In the hydrological cycle, rainfall is one of the essential parameters for human as well as for plants and in the exchange process. The world’s climate rainfall is an important parameter which changing and its effect being felt the world over due to global warming. According to climate models, rainfall/precipitation patterns will shift in two major ways. The first shift is in strengthening the current patterns, which means the wet regions would get wetter and dry regions would get drier. This is because warmer air as a result global warming traps more water vapour, and scientists predict that this additional water will fall in the already wet parts of the earth. The second shift is due to changes in atmospheric circulation, the narrow zones in seas and oceans along which storms travel, driven by prevailing winds because of storm tracks would move away from the equator and towards the poles.

Precipitation occurring due to typhoons is high in a shorter time, but the diurnal cycle is not fully understood. The typhoon induced precipitation is of interest for its destruction [1] and coastal regions are experiencing heavy rainfall which is similar to seasonal precipitation [2] [3]. The internal precipitation process is directly related to latent heat release during typhoon also depends on the intensity, duration and size of the typhoon [4] [5] [6]. Occurrence of precipitation on a particular day is connected with regional or large-scale thermal and dynamical conditions [7] and it has been studied using observations and model studies. Diurnal variations of precipitation have large differences between open oceans and the continents [8].

Convectional rainfall occurs when the insolation heats the earth’s surface and causes water to evaporate and as it rises, it cools. The air reaches a point the water vapor condenses and turns back to a liquid form leads to the development of clouds. As the clouds continue to grow the weight of the water droplets can eventually lead to precipitation, since heating of the lower atmosphere produces available potential energy that can be released by convective overturning [7] [9] to produce convective precipitation. Over the oceanic areas deep convection reaches maximum in the early morning [10], however the cause for deep convection still debatable.

Presently available theories are: the dynamical processes of the differential radiative heating between the convective and the surrounding cloud-free region [11]; the upper level absorption of shortwave radiation leading to convective anvils during daytime and longwave cooling at cloud top at night, which influence relative humidity and stability [12]; due to nighttime longwave cooling, increase in relative humidity and reduced entrainment [8]; and the daily variations in the surface layer over the ocean is more complex mechanism [13] [14]. The inner and outer rainbands in the spiral rainbands divided according to their locations and characteristics [15]. Commonly known as thunderstorms, convective storms are the atmospheric phenomenon responsible for weather hazards such as lightning, heavy rain, hail, and tornadoes. The tendency of convective storms to generate these hazards depends in part on whether the convection organizes into mesoscale convective systems such as squall lines, or into the long-lived convective cells known as supercells; these storm types have unique dynamical structure that is largely controlled by the three-dimensional humidity, temperature, and wind in the environment of the developing convection.

Orographic rainfall is rain that is produced from the lifting of moist air over a mountain. The moist air rises and cools, producing orographic clouds, which are the source of the rain. Most orographic rain falls upwind of the mountain range, with some also falling a short distance downwind. Orographic precipitation, also known as relief precipitation, typically affects mountainous areas near coastlines. This process can produce any type of precipitation, including snow, sleet, hail or freezing drizzle.

Zhoushan is located in the eastern China Sea, a subtropical marine climate, geographical regionalization. Zhoushan is warm and humid climate situated in the geographical co-ordinates between 29˚N - 32˚N; 121˚E - 123˚E consisting of 1390 islands and rainfall is the main source of water resource. Zhoushan consisting of scattered islands, fresh water area is limited and rainfall is important and main source for living and agriculture. There are many factors that influence precipitation rate. Zhoushan experiences seasonal and annual variations. As Zhoushan is situated in the sea, the source of rainfall is having different types, such as convectional rain, topographic effect and due to typhoons occurring over northwestern Pacific Ocean. This work has been studied to understand the seasonal, annual rainfall pattern and the effect of typhoons on heavy rainfall occurred over Zhoushan.

Monthly rainfall data has been obtained from Global Precipitation Climatology Project (GPCP) merged rainfall data to check the monthly, seasonal, annual and decadal variations set over Zhoushan during the period 1979-2018. GPCP stands for Global Precipitation Climatology Project, was established by the World Climate Research Program (WCRP) to develop a more complete understanding of the spatial and temporal patterns of global precipitation. The general approach is to combine the precipitation information available from each of several sources into a final merged product, taking advantage of the strengths of each data type. GPCP provides (1˚ spatial resolution) global precipitation products with satellite and gauge information [16]. We have chosen some typhoons to find out the rainfall pattern over Zhoushan during the typhoon time. Tropical Rainfall Measuring Mission (TRMM) data has been used to find out the daily rainfall occurred during the typhoon. TRMM-Multi-satellite Precipitation Analysis (TMPA) 3B42 precipitation product version 7 [17] covering the globe from 50˚S to 50˚N and has spatial resolution of 0.25˚ grid. TRMM 3B42 rainfall data has been frequently used for typhoon rainfall analysis regionally and globally [3] [18] [19]. We have chosen two different data sets for monthly and daily because, TRMM/ TMI data is better than GPCP for the daily data for the precipitation measurement of typhoon. For this study we used the typhoon track information from JTWC [20].

Calculation of Standardized anomalies:

We calculated the standardized anomalies for better understanding of seasonal, annual rainfall and presented. As it is well known that during the typhoon time heavy rainfall occurs, which almost seasonal rainfall is. The of standardized anomalies calculated as follows

Mean x ¯ = 1 n ∑ i = 1 n x i

Standard deviation σ = 1 n − 1 ∑ i = 1 n ( x i − x ¯ ) 2

Standardized anomaly = x − x ¯ σ

where x is the actual value, n is the number of points.

The formation mechanisms of the TC inner core and the outer rainbands are different. Within the inner core of the storm, the precipitation bands usually have growing convective cells on the upwind end and stratiform precipitation on the downwind end, thus displaying a distinct precipitation life cycle. The rainbands located far from the storm center are buoyancy-driven convective elements that appear influenced, but not constrained, by the cyclonic vortex circulation [28]. Several observational studies [29] [30] [31] suggest an increasing trend in TC rainfall and intensity in various regions. In this paper random case studies have been investigate the diurnal variations of TC using TRMM precipitation.

Zhoushan is an island with different topography and convective rainfall patterns, however the main rainfall occurring over Zhoushan due to typhoons (landfall or passing through). Zhoushan is having rainfall over the summer mainly due to the typhoons. In recent years, there is an increasing trend in typhoons passing through Zhoushan with higher intensity. There is a decreasing trend in spring season due to the weather pattern extending. Annual rainfall pattern showing increasing trend, even though there are El Nino/La Nina, the rainfall over Zhoushan is not following. Decadal rainfall clearly indicating higher rainfall the recent decade (2009-2018) and there is increasing trend. Increase in rainfall in the recent decade is due to the higher intensity typhoons are passing through. The case studies are indicating that Zhoushan experienced rainfall due to typhoon passing through; however rainfall depends on the intensity of the typhoon.

The authors express their gratitude to GPCP for monthly rainfall data and the NASA/Goddard Space Flight Center for providing TRMM daily rainfall data. The authors thank the editor and the anonymous reviewers for their comments that improved the original manuscript.

The authors declare no conflicts of interest regarding the publication of this paper.

Wang, L.Z., He, K.K., Zhu, W.Y. and Subrahmanyam, M.V. (2019) Rainfall Variations over Zhoushan during Last Four Decades. Open Access Library Journal, 6: e5819. https://doi.org/10.4236/oalib.1105819