surface-based observations is very low. This demonstrates the value of space-based rainfall estimates for drought detection and monitoring, especially in regions where rain gauge observations are limited or unavailable.
Case study: Heavy rains in Australia
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Figure 3. Australian rainfall deciles for December 2010 derived from the Australian Bureau of Meteorology rain gauge observations. The extreme rainfall observed over parts of western and eastern Australia in December 2010 were associated with the 2010-2011 La Niña event. |
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Figure 4. JAXA GSMaP (left) and CPC/NOAA CMORPH (right) monthly rainfall percentiles for December 2010. |
This second case study looks at two heavy precipitation events over Australia in December 2010 and January 2011, which caused widespread flooding.
An "extreme rainfall" event is defined as occurring when mean rainfall for a specified period is higher than a certain percentile threshold, for example in the 90th to 99th percentile. Australia experienced such extreme rainfall during the La Niña event in 2010/2011 – 2011 was the third-wettest year since national rainfall records began in Australia in 1900. Averaged across Australia, both years experienced rainfall well above average: 690 mm (225 mm above the long-term average of 465 mm) in 2010 and 699 mm (234 mm above the long-term average of 465 mm) in 2011.
The 2010/2011 La Niña event had a significant impact on Australian rainfall. La Niña is typically associated with increased rainfall in northern and eastern Australia. During the 2010/2011 La Niña, most of mainland Australia experienced significantly higher than average rainfall over the nine months from July 2010 to March 2011. A number of new Australian rainfall records were set: wettest September, December and March on record and second-wettest October and February. The record-breaking rainfall during the 2010/2011 La Niña led to widespread flooding in many regions between September 2010 and March 2011 including southeast Queensland, large areas of northern and western Victoria, New South Wales, northwestern Western Australia and eastern Tasmania.
JAXA GSMaP and CPC/NOAA CMORPH monthly rainfall percentiles for December 2010 (see figure 4 on page 62) were used to examine the widespread flooding that occurred in the Australian State of Queensland in December 2010. A monsoonal trough crossed the coast from the Coral Sea on 23 December, bringing torrential rainfall to a large area of Queensland from the Gulf of Carpentaria to the Gold Coast. This was followed by intense rainfall on 25 December, due to the landfall of tropical cyclone Tasha. By 28 December, nearly half of Queensland was flooded; economic losses reached A$6 billion. The areas of rainfall above the 95th percentile shown in both the GSMaP and the CMORPH analyses maps corresponded to the "very much above average" rainfall deciles derived from BOM rain gauge observations (Figure 3). Furthermore, the weekly rainfall percentiles for 20–26 December (shown in Figure 5) demonstrate that the extreme rainfall event in Queensland was well detected using the two satellite precipitation products.
An episode of heavy precipitation over the Australian state of Victoria in January 2011 was the second event examined. High intensity rainfall between 12–14 January caused major flooding across much of the western and central parts of Victoria. The 2011 flood event was described as one of the biggest floods in the state's history, it affected over 50 communities. Over 1 730 properties were flooded and over 17 000 homes lost electricity supply. Total damages amounted to A$ 2 billion.
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Figure 5. JAXA GSMaP (left) and CPC / NOAA CMORPH (right) weekly rainfall percentiles for 20-26 December 2010. |
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Figure 6. JAXA GSMaP (left) and CPC/NOAA CMORPH (right) weekly rainfall percentiles for 10-16 January 2011. |
JAXA GSMaP and CPC/NOAA CMORPH weekly rainfall percentiles for 10–16 January 2011 (Figure 6) show areas of rainfall above the 99th percentile, clearly indicating that parts of Victoria were affected by extreme rainfall. In all of these cases, the GSMaP and CMORPH showed very similar results in indicating the overall spatial patterns and magnitude of the extreme events, although small differences exist due to differences in both the products and the periods defining the base extreme climatology.
These examples demonstrate that space-based observations provide valuable information for monitoring heavy rainfall.
Conclusions
It is of vital importance to sustain in situ rain gauge networks, however, the first results of the Demonstration Project in Asia-Pacific demonstrate that space-based estimates of extreme precipitation are an effective solution to enhance capacity of RCCs and NMHSs for monitoring drought and heavy rainfall. Such capacity would enable service providers to assist Governments and local communities with informed decision making in adaptation to climate variability and change. Recognizing the achievements of Demonstration Project in assisting RCCs and NMHSs in East Asia and the Pacific, the Eighteenth World Meteorological Congress (Cg-18) adopted the Space-based Weather and Climate Extremes Monitoring (SWCEM) Implementation Plan (see [6] for detail). It further endorsed its implementation from 1 January 2020 to mark the transition of the project into an operational phase. The Cg-18 also requested WMO technical commissions and relevant Regional Associations to consider the possibility of implementing similar demonstration projects in Africa and South America.
Authors
Yuriy Kuleshov, The Australian Bureau of Meteorology, Australia
Takuji Kubota, The Earth Observation Research Center, Japan Aerospace Exploration Agency (JAXA), Japan
Tomoko Tashima, The Earth Observation Research Center, Japan Aerospace Exploration Agency (JAXA), Japan
Pingping Xie, The Climate Prediction Center, National Oceanic and Atmospheric Administration (NOAA), USA
Toshiyuki Kurino, WMO Secretariat
Peer Hechler, WMO Secretariat
Lisa V. Alexander, UNSW Sydney, Australia
References
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