Kelly Howell analyzed water vapor and rainfall climatologies using blended total precipitable water and rainfall rate datasets covering 2006-2008. Annual mean total precipitable water was 24.94 mm, with maxima in the warm pool and intertropical convergence zone. Oceans received more frequent and heavier rainfall than land, with peaks in the warm pool and South Pacific convergence zone. Higher rainfall amounts and frequencies occurred at higher total precipitable water values globally. Regional distributions showed variability depending on location.
10. Annual Mean TPW (Trenberth 1998) Mean global TPW is 24.52 mm (Trenberth et al. 2003). TPW maximizes north of the equator around 45 mm.
11. Annual Mean TPW The global mean TPW is 24.94 mm, with a maximum of approximately 45 mm north of the equator. West Pacific Warm Pool SPCZ ITCZ Oceanic Deserts
12. Seasonal Mean TPW (Ferraro et al. 1996) The seasonal changes in TPW are evident, particularly in the west Pacific warm pool during the JJA monsoon period and in the eastward advancement of the SPCZ during DJF. In addition, the northernmost latitudes display their highest TPW values during JJA and the southernmost latitudes display their highest TPW values during DJF, following the seasonal changes in solar insolation.
13. Seasonal Mean TPW The seasonal TPW distributions are similar to previous findings, with the SPCZ extending its farthest eastward during DJF and TPW highs around southeast Asia during the JJA monsoon period. The presence of a double ITCZ can be detected in the eastern tropical Pacific during MAM. However, TPW values in the ITCZ are lower than the findings by Ferraro et al. (1996).
15. Global Mean RR in mm day -1 (Legates and Willmott 1990) Over land, rainfall maximizes over the Amazon Basin and the African rainforests. Over the ocean, the most rainfall occurs in the ITCZ, the west Pacific warm pool, and the SPCZ. This plot was constructed based on rain gauge measurements. On average, ocean surfaces receive the most rainfall while land surfaces receive the least. Source: Xie and Arkin (1997). Source Surface Type RR [mm day -1 ] CMAP Ocean 3.02 Land 1.86 Globe 2.69 Jaeger (1976) Ocean 2.91 Land 2.01 Globe 2.66 LW (1990) Ocean 3.15 Land 1.97 Globe 2.82
16. Annual Mean RR in mm day -1 The oceans receive more rainfall than land surfaces. The ITCZ receives the most rainfall; the oceanic deserts receive the least. Surface Type RR [mm day -1 ] Ocean 2.68 Land 2.57 All Surfaces 2.63
17. Annual Mean RR in mm year -1 (Ferraro et al. 1996) Over land, rainfall maximizes just south of the equator. Over the oceans, there is a double peak in the tropics, maximizing north of the equator. The solid lines indicate Ferraro et al.’s (1996) findings; the dashed lines indicate findings from Legates and Willmott (1990).
18. Annual Mean RR in mm year -1 The zonal mean RR distributions over ocean and land display similar trends to the findings by Ferraro et al. (1996), although these estimations are slightly higher. Latitude (degrees N)
19. Annual Mean Rainfall Frequency The zonal distributions of rainfall frequency are analogous to the zonal distributions of rainfall. Overall, the oceans receive rain more frequently (11.04 %) than land surfaces (8.58 %). Latitude (degrees N) Rainfall frequency = 100× Latitude Zone Ocean Frequency Land Frequency 60-45 o N 10.99 % 4.68 % 45-30 o N 12.72 % 5.65 % 30-15 o N 7.56 % 5.15 % 15-0 o N 16.48 % 13.91 % 0-15 o S 11.02 % 16.41 % 15-30 o S 7.63 % 6.90 % 30-45 o S 11.32 % 5.91 % 45-60 o S 10.23 % 5.19 %
20. Seasonal Mean RR in mm month -1 (Ferraro et al. 1996) Seasonal mean rainfall tends to follow the patterns of seasonal mean TPW, with the SPCZ extending its farthest eastward in DJF and monsoonal rains occurring over southeast Asia during JJA. Also notable is the presence of a southern branch of the ITCZ during MAM.
21. Seasonal Mean RR in mm month -1 Seasonal mean rainfall estimates from CMORPH are similar to the findings by Ferraro et al. (1996). The NH and SH land areas show large changes between DJF and JJA: the winter hemisphere’s land areas receive very little rainfall.
22. Seasonal Mean Rainfall Frequency (Ferraro et al. 1996) Seasonal rainfall frequency patterns mimic rainfall patterns, with more frequent rain occurring over the areas that typically receive more rainfall.
23. Seasonal Mean Rainfall Frequency The global frequency distribution estimates are much higher than those found by Ferraro et al. (1996), although the same patterns are present. Using the 0.1 mm hr -1 threshold, global frequencies hover around 10%. Season 0.1 mm hr -1 0.5 mm hr -1 1.0 mm hr -1 DJF 10.31 % 4.88 % 2.76 % MAM 10.30 % 4.79 % 2.76 % JJA 10.17 % 5.00 % 2.92 % SON 10.09 % 4.95 % 2.88 %
25. TPW Threshold for Rainfall? Average rainfall rate versus column water vapor for the eastern Pacific at various tropospheric temperatures. 25% of the rainfall occurs for TPW values above a ‘critical value.’ Time series of (a) RR in mm hr -1 , (b) TPW in mm, and (c) global solar radiation at the Koto Tabang GPS station during JJA 2001. Rainfall does not tend to occur at times with relatively low TPW. Source: Wu et al. (2003) Source: Neelin et al. (2009)
26. Regional Studies The following plots were constructed using the data at each of these grid points analyzed over all 35 months. Location Longitude Latitude East of Florida 70 o W 30 o N Indian Ocean 75 o E 8 o S East of Japan 142 o E 35 o N South of Panama 83 o W 4 o N South Atlantic Ocean 22 o W 45 o S North Atlantic Ocean 30 o W 50 o N West Pacific 155 o E 6 o N Southeastern Pacific 120 o W 8 o S SPCZ 170 o E 10 o S
27. TPW Distributions Annual mean TPW is highly variable in the midlatitude location, relatively high in the tropical location, and relatively low in the oceanic desert location. TPW (mm) Location Mean TPW East of Florida 30.66 mm West Pacific 54.28 mm Southeastern Pacific 29.62 mm
28. RR Distributions Equation of exponential decay. A more negative slope indicates a faster rate of decay (i.e., there are relatively few heavy rain events). More negative slopes tend to be associated with lower rainfall frequencies. RR [mm hr -1 ] Location Slope of Fit Rainfall Frequency East of Florida -0.58 11.11 % West Pacific -0.516 31.20 % Southeastern Pacific -1.556 2.29 %
29. RR vs TPW The shapes of these distributions approximate the TPW distributions. In general, the higher RRs occur at the more frequently occurring TPW values. However, this is not the case in the southeastern Pacific, where the highest RRs occur at higher TPW values. TPW (mm)
30. Probability of Rainfall RR ≥ 0.1 mm hr -1 RR ≥ 3.0 mm hr -1 As rain intensity increases, rainfall becomes less likely at lower TPW values. TPW (mm)
32. RR Distribution by TPW Range At higher TPW values, rainfall is more probable and there is a higher proportion of heavier rainfall. RR [mm hr -1 ] TPW Range Probability of Rainfall 0-15 mm 2.34 % 15-30 mm 7.64 % 30-45 mm 16.59 % 45-60 mm 29.89 % 60-75 mm 45.89 %
33. RR Distribution by TPW Range The RR distributions are not strictly exponential, but an exponential fit is a consistent representation of the distribution. Steeper slopes are associated with drier environments. RR [mm hr -1 ] TPW Range Slope of RR Distribution 0-15 mm -1.01 15-30 mm -0.90 30-45 mm -0.70 45-60 mm -0.59 60-75 mm -0.49
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