Seasonal Cycles of Precipitation and Precipitable Water

1. Seasonal Cycles of Precipitation and Precipitable Water and Their Use in Monsoon Onset and Retreat Er Lu and Xubin Zeng Dept of Atmospheric Sciences, University of Arizona

2. Purpose of Study • Understand the seasonal cycles of precipitation (P) and precipitable water (W) and their relations. • Propose a globally unified monsoon index based on W, and use the P~W relations to clarify the issues of the index.

3. Background • P & W: 2-D, observational, hydrological quantities • P ~ dW/dt (water balance equation) • P ~ W ?

4. Synoptic P & W correlation Synoptic P & W are positively correlated during the year or any season. P & W analysis (1988-97, daily) • P: NCEP/CPC • W: NASA/DAAC

5. Seasonal Cycles of P & W Standard deviation of daily P Out-of-phase W P In-phase Not correlated SE (Southeastern U.S.) SM (South Mexico) WC (Western Coast of the U.S.)

6. Part 1: Understand the Same Seasonal Pattern of W Spatial coherence of seasonal W W So, W continentally increases from winter to summer.

8. Suppose RH takes values in [0,1] with an even possibility, then the areas of the domains above and below the contour of delta(Ts) represent, respectively, the possibilities of having an increase and decrease of W. When delta(Ts) is large, as in seasonal changes of mid-high latitudes, the possibility of having an increase of W is high, and that of having a decrease of W is low. So, W tends to increase from winter to summer. Contours are delta(Ts) obtained from delta(W)=0

9. Also, when delta(Ts) is large, in order to have a decrease of W, the summer RHs2 needs to become extremely small, which can only be satisfied for an isolated atmospheric column. For the real ocean-land-atmosphere system, the summer high surface temperature can lead to enough increase of W through evaporation and thermally–induced circulation that converges water vapor.

10. Water balance equation Assume in rainy time C+E is offset by P, so only dry time is included in the integration. Divide the water vapor convergence into a dynamic part and a thermally-induced part, which is related to the change rate of surface temperature: For synoptic change or seasonal change over tropics, dynamic part is the dominant, so W can increase or decrease. For seasonal change of mid-high latitudes, in which delta(Ts) is large, W always increases from winter to summer.

11. Small delta(Ts): seasonal change over tropics or synoptic change Large delta(Ts): seasonal change of mid-high latitudes W and Ts have weak correlation W and Ts have strong correlation Dynamic Thermodynamic

12. Stephens (1990, J. Climate): Seasonal W over oceans was prescribed from SST. Correlation between seasonal W and SST is strong in some regions, but weak in others. Our inference: Prescription of seasonal W from surface temperature can be made over both oceans and lands, but only for mid-high latitudes, not tropics.

13. Part 2: Understand Different P Seasonality Regimes P seasonality regimes (Finkelstein & Truppi 1991) P

14. Favorable circulation condition in WC (wettest rainy seasons – driest rainy seasons) P, E (evaporation), and C (vapor convergence) in rainy and dry seasons Evaporation is not important to winter P in WC, but may be to others. North American Regional Reanalysis (NARR) The formation of precipitation is affected by complex dynamic and thermodynamic conditions (e.g., evaporation and circulations), and, in different seasons and locations, these conditions can vary greatly. We attempt to understand the different precipitation seasonality regimes from the basic atmospheric fields.

15. Seasonal tendency of P can be reflected from RH

16. RH varies with water vapor and temperature, but how to compare the changes of water vapor and temperature?

17. Change of temperature Change of water vapor Change of saturation extent (precipitation) If , then P does not change much.

18. WC: The change of temperature from winter to summer is much greater than the change of water vapor. So, relative to summer, the coldness of the winter air is much more significant than the dryness, which makes the winter have a large saturation extent and thus precipitation. SM: The much more significant moistness of the summer air than its warmness is important to the summer monsoon precipitation. SE: The changes of water vapor and temperature are roughly equivalent, so precipitation occurs throughout the year. Lu & Zeng 2005, GRL, in press

19. Different Relations between Seasonal P and W Seasonal P & W correlation SM In-phase WC Out-of-phase SE Not correlated

20. Part 3: A Globally Unified Monsoon Index by Using W Principle • The annual cycle of W is used to represent the annual monsoon process including the summer and winter monsoons and the two transitional periods between them. • The monsoon onset and retreat are regarded as phase-locked phenomena, with each corresponding to a relatively fixed stage of the annual cycle. Zeng & Lu 2004, J. Climate

21. Onset Date

22. Retreat Date

23. Is it reasonable to use W to determine monsoon onset/retreat (since conventionally P is used)? • Since the in-phase relation between seasonal P and W, we can use W to determine the climate monsoon onset/retreat dates. • The significant positive correlation between synoptic P and W during monsoon onset/retreat seasons implies that a larger (smaller) daily W statistically corresponds to a larger (smaller) daily P. So, an earlier (later) onset/retreat determined from P can also be determined from W. Therefore, the interannual variations of monsoon onset/retreat can also be determined from W. • Why W cannot correctly determine the global monsoon regions? W always increases from winter to summer in mid-high latitudes. The large increase of W can appear in both the monsoon and the nonmonsoon regions. So, the global monsoon regions cannot be obtained from W.

24. Summary • The W, as long as not in tropics (no matter in monsoon or nonmonsoon regions), always increases from winter to summer due to the large annual range of temperature. • In regions not in tropics, though water vapor always increases from winter to summer, P may have different seasonality regimes, depending on the comparison of the change of water vapor with that of temperature. In monsoon regions, the change of water vapor is much greater than the change of temperature, so P also increases from winter to summer. In nonmonsoon regions, the change of temperature can be equivalent to or much greater than the change of water vapor, so P can occur throughout the year or mainly in winter. • The positive correlations between P and W at seasonal and synoptic scales in monsoon regions suggest that it is suitable to use W to indicate both the means and the interannual variations of the monsoon onset and retreat. However, the global monsoon regions cannot be determined from the seasonal change of W, since W also increases from winter to summer in nonmonsoon regions.