1. The influence of atmospheric circulation on the occurrence of hail in China
Li Mingxin
Department of Atmospheric and Oceanic Sciences, Peking University, Beijing

2. Background
Trends of hail occurrence
Kunz et al. 2009
In light of climate change , it is still a matter of debate whether the number and/or severity of hailstorms has increased over the last decades and is expected to increase in the future.
Cao 2008
Xie & Zhang
et. al 2008
Changnon & Changnon（2000)
Brooks and Dotzek 2008
It has been found that hail occurrence show different trend in the different area of the world. IN North America, an increasing trend was found in Otarria of Cananda, and decreasing trend in United states. I Ueope, Kunz 2009 showed decreasing hail occurrence trend in south of German. In Asia, my student and I found the trend of hail is dereaseing in north China but flat in South China. In the mean while, It has been found that the number of severe storm or thunder storm did not show significant trend in Unite and German, respectivally. Recently we do found the hail frequency are dereasing in Tibetan Plataeu. How to understand the potential effects of global-scale changes on local scale weather is a persistant challenge for reasarch.
Understanding the potential effects of global-scale changes on local-scale hail weather is a persistent challenge for research.

3. Challenge: the stark mismatch of scale
1 insufficient direct monitoring systems
2 the spatial and temporal resolution of numerical models
One of the challenges inherent to understanding hail changes
in the context of global climate change is the stark mismatch
of scale. Climate models simulate large-scale patterns of change.
It requires novel research approaches to connect the questions and processes across both weather and climate scales
It requires novel research approaches to connect the questions and processes across both weather and climate scales

4. Objective
to investigate the effect of large scale circulation to the occurrence of hail and its long term trend in China

5. Methodology and Data
Objective classification(PCA: T-mode Principal component analysis)
Huth (2008) compared the classification methods and proposed that the performance of the T-mode PCA is best in terms of its reproduction of predefined types, its temporal and spatial stability, and its less dependence on pre-set parameters.
Datasets
NCEP/NCAR reanalysis
Hail data at 753 surface stations from information center of CMA

6. Seasonal variation of 5-day annual mean hail frequency in China 1960-2012
548 Station

7. Seasonal variation and area division
Clocks represent for seasonal variation
Small dots show the annual mean hail frequency
Four areas are marked with I II III and IV
Warm season : Apr—Oct (I III IV)
Cold season : Oct—Apr (II)

8. Variation of annual mean hail frequency of different regions 1960 -2012
(a) shows the MHD (annual station averaged hail days) in China and a sharp decrease is
significant from early 80th.
(b) All the four regions had experienced a decrease in the past several decades.
But the decrease in South is not that significant than that in other three areas.

9. Results of circulation classification in Northern China(1960-2012)
We classify all the days into five circulation types according to 500hPa geopotential height.
Above is the annual hail frequency under each type in north.

10. Results of circulation classification in Northern China(1960-2012)
Type type 3
GPH500hPa
SLP
There are powerful low pressure system in both type but with different location.

11. Results of circulation classification in Northern China(1960-2012)
Type type 3
Circulation type frequency
no obvious long-term trend in circulation frequency
Hail decrease greatly from early 90s.
Ratio
(Hail/circulation)

12. Results of circulation classification in Northern China(1960-2012)
Type type 3
SLP before 90s
Weather system became weaker after 90s
SLP after 90s
Type 2 and type 3 are more related with hailstorm. The Weakened weather systems from 90s mainly associated with the hail frequency decrease in the north. The increased CIN after 1990s may inhibit generation of hail.
Bigger CIN
PDF of CIN

13. Thank you