On the factors that may explain changes in the rainfall regime
The volume of rainwater in an average year in Israel is about seven billion cubic meters. Of this amount, less than two billion are actually collected in the water reservoirs and can be used. The rest, about 70% of the rainwater, returns to the atmosphere by direct evaporation from the ground or through evaporation from vegetation, about 5% flows in streams and about 25% seeps into the groundwater.
Is it possible to trust the predictions of the meteorological models, according to which the amount of rain in Israel will decrease by 30%-40% in the next 100 years? This is what Dr. Baruch Ziv, a meteorologist, forecaster, climate researcher and lecturer in the Department of Geophysics at the Open University, and Prof. Hadas Saaroni, a climate researcher who serves as a full professor in the Department of Geography and the Human Environment at Tel Aviv University, wanted to check. Dr. Ziv and Prof. Saaroni research weather systems in the Mediterranean region, especially in the east (such as the Persian Gulf in the summer and the Cypriot Depression in the rainy season), which affect Israel, and the changes that occur in them due to global warming.
What is the question? How can the forecast on the amount of rain in the future be improved?
"Climate forecasting is a complicated and expensive process and not always reliable," says Dr. Ziv. "Since the 80s, forecasters have been able to accurately predict about five days ahead with the help of computer models of the atmosphere, on the basis of which a future simulation is made. In this way, a series of successive maps of the coming days is obtained, and if a mistake is made, it is dragged forward and disrupts the forecasts for the future. In predicting the future rainfall, a detailed daily forecast is not required, but rather statistical information about the weather systems and the phenomena caused by them. For example, if the number of barometric depressions, which cause rain, their depth and characteristics will change; And above all, what affects these depressions, and hence, what will be the weather and the rainfall regime that will prevail here in the coming years. A prominent finding in the forecasts of these meteorological models (which were released until the year 2100), is a gradual and consistent decrease in the number of depressions in our region, which should lead to a significant reduction in the amount of rain and changes in the rain regime (in the number of rainy days, the duration of the rainy season, etc.). But the predictions of these models are not reliable, as they have difficulty in accurately reconstructing the present-day depressions of the Mediterranean Sea. In addition, the amounts of rain they predict are based on statistical schemes applied to the maps and not on a direct calculation." The Cypriot Depression - the main cause of Israel's rains. Image source: MODIS website
In the past, Dr. Ziv and Prof. Saaroni participated in a project in which the Cypriot sinkholes were classified according to their location and strength. This classification is still used today as a basis for climate studies - such as rain forecasting. The Cypriot depression is a barometric depression whose focus is in the northeastern corner of the Mediterranean Sea, between the island of Cyprus and the coasts of northern Syria and the slopes of the Turkish mountains, and is the main cause of Israel's rains (about 90%). Dr. Ziv: "We clearly saw that given the location and strength of a Cypriot trough, one should expect, on average, rain in the north, on the coast or in the south, depending on the characteristics of the trough. However, when we drew days belonging to the same depression type we got different maps of daily rainfall. An attempt to predict the daily amount of rain according to 'analogs' - copying the amount received in the past on similar days in terms of the features of the outlet - yielded inaccurate forecasts."
In their current study, which won a grant from the National Science Foundation, the researchers sought to upgrade the process of predicting rain against the background of pessimistic forecasts, and implemented an automatic algorithm in the rain maps that follows what happened to the Cypriot trough in the atmosphere (not only at sea level, where the troughs are identified). "The atmosphere also has depth, it is not two-dimensional. Therefore, the classification of depressions until now, only according to their location and their pressure at sea level, was not reliable enough and thus the future rain trends were not observed correctly and maybe that is why the forecasts are so severe. The warming can cause changes in the rain regime, but this does not necessarily lead to drying out of the area. That's why forecasts that do not include the connection to the higher strata and distant areas, fail to yield reliable results at the national level," says Dr. Ziv.
Using the upgraded rain maps, the researchers examined the changes in the Cypriot depression over 46 years, and the way in which these changes affected the amount of rainfall in the country. Dr. Ziv adds: "We saw that there was no change in the amount of precipitation in Israel throughout these years; In the coastal plain it even rose slightly and in contrast it decreased in the north and in the mountains. That is, there was a change in the distribution of the rain, but not in its quantity." In addition, the researchers calculated the average rainfall intensity per day and found that, parallel to the reduction in the number of Cypriot depression days and rainy days, the rainfall intensity increased. The increase they found in the average daily amount of rain in recent years may explain why the annual amount of rain has not changed in the last decades.
Using the upgraded rain maps, the researchers examined the changes in the Cypriot depression over 46 years, and the way in which these changes affected the amount of rainfall in the country.
However, "the future rainfall regime in Israel is not yet fully clear and the Cypriot depressions must be examined in depth, and especially the mechanisms that create them and determine their movement paths. Changes in the trajectory of the barometric depressions that dominate the Atlantic Ocean and Northern Europe, may change the distribution of depressions in the Mediterranean Sea and in the Cyprus region. This connection is not simple and it is even being investigated in this project", concludes Dr. Ziv.
Life itself:
Dr. Baruch Ziv, 75, born in Kfar Giladi, married + three children and six grandchildren, lives in Petah Tikva. He has been involved in meteorology since the age of nine ("thanks to a teacher who let me manage a meteorological station in Kfar Giladi and made me love the profession"). In his spare time he likes to listen to music, sing and play. Storm myrtle
Prof. Hadas Saaroni, In her 60s, lives in Tel Aviv, mother of Gal and Or (Gal, affected by cerebral palsy, lives in the anthroposophical village Beit Uri in Afula, which has been upholding the principles of sustainability for more than 50 years). A fan of weather since childhood (while delving into understanding its mechanisms, its influence and importance to a person), nature and trips, volunteers at "Parents for Parents: Akim's support and assistance center", and an activist for society and the environment.
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