Northwestern Pacific is the only ocean which has the most typhoon formation. The study of typhoon has far reaching significance today. Typhoon can relieve drought and make temperature drop substantially. Even we were suffering continuous high temperature in summer, the temperature would decrease immediately accompanied with typhoon. By using MODIS and weather station data to calculate the vegetation index, we analyze the drought characteristics of Shanghai during Saomai period, so that we can show the changes from the two aspects of the vegetation growth and the surface temperature. On the other hand, through the relative humidity data of Typhoon Saomai, we can find that the vegetation index and the relative humidity have been increased significantly. Typhoon rain also has its beneficial agricultural production side. It can lift the drought or ease the drought. It also provides abundant water resources for the growth of crops. In addition, the typhoon for the adjustment of the Earth’s heat, is contributed to maintain heat balance. Therefore, the typhoon to bring the changes in hydrothermal environment on the objective assessment of its impact and timely use of typhoon resources are of great significance.
Drought is the common disaster in China, which has a severe impact on human production and life, so typhoon becomes an urgent need for the influence of drought and high temperature, especially in the condition of the effect from the appearance of over 40 times’ high temperature before the mid-August, 2013 in Shanghai. This thesis will assess how typhoon impact the water and heat environment in Shanghai from its relieving function based on the related data like daily rainfall amounts, relative moisture and MODIS [
With the support of Shanghai satellite and remote sensing application center, we evaluated the remote sensing of how typhoon relieve Shanghai hydrothermal environment and deducting the influence of high temperature, including comparing the measured rainfall data from meteorological stations three months, two months, one month and half a month before typhoon with data in the same period in history to calculate precipitation anomaly for analyzing water and heat environment in Shanghai before the impact of, then using remote sensing image to calculate normalized difference vegetation index, combining the calculation of normalized difference vegetation index to analyze how typhoon alleviate hydrothermal environment in Shanghai, finally, utilizing the change of the relative humidity to reflect the hydrothermal environment before and after the typhoon [
Hydrothermal environment is a natural phenomenon of lacking water in a certain area and a certain period of time. Its formation and impact are not only related to natural environmental factors, but also with human social factors (this article excluding human factors). Hydrothermal environment disaster refers to the phenomenon that the specific annual, seasonal, or monthly precipitation is significantly less, which results in greater harm to production and human life in the region [
The percentage of precipitation anomaly is the most commonly used physical quantity [
In this formula: P refers to a station for a period of precipitation (mm);
This study calculated the percentage of precipitation of 11 meteorological stations in Shanghai during the first half month, one month, two months and three months respectively in 2005, compared with the average annual precipitation in 1976-2005. The hydrothermal environment fluctuation at each station can be seen from the picture of precipitation anomaly. The 11 stations distributed evenly in Shanghai, so it can be more comprehensively reflect how typhoon impact the hydrothermal environment in various regions of Shanghai. In addition, although the terrain is low in Shanghai, but due to the Shanghai area is often in the weather system and the edge of the typhoon affected areas with relatively large regional climate discrepancy, precipitation distribution from spatial and temporal side is not average.
What can be seen from
Precipitation anomaly percentage is negative at most meteorological stations
in Shanghai one months before the typhoon, negative precipitation in northern Shanghai Anomaly value is greater than the negative half a month before the typhoon rainfall anomaly, positive anomaly value is less than half a month before the typhoon rainfall anomaly, while the negative anomaly value in southern meteorological station was increasing and positive anomaly was reducing. Making a contrast to the northern meteorological station, the precipitation anomaly percentage in Jiading varied 1% half a month before the typhoon to −22% a month before the typhoon; Nanhui’s changed from −16% to −38%; Pudong’s varied from 9% to −9%; Jinshan’s transferred from 121% to 24%; Xujiahui’s altered from 13% to −3%; Chongming’s increased from −12% to 4%; Minhang’s rose from −28% to −25%; Songjiang’s went up from −14% to −9%; Fengxian’s changed from −61% to −41%; the precipitation anomaly in Baoshan was from −58% to −28%; Qingpu’s transformed from −14% to −9%; The most increased area followed Jinshan, Xujiahui in the south, then Chongming in the north, ascended by 97%, 16% and 16% separately, indicating that the three sites had less precipitation during this period of time, while the most decreased followed by Baoshan, Jiading in the north and Nanhui, Pudong in the south, respectively declined by 30%, 23% and 22%, 18%, indicating that the three sites had more precipitation in this period.
It shows a negative precipitation anomaly in the northern station two and three months before typhoon. It can be seen that the average rainfall in the northern region of Shanghai half of one month, one, two and three months before typhoon are less than the same period of 30 years from the analysis of precipitation anomaly percentage before typhoon because the water and heat environment is a long-term accumulation of effect, while these areas had been in a hydrothermal environment condition for a long period; however, the southern part of Shanghai appears an opposite state, but compared to the history of the two and three months, the anomaly is still less than normal. All this illustrated that the degree of hydrothermal environment in the vast majority of parts of Shanghai was significantly worsen from May to July in 2005 than in previous years.
Although 11 rainfall meteorological stations data is single point data, owing to the small area of Shanghai, it can reflect the rainfall condition, so we can take advantage of the rainfall data on August 5 - 8 to know the rainfall data of Typhoon Saomai, as shown in
In the process of vegetation photosynthesis, the absorption of blue light and red light band is strong, and the energy of the vegetation reflection among the visible light is very low. However, in the near infrared band, the absorption rate of
vegetation is very low, most of which are scattered by vegetation. Because of the difference of structure and optical properties, the scattering degree is different [
The change of average NDVL from 11 meteorological stations in Shanghai before and after Typhoon Saomai can be seen in
Because the different vegetation has different reactions to the hydrothermal environment and precipitation, according to the statistics of MODIS land cover
product classification, the vegetation area of Shanghai is divided into green land (forest), grassland and cultivated land of three classes (
Relative humidity can represent the physical quantity of dry and wet degree,
Name of station | NDVI before typhoon | NDVI after typhoon | Discrepancy |
---|---|---|---|
Minhang | 0.24 | 0.35 | 0.11 |
Baoshan | 0.12 | 0.24 | 0.12 |
Jiading | 0.14 | 0.38 | 0.24 |
Chongming | 0.15 | 0.53 | 0.38 |
Xujiahui | 0.12 | 0.26 | 0.14 |
Nanhui | 0.41 | 0.72 | 0.31 |
Pudong | 0.18 | 0.34 | 0.16 |
Jinshan | 0.36 | 0.82 | 0.46 |
Qingpu | 0.27 | 0.67 | 0.40 |
Songjiang | 0.35 | 0.54 | 0.19 |
Fengxian | 0.38 | 0.84 | 0.46 |
which is an important part of atmospheric circulation. It is a significant factor affecting growth that can affect the productivity of vegetation, meanwhile, it is also an important indicator to monitor the hydrothermal environment.
Because the Shanghai area is small, with relatively low accuracy of inversion data (resolution about 40 km), soil moisture cannot inverse, especially the heavy cloud-layer weather, the relative humidity of satellite remote sensing data measured are unable to solve both from scale and resolution and capacity for Shanghai, and Shanghai is only one site soil moisture observation that cannot fully represent Shanghai entirely, we carefully considered the relative humidity of the conventional determination as the characterization due to impact on the water and heat environment and high temperature that Saomai brought, and 11 sites were evenly distributed which also more comprehensive on behalf of the relative humidity change entirely Shanghai. So this study takes the average relative humidity data in August 1st, August 2nd, August 3rd and August 4th from 11 stations as relative humidity before typhoon in Shanghai and data in August 5th, August 6th, August 7th and August 8th as the relative humidity of Shanghai after typhoon, the changes of relative humidity before and after the typhoon in Shanghai as shown in
From
while Chongming county increased the least, an increase of 8%.
The relative humidity in Shanghai has the same change tendency in general. After Typhoon Saomai, the average and relative humidity of 11 stations with observation data increased. The rainfall it brought also increased the average relative humidity of different land cover types (farmland, woodland, grassland) to various degree. The green land (Woodland) is more obvious, which is consistent with the change of vegetation water supply index.
Utilizing 4 days’ compound relative humidity data mentioned above from 11 stations before and after the typhoon to research the local relative humidity, and extract, analyze the influence of relative humidity on 11 meteorological stations (
Name of station | August 4th | August 5th - 8th | Discrepancy |
---|---|---|---|
Minhang | 75 | 90 | 15 |
Baoshan | 75 | 87 | 12 |
Jiading | 78 | 90 | 12 |
Chongming | 78 | 86 | 8 |
Xujiahui | 74 | 91 | 17 |
Nanhui | 81 | 87 | 6 |
Pudong | 75 | 85 | 10 |
Jinshan | 78 | 86 | 8 |
Qingpu | 73 | 84 | 11 |
Songjiang | 72 | 84 | 12 |
Fengxian | 81 | 91 | 10 |
Because the Typhoon Saomai landed in Zhejiang province on 1 a.m, 6 August, but on August 5th, Shanghai is located in the range of the seven level within the circle, heavy rain occurred in Shanghai influenced by the Typhoon Saomai, and in August 4th, most measured rainfall is 0, so we take August 5th as the first day being affected by typhoon Comparison of relative humidity of all stations in Shanghai during the previous day (August 4th) and relative humidity during typhoon rainfall (August 5 - 8) can be seen in
Typhoon “Sang Mei” brings precipitation to increase relative humidity and alleviate hydrothermal environment. In order to understand the relationship between precipitation and relative humidity, this study used (August 1, 2005 August 8th) daily measured rainfall and relative humidity of the meteorological station before and after the typhoon to analyze the relationship between the typhoon rainfall and relative humidity. Because the relative humidity in the same day has the fluctuation, this research will analyze each meteorological station date with the same day rainfall amount (
Xujiahui has the most rainfall among 11 meteorological stations in Shanghai up to 306.5 mm from August 5th to 8th, which the rainfall in August 7th was
180.8 mm, its relative humidity reached 69% from August 3rd to 90% on August 7th. After typhoon, meteorological stations in relative humidity was highest in Fengxian and Xujiahui, had reached the highest humidity 91% on August 6th, Fengxian meteorological station accumulated 235.7 mm rainfall in August 5 - 8 days while relative humidity was 74% on August 3rd, increased by 17%. Although Jinshan, Nanhui, Chongming also received an effect of typhoon precipitation, but the relative humidity increases less obviously than Jiading, Xujiahui, Min Hang, mainly because the region is located in the coastal or Yangtze River surrounded by the effects of precipitation and relative humidity can easily influenced by the River and sea. 11 meteorological stations in
Because the change of relative humidity of each station is single point data, the change of relative humidity of Shanghai is reflected by all meteorological stations. In order to understand the trend of relative humidity influenced before and after typhoon, we analyze the impact of Typhoon on the relative humidity of Shanghai. Relative humidity is the average relative humidity of 11 meteorological stations in the city, and rainfall is the average daily rainfall of the 11 meteorological stations. What can be seen from
Because the rainfall at separate stations three months, two months and a month ago all cannot reach the same period in 1976-2005 years, so the relative humidity level is similar to the same period in the northern region. When typhoon arrived, the average relative humidity in the north and south rose sharply. A good amount of rainfall that typhoon brought, to a certain extent, alleviate the water environment in Shanghai.
In summary, after the Typhoon Saomai, it brought city plenty of rainfall, and the average humidity increased after typhoon, the climate is relatively humid, more lush vegetation growth in Shanghai, which increased ground infiltration, enhanced transpiration, reduced canopy temperature at the same time, so vegetation index also increased. Relative humidity and water supply index increased, too. Meanwhile the hydrothermal environment before typhoon was alleviated.
1) Using the measured rainfall data in Shanghai to calculate the hydrothermal environment of Shanghai before and after the typhoon Saomai, the results show that the hydrothermal environment of Shanghai has been significantly alleviated by typhoon;
2) Making a contrast before and after typhoon from the vegetation index and water supply index, the vegetation growth index of Shanghai area has been greatly improved by typhoon, and the vegetation hydrothermal environment has been greatly alleviated;
3) Utilizing the change of relative humidity at the 11 sites in Shanghai before and after Typhoon Saomai to analyze how typhoon relieve hydrothermal environment. What’s more, affected by the typhoon, relative humidity in Shanghai increase 20 values, to a certain degree, it also reflects that the typhoon eased the hydrothermal environment in Shanghai.
Guo, R. and Weng, Y.Y. (2017) Analysis of the Positive Effect from the Typhoon Saomai to the Hydrothermal Environment of Shanghai. Journal of Geoscience and Environment Protection, 5, 221-234. https://doi.org/10.4236/gep.2017.58018