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 Different precipitation seasonality regimes, although produced by different surface and atmospheric conditions, can be understood from the basic atmospheric fields. By comparing the change of water vapor from winter to summer with the change of temperature using the NARR reanalysis, three seasonality cases over the United States and Mexico are analyzed. In the western coast of the U.S., 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. In contrast, over South Mexico, the much more significant moistness of the summer air than its warmness is important to the summer monsoon precipitation. In the southeastern U.S. where precipitation occurs throughout the year, the changes of water vapor and temperature are roughly equivalent.
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 The precipitation seasonality and its spatial difference is a fundamental issue of the climate, and has been studied over a century through analyzing the precipitations from observational records and model outputs with various statistical techniques [e.g., Greely, 1893; Henry, 1897; Horn and Bryson, 1960; Hsu and Wallace, 1976; Walsh et al., 1982; Kirkyla and Hameed, 1989; Finkelstein and Truppi, 1991]. The formation of precipitation is affected by complex dynamic and thermodynamic conditions (e.g., the surface characteristics and the atmospheric circulations), and, in different seasons and locations, these conditions can vary greatly. In this study, we attempt to provide an understanding of the different precipitation seasonality regimes in a unified manner from the basic atmospheric fields, instead of analyzing their specific surface and circulation characteristics though they are important, since, at a seasonal scale, the different dynamic and thermodynamic factors influencing the precipitation can be reflected in the atmospheric states.
 Water vapor is required for precipitation; however, in some areas (e.g., the western coast of the U.S.), the largest precipitation of the year occurs when the water vapor is the least, and contrarily when water vapor is the largest, there is almost no precipitation. Our analysis indicates that the air temperature is the key to this case. Bretherton et al.  also pointed out that the monthly mean precipitation over the tropical oceans can be well determined from the water vapor and temperature fields. The comparative strengths of water vapor and temperature in winter and summer are analyzed in this study for the three major precipitation seasonality regimes over the continental United States and Mexico (US-Mexico).
 The daily precipitation analysis used in section 3 is created by the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric Administration and available at http://www.cpc.ncep.noaa.gov/products/precip/realtime/retro.html. The dataset starts from 1948 and covers the US-Mexico with 1° × 1° horizontal resolution. More description of the data is given by Higgins et al. . The daily precipitable water analysis used for making a comparison with precipitation is taken from the NASA Water Vapor Project [Randel et al., 1996], and is currently available for 12 years (1988–1999). The specific humidity, temperature, and precipitation used in section 4 are taken from the North American Regional Reanalysis (NARR), which was developed by the National Centers for Environmental Prediction and is available at http://wwwt.emc.ncep.noaa.gov/mmb/rreanl/index.html. The dataset includes 25 years (1979–2003) and has resolutions of 3 hours in time, 32 km in horizontal, and 29 layers in vertical. F. Mesinger et al. (North American Regional Reanalysis, submitted to Bulletin of the American Meteorological Society, 2005) present a detailed introduction and reliability evaluation of the data.
3. Precipitation Seasonality Cases
 The precipitation seasonality can be classified as numerous regimes depending on the given standards, and, even in the United States, can be divided into six regimes as suggested by Boyle . For simplicity, in this study, we will focus on the three major precipitation seasonality regimes, which, though have already been known in the studies by Hsu and Wallace , Finkelstein and Truppi , and many others, are illustrated here. Figure 1 is the standard deviation of the ten-year (1988–97) averaged annual time series of the daily precipitation analysis in the US-Mexico, which is useful in locating the strong precipitation seasonality areas. The maximal precipitation variations within a year are over the western coast of the U.S. (WC), South Mexico (SM), and the southeastern U.S. (SE). Figure 2 shows the ten-year (1988–97) averaged annual cycles of the precipitation and precipitable water analyses at the centers of these maximal variations: 123°W/46°N in WC, 105°W/21°N in SM, and 90°W/33°N in SE. For convenience, these three centers will be denoted hereafter by WC, SM, and SE, respectively. The precipitations in WC (Figure 2a) and SM (Figure 2b) both have strong seasonal variations, but the former decreases from winter to summer, while the latter increases from winter to summer. The precipitation in SE can occur throughout the year (Figure 2c), but is dominated by the day-to-day variation, and the seasonal change is rather weak. Since the monthly mean precipitations of February and August can reflect the major difference of the precipitations between winter and summer in all the three regimes as shown in Figure 2, they will be used in this study to represent the winter and summer precipitations. The precipitable water, in contrast, exhibits a same seasonal pattern in the three regimes (Figure 2). The relation between precipitation and precipitable water is examined in our follow-up study, and it is shown analytically that the precipitable water over the continent is dominated by the large seasonal change of the surface temperature, thus always increases from winter to summer.
4. Understanding From Water Vapor and Temperature Fields
 For a single precipitation process, the saturation of the water vapor in a layer or layers of the atmosphere is a critical condition for the formation of precipitation. At the seasonal or monthly timescale, should an overall higher saturation of the atmosphere correspond, to a great extent, to a larger precipitation? Generally, during the period studied (e.g., one month), if the total raining time is long (short), a large (small) precipitation and a large (small) mean saturation extent can both be expected, so the precipitation and the mean saturation extent of the period should be well positively correlated. Bretherton et al.  found that the monthly mean precipitation over the tropical oceans can be well represented by the relative humidity calculated from the monthly mean precipitable water and the water vapor capacity of the atmosphere. Figures 3a–3c show the profiles of the 25-year averaged monthly mean relative humidity in February and August of the three regimes. In WC, the saturation extent in the rainy February is much greater than in the dry August at most of the levels (Figure 3a), and in SM, the saturation extent in the rainy August is much greater than in the dry February at all levels (Figure 3b). In SE, corresponding to the year-round precipitation, the saturation extent overall does not change much, and maintains a moderate level (Figure 3c). So, for the three seasonality regimes, the seasonal changes of the saturation extent from winter to summer are well consistent with the changes of the precipitation. Figures 3d–3f further provide the profiles of the correlation between the monthly mean relative humidity at a height and the monthly mean precipitation in February and August for the 25 years. The correlations are positive at almost all levels of the two months in the three regimes, and greater than 0.4, the correlation at a 95% significance level, in most levels of all the rainy months (Figures 3d–3f). The correlation of the dry August in WC is also significant at the middle levels (Figure 3d). These results imply that the monthly mean saturation extent can not only well reflect the seasonal tendency (increase, decrease, or no much change) of the monthly mean precipitation, but also indicate to a great extent the interannual variation of the monthly mean precipitation. Therefore, we can use the saturation extent to represent the precipitation in the following analysis. The correlation in the dry February of SM is small (Figure 3e), and the reason is that the precipitation of the dry month only falls in a small portion of the time with a high relative humidity, but the monthly mean relative humidity can be influenced by the most part of the month that has no precipitation. To ensure the saturation extent can well reflect the changes of precipitation in those weak seasonality regimes, a pentad mean or running mean maybe more suitable.
 The relative humidity is a function of water vapor and temperature, so the seasonal change of the precipitation can be evaluated from water vapor and temperature fields. Figure 4 presents the profiles of the 25-year averaged monthly mean specific humidity and temperature in February and August for the three seasonality regimes. Note that the water vapor and temperature both increase from winter to summer at all levels and for all the three regimes. However, since they are different variables, it is difficult to identify, from Figure 4, which increases more significantly from winter to summer and contributes more to the seasonal change of precipitation. So, we need an approach to measure the seasonal changes of the two variables. Here, an analytical method is used.
 The change of the saturation extent, indicated by relative humidity (RH), from winter (win) to summer (sum) can be represented by Csat ≡ RHsum/RHwin. Using the definition of relative humidity RH = e/es (T), where e is vapor pressure and es (T) is the saturation vapor pressure at temperature T, it can be written as Csat = Cvap/Ctem, where
are defined as the change of water vapor and the change of temperature, respectively, from winter to summer. If Cvap = Ctem, that is, the change of water vapor from winter to summer is equivalent to the change of temperature, then Csat = 1, which means the saturation extent does not change from winter to summer, and, based on the consistence of the seasonal changes of precipitation and saturation extent from winter to summer, it can be inferred that the precipitation does not change much from winter to summer. When the change of water vapor from winter to summer is greater than the change of temperature (Cvap > Ctem), the saturation extent and precipitation increase from winter to summer (Csat > 1). When the change of temperature is greater (Ctem > Cvap), the saturation extent and precipitation decrease (Csat < 1). Using the expression of specific humidity q = ɛ e/p, where p is pressure and ɛ the ratio of the gas constants for dry air and water vapor, then Cvap can be written as Cvap = qsum/qwin for the change at a pressure surface.
 To better understand the meaning of Cvap and Ctem, (1) and (2) can be rewritten by defining the dew point temperature Td through e ≡ es (Td) and expressing the saturation vapor pressure as es (T) = A exp (−L/RvT) (the integrated form of the Clusius-Clapeyron equation), in which Rv is the gas constant for water vapor, L the latent heat of vaporization, and A the integration constant. Then, (1) and (2) become
where a = L/(RvTdsumTdwin) and b = L/(RvTsumTwin) vary little with temperature and dew point within their normal ranges, and can be taken as constants. So, Cvap and Ctem can reflect the differences of dew point and temperature, respectively, between winter and summer. The Csat can be written as
It shows how the differences of dew point and temperature contribute to the change of saturation extent.
Figures 5a–5c present the profiles of Cvap and Ctem calculated with the 25-year averaged monthly mean data of February and August for the three regimes. In WC (Figure 5a), the change of temperature (Ctem) is much greater than the change of water vapor (Cvap) in the layer from 900 to 300 hPa. This means that, relatively, the winter atmosphere over WC is cold but not so dry, and the summer atmosphere is warm but not so moist. Therefore, the winter has a higher relative humidity and becomes the rainy season of the year, while the summer has a lower relative humidity with little precipitation. In this interesting precipitation seasonality regime shown in Figures 2a and 4a, when the water vapor (e, q, or precipitable water) is the least of the year in winter, the precipitation is the largest, while when the water vapor is the largest in summer, the precipitation is the least. So, the effect of the temperature is the key to the opposite seasonal behaviors of the water vapor and the precipitation. In SM (Figure 5b), the change of water vapor (Cvap) from winter to summer is much greater than the change of temperature (Ctem) in all levels, especially in the middle layer (500 hPa). This implies that, relatively, the winter is dry but not so cold (the temperature is actually very close to that of summer), which makes the winter saturation and precipitation very small. The summer is rather moist but not particularly hot, so the atmosphere is easy to saturate, and the huge summer monsoon precipitation forms. The averaged relative humidity in summer is greater than 45% at all levels and can reach 80% at the lowest level (Figure 3b). In SE, where precipitation occurs throughout the year, the changes of water vapor and temperature from winter to summer are roughly equivalent. The configuration of the profiles of Cvap and Ctem is kind of a combination of the above two cases. A careful examination of the changes of water vapor and temperature in the upper and lower levels may need a pentad or running mean for the data. The profiles of Csat in Figures 5d–5f illustrate the significance of the change of water vapor from winter to summer comparative with the change of temperature with aid of the reference line of Csat = 1.
 The precipitation seasonality can vary from place to place across the world. In this study, the three major precipitation seasonality regimes over the US-Mexico are understood from the comparative strengths of the water vapor and temperature in winter and summer. Our analysis suggests that, when understanding the precipitation seasonality, especially the regime with a negative correlation between the precipitation and water vapor as shown in Figure 2a, the effect of temperature, in addition to water vapor, should be taken into account. The water vapor, described by e, q, or precipitable water, and temperature both increase from winter to summer (Figures 2 and 4). The key here is how to measure and compare the changes of the different variables of water vapor and temperature. For this purpose, two dimensionless numbers, Cvap and Ctem, are defined in this study.
 In WC where winter is the rainy season, the change of temperature (Ctem) from winter to summer is much greater than the change of water vapor (Cvap), suggesting that the winter air is cold but not so dry and the summer air is warm but not so moist. So, the saturation extent and precipitation are larger in winter and smaller in summer. In SM where summer is the rainy season, the change of water vapor (Cvap) is much greater than the change of temperature (Ctem), which means the winter air is dry but not so cold and the summer air is moist but not that hot. So, the saturation and precipitation are smaller in winter but larger in summer. In SE, where precipitation prevails the entire year, the changes of water vapor and temperature from winter to summer are overall equivalent. To better compare the changes of water vapor and temperature in the weak precipitation seasonality regimes, a pentad mean or running mean maybe more suitable.
 It should be noted that, for the precipitation, the water vapor and temperature are not the cause, but an atmospheric condition. In different places and seasons, the precipitation may be affected by different surface conditions and different dynamic and thermodynamic processes involved in the atmosphere. However, at the seasonal scale, all the physical factors influencing the precipitation can be reflected in the water vapor and temperature fields, which makes it possible for us to understand the different precipitation seasonality regimes through them. The dynamic analysis of the rainy season precipitation of the three regimes, e.g., their dominant atmospheric circulations and systems, will be presented separately.
 This work was supported by NOAA under grant NA05OAR4310008 and NASA under grant NNG04GL25G. The North American Regional Reanalysis, precipitation analysis, and precipitable water analysis are distributed by the NCEP/EMC, the NCEP/CPC, and the NASA/DAAC, respectively.