Dynamical Effect of Land Surface Processes on Summer Precipitation over the Southwestern United States

Transcription

1 496 JOURNAL OF CLIMATE VOLUME 16 Dynamical Effect of Land Surface Processes on Summer Precipitation over the Southwestern United States MASAO KANAMITSU Climate Research Division, Scripps Institution of Oceanography, La Jolla, California KINGTSE C. MO NOAA/NCEP/NWS/Climate Prediction Center, Camp Springs, Maryland (Manuscript received 26 March 2002, in final form 10 July 2002) ABSTRACT The physical mechanism of summertime precipitation over Arizona and New Mexico (AZNM) is examined using regional model experiments. Two sets of regional model simulations with different physics packages produce very different precipitation (P) over the Southwest including AZNM. The better simulation that produces a wet monsoon similar to the observations has larger evaporation (E) over AZNM and stronger moisture flux from the Gulf of California into AZNM. Diagnostics of the simulations suggested that the increase in precipitation is not due to the increase in evaporation locally but rather to the change in moisture flux. Regional model experiments were then designed to isolate the impact of local E and the large-scale flow. Both regional model experiments and diagnostics support the following physical mechanism: There is an increase in E in the realistic simulation due to the change in land surface physics. This increase in E is compensated by the decrease in sensible heat, which leads to the colder land surface. Associated with this cooling, the surface pressure raises and the Southwest heat low weakens due to the increase in the surface pressure. This alters the large-scale low-level circulation and increases the occurrence of the low-level moisture surge events from the Gulf of California into AZNM, and accordingly, increases P. The mechanism is also found in observations of day-to-day variation of precipitation over AZNM. 1. Introduction The change in moisture content of the atmosphere is determined by two physical processes; evaporation from surface (E) and moisture flux convergence (C). These processes are connected through soil moisture (S) over land via atmospheric circulation and boundary layer physics. The increase of atmospheric moisture in the area can lead to an increase in precipitation (P) asa result of supersaturation or the enhancements of convective activity. Understanding the mechanisms of the change in P is difficult since there are complex relationships between E, C, and S. For example, if precipitation increases, soil moisture increases. This in turn increases evaporation, but at the same time, lowers the surface temperature. This changes the surface pressure and consequently the flow field, which changes moisture convergence and accordingly precipitation. Furthermore, these relation- Corresponding author address: Dr. Masao Kanamitsu, Scripps Institution of Oceanography, MC 0224 CRD/SIO/UCSD, La Jolla, CA kana@ucsd.edu ships are functions of geographic location and season, presenting additional difficulties to diagnose. Many studies showed that the increase of P is through the increase of atmospheric moisture content through the increase of E, but the relationship between E and P is still not well studied. Mechanisms for the increase of P in the Tropics are often explained by the changes in albedo as suggested by Charney (1975), and later by Dirmeyer and Shukla (1994) and Hahmann and Dickinson (1997). Charney (1975) proposed a mechanism that an increase in albedo reduces heat flux, stabilizes the atmosphere stratification, and reduces convection. Betts et al. (1996) discussed the importance of the Bowen ratio for medium range numerical weather forecasts. The larger Bowen ratio creates a deeper boundary layer and increases entrainment of cold and dry free atmospheric air at the top of the boundary layer. This tends to suppress convection particularly in the afternoon. In contrast to the role of E, some observational studies emphasize the importance of large-scale moisture flux. Hales (1972, 1974), Brenner (1974), and model experiments by Stensrud et al. (1995, 1997) and Anderson et al. (2000a,b) indicated that most monsoon rainfall over Arizona and New Mexico (AZNM) is due to convective 2003 American Meteorological Society

2 1FEBRUARY 2003 KANAMITSU AND MO 497 Experiment name Resolution Area SiB TABLE 1. List of numerical experiments performed in this study. Physics and surface parameters SiB Lateral boundary forcing Land surface tempurature Initial conditions 1 Jul Jul 1999 SiB 20 km AZNM SiB 1 Jul km AZNM 1 Jul 1999 SiB 20 km AZNM SiB 1 Jul 1999 Downscaling 1 Downscaling 2 Downscaling 3 Sensitivity 1 Sensitivity 2 Sensitivity 3 Sensitivity 4 SiB SiB 1 Aug Jul Aug Jul Jul Aug Aug 1990 activity in the afternoon, which is associated with largerscale strong moist and relatively cold surge events from the Gulf of California. This surge acts as a source of moisture for the convective precipitation over warm and semi-arid AZNM. Diagnostic studies by Douglas et al. (1993) and Douglas (1995) using station observations showed that the low-level jet from the Gulf of California supplies moisture needed for rainfall over the Southwest. Occasionally, a combined effect of midlatitude disturbances and surge event produces heavy rainfall events over AZNM (Hales 1972). Using model simulations and observations, we will examine the relationship between E and moisture flux, and demonstrate a mechanism that controls precipitation over AZNM. The regional model and the physical processes used, and the list of experiments is described in section 2. The comparison and the detailed analysis of two major experiments that motivated this study are presented in section 3. In section 4, results from experiments that separate the effect of local evaporation and the change in atmospheric circulation over AZNM are discussed. More observational evidence is presented in section 5 and the conclusions and discussions are given in section Model details, list of experiments, and data used The regional model used in this study is the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM; Juang and Kanamitsu 1994; Juang et al. 1997). The physics of the model come from the NCEP Department of Energy (DOE) reanalysis 2 (R-2) global model (Kanamitsu et al. 2002) and include the relaxed Arakawa Schubert convective parameterization (Moorthi and Suarez 1992), nonlocal vertical diffusion, Oregon State University hydrology model (Pan and Marht 1990), and shortwave and longwave radiation with interactive clouds. The initial condition was taken from R-2. Data are interpolated from the T62-resolution reanalysis grid to the RSM grid. The initial conditions include the atmospheric fields as well as soil moisture, land surface temperature, and snow depth. Sea surface temperatures (SSTs) are interpolated from weekly SST analysis derived from the in situ and satellite data using the optimum interpolation method of Reynolds and Smith (1994). Two horizontal resolutions are used: for control experiments and 20 km for the double-nesting experiments (to be explained later). Vertical resolution is 28 levels for all experiments. The domain covers the contiguous United States (20 60 N, W) on the polar projection grid for the control, and AZNM (32 36 N, W) for the double-nesting experiments. The model orography is shown later in Fig. 1d for 50- km resolution and Fig. 6d for 20-km resolution. The results from 12 numerical integration experiments are described in this study (Table 1). These experiments are classified into the following four groups (the table uses double horizontal line to separate the experiments in the four groups). 1) Group 1 (control experiments). The first two experiments labeled SiB [for the Simple Biosphere (SiB) model used for land surface atmosphere interaction] and [for the United States Geological Survey () data used for defining land surface properties] are the control experiments that motivated this study. They both ran on a 50-km grid over the continental United States but with different physics packages (to be explained below). The experiments started from 1 July 1999 and lasted for one month. 2) Group 2 (double-nesting experiments). The next three experiments are specifically designed to separate the effect of local E and change in the moisture convergence C. They were nested in the outputs of the control experiments in group 1. These experiments were performed with the 20-km RSM. 3) Group 3 (downscaling experiments).the third group of experiments, the next three integrations, are performed to examine the two contrasting years, 1995 and 1999, and are done with the same physics as

3 498 JOURNAL OF CLIMATE VOLUME 16 FIG. 1. Mean precipitation for Jul 1999: (a) from the observed gridded precipitation based on the gauge data, (b) from the SiB experiment, (c) from the experiment. Contour intervals are 0.5, 1, 2, 4, 8, and 12 mm day 1. Areas where values are greater than 0.5 (4) mm day 1 are shaded light (dark). (d) Surface (terrain) height for the 50-km RSM. Contour interval is 200 m. but with differing initial conditions and boundary forcing. These three experiments are used to downscale R-2 to obtain finer analysis over AZNM, and are not a part of the previous five sensitivity experiments. 4) Group 4 (sensitivity experiments).these experiments address the question of whether the difference between the and SiB control experiments are due to the domain used or whether they are only true for July To address the domain sensitivity, we shifted the domain southward to cover Northern Mexico and performed the and SiB experiments for July 1995 and August The SiB experiment used a physics package similar to the R-2 version of the global model (Kanamitsu et al. 2002). The surface parameters, namely, vegetation type, vegetation cover, soil type, albedo, and surface roughness, are produced by a simple SiB model on a relatively coarse 2 2 latitude longitude grid (Xue et al. 1991). The formulation of direct evaporation from bare soil is based on Mahrt and Pan (1984) and is a function of soil water conductivity and diffusivity at the land surface. The experiment used finer resolution (30 ) vegetation type, vegetation cover, and soil type, and was derived from data (Chen and Dudhia 2001a). Albedo and surface roughness are a function of vegetation type (Chen and Dudhia 2001a,b). The formation of evaporation in this experiment is modified in order to use the data, because it lacks some parameters necessary for a full Mahrt and Pan formulation. We used a more conventional formulation in which evaporation is a function of soil moisture and potential evaporation (Mahfouf and Noilhan 1991; Chen and Dudhia 2001a). The Southwest monsoon event of July 1999 was chosen for detailed analysis. This year was characterized by a relatively wet monsoon as indicated by the 0.25 precipitation analysis of Higgins et al. (2000) (Fig. 1a). There was about 2 mm day 1 rainfall over AZNM with a maximum of about 4 mm day 1 rainfall along the state line of Arizona and New Mexico. The RSM was integrated for one month, forced by R-2 at the lateral boundaries, for 50-km resolution experiments and outputs were produced every 6 h. Outputs from these 50-km RSM experiments were used to laterally force the 20-km RSM simulations. Note that since

4 1FEBRUARY 2003 KANAMITSU AND MO 499 FIG. 2. (a) Daily precipitation averaged over the area (32 36 N, W). The unit is mm day 1. (b) Six-hour accumulation of precipitation for the SiB (dark circles) and the (open circles) experiments. the lateral forcing is via analysis, these experiments are simulations and not forecasts. The latent heat, sensible heat, and radiation fluxes and precipitation used in the diagnostics were accumulated for the 6-h period. 3. The results from SiB and experiments The monthly mean precipitation simulated by the SiB experiment is too dry over the AZNM (Fig. 1b), and fails to capture the pattern and intensity of the monsoon rainfall. The model shows no rain over Texas, and dry conditions along the Gulf of Mexico. Rainfall over the Southwest is confined over western New Mexico and eastern Arizona. The precipitation simulation improves significantly for the experiment. The wet AZNM monsoon of 1999 is well simulated by the model (Fig. 1c). There is a broad area of rainfall over AZNM, which captures the observed maximum of 4 mm day 1 along the Arizona New Mexico state line. The maximum band of rain along the Gulf of Mexico is well simulated and Texas is not as dry. The differences in time-mean geographical distribution of precipitation between the and the SiB experiments (Fig. 3a) shows that the experiment is generally wetter over most areas, especially over the Southwest. The increase of rainfall is as large as 2 3 mm day 1. The daily variation of 6-hourly accumulation of rainfall over AZNM (32 36 N, W) from models (Fig. 2b, open circles) and daily accumulation from observations (Fig. 2a) indicates that the experiment captures many of the rainfall episodes, although the model does not always capture extreme daily rainfall amounts (note the different scale of the vertical axis between Figs. 2a and 2b). For the SiB experiment (dark circles), there are only three rainfall events and the duration of each event is less than 2 days. The difference between and SiB is largely due to the frequency of rainfall events. The improvement of the experiment upon SiB is due to several changes made to the model, namely, the vegetation type, vegetation cover, soil type, albedo, surface roughness, and the formulation of the evaporation from soil. Sensitivity experiments indicate that the most crucial physics responsible for the improvement in the experiment is the formulation of evaporation from bare land (K.C. Mo, M. Kanamitsu, H.-M. Juang, and S.-Y. Hong 2002, unpublished manuscript). The change in P between SiB and over AZNM is approximately equal to the change in E (values ranging from 0.5 to 2 mm day 1 ), and the contribution of the moisture convergence (P E) is about half or less (Fig. 3d) over the same area. The straightforward interpretation of this result is that the increase of precipitation in the experiment is due to the local increase in E. However, such a conclusion can be misleading since E itself is dependent on P and moreover, P is dependent on moisture convergence, as discussed in the introduction. Differences in the downward shortwave and longwave radiation at the surface over AZNM between the SiB and experiments are found to be small and the difference in albedo is also small (not shown). In order to satisfy the energy balance requirement at the land surface, the change of latent heat flux is compensated for by the change of the sensible heat flux. Accordingly, the mean latent and sensible heat fluxes have similar patterns and same magnitude but opposite sign (Figs. 3b and 3e). The difference is not caused by one or two large events, but occurs throughout the integration. The decrease in sensible heat flux in the experiment resulted in colder surface temperature (Fig. 3f). The experiment s temperature is colder by 4 C than that of the SiB. The western part of the continental United States is dominated by low pressure at the surface during the summer monsoon months (Tang and Reiter 1984). This is a heat low generated by the dry warm surface, and in a sense, a direct response of atmospheric motion to surface heating. Our examination of the and SiB experiments suggests that, in response to the colder surface temperature simulated by the experiment (Fig. 3f ), intensity of the heat low is reduced as indicated by the difference in 850-hPa heights (Fig. 3c). Associated with the change in the low-level pressure fields, moisture flux into the AZNM changes. Figure 4a presents monthly mean vertically integrated moisture flux Q of the experiment (Q was calculated from 6-h outputs so the transient terms within 6 h were neglected). Over AZNM, the moisture supply is mainly transported by the low-level jet from the Gulf of California with limited amounts of moisture from the Gulf

5 500 JOURNAL OF CLIMATE VOLUME 16 FIG. 3. (a) The precipitation difference for Jul 1999 between the and the SiB experiments. Contour interval is 1 mm day 1, zero contours are omitted, but contours 0.5 and 0.5 mm day 1 are added. Positive values are shaded. (b) Same as (a) but for E. (c) Same as (a) but for 850-hPa height with contour intervals of 2 m. (d) Same as (a) but for P E. (e) Same as (a) but for the mean sensible heat flux difference. Contour interval is 20 W m 2. (f) Same as (a) but for mean temperature 2 m above the surface; contour interval is 1 C. of Mexico, in agreement with the work by Schmitz and Mullen (1996) and Higgins et al. (1998). The difference of moisture fluxes between the and SiB experiments shows anomalous anticyclonic circulation (Fig. 4b), which increases the moisture flux from the Gulf of California into AZNM. Since vertically integrated moisture flux is dominated by low-level winds, this anomaly in moisture flux is a reflection of the weakening of the heat low circulation (Fig. 3c) due to the colder surface temperature (Fig. 3f). The stronger moisture flux from the south into the AZNM area enhances the precipitation events. This can be illustrated by the cross section of the meridional moisture flux (q ) averaged over W near the entrance of the moisture flux into AZNM at 32 N (Fig. 5a). For both experiments, the maximum flux is located at 925 hpa. Contributions to the integrated moisture fluxes above 700 hpa are small, thus supporting the evidence that changes in low-level circulation are responsible for the increase in moisture flux. There is a good correspondence between the surge events and rainfall for both experiments (Fig. 2). After the strong surge events of 6, 13, 17, 21, and 27 July the mixing ratio at the surface between the two experiments diverges (Fig. 5c). There are more surge events in the experiment. This is closely correlated with the enhanced convective activity over AZNM during the afternoon. For the SiB experiment, only three surge events occur, corresponding to three short rainfall episodes, and overall much less rain. Comparison of these two experiments strongly suggests that the difference in precipitation between the two is due to the change in large-scale circulation, and

6 1FEBRUARY 2003 KANAMITSU AND MO 501 FIG. 4. (a) One-month mean vertically integrated moisture flux vector and meridional moisture flux (q ) averaged for the experiment. The meridional component is contoured every 40 kg m 1 s 1 and positive values greater than 40 kg m 1 s 1 are shaded. The unit of the vector is 300 kg m 1 s 1. (b) Same as (a) but for the difference between the and the SiB experiment. The meridional flux is contoured 20 kg m 1 s 1 and the unit vector is 100 kg m 1 s 1. not due to the change in local evaporation. The change in evaporation is the result of the increased precipitation. A good correspondence between negative area of P E (Fig. 3d) difference, positive area of 850-hPa height (Fig. 3c) difference, and negative area of sensible heat flux and near-surface temperature differences are the basis for this conclusion. In the next section, we will further elaborate on this mechanism using model experiments. There is a question regarding the general applicability of the results described above. The results may be more strongly dependent on other factors, such as the selection of the initial years, and the choice of domain. In order to address these questions, several additional experiments were performed and the results are presented in the appendix. In summary, the experimental results did not depend on initial years or on the selection of the domain. FIG. 5. (a) Cross section of the meridional moisture flux (q ) averaged over longitude ( W) at 32 N for the experiment. Contour interval is 10 g kg 1 ms 1. Zero contours are omitted and positive values are shaded. (b) Same as (a) but for the SiB. (c) The mixing ratio at the surface averaged for the area (32 34 N, W), for the (open circles) and the SiB (dark circles). 4. Experiments to differentiate effects of local E and synoptic-scale moisture convergence on P The simple diagnostics performed in the previous section are not conclusive because it is not possible to separate the effect of E and C on P since these three are closely related through dynamical, thermodynamical, and soil moisture interactions. In order to understand the cause and consequence relationship, it is necessary to design experiments that separate these interactions. Applying a one-way nested regional model is a convenient way to turn off the interaction between the change in atmospheric circulation and the local change in E. One-way nesting does not allow regional model forcing to feed back into the larger scale. If we set the area of regional model that covers only the AZNM area and laterally force the model with and SiB 50- km simulations, we practically turn off the interaction between the AZNM scale and larger synoptic scale that

7 502 JOURNAL OF CLIMATE VOLUME 16 covers the western United States and Mexico. In regional modeling terminology, this is the double-nested downscaling experiment. Since the AZNM area (32 36 N, W) is small, we chose 20 km as the horizontal resolution. The vertical levels are the same in the and SiB experiments. The terrain represented by surface height for a 20-km double-nested model (Fig. 6d) recognizes high mountains over New Mexico and valleys over AZNM. In comparison, the 50- km model shows a much smoother terrain (Fig. 1d). The difference of terrain is not important for this experiment. The domain is intentionally minimized to assure that the moisture fluxes are supplied by the 50-km model outputs through the boundary conditions (which are supplied every 6 h). In this configuration, the increase in P within the domain will influence E, but change in E within the domain is unable to influence the synopticscale circulation covering the AZNM area and its associated moisture fluxes. The following three experiments were performed. The first 20-km experiment (labeled ) uses the model physics configuration laterally forced by the 50-km simulation. This experiment is regarded as a control experiment. It can be viewed as a downscaling experiment of the 50-km simulation since the difference between the and experiments is only the resolution and domain. This simulation should produce precipitation similar to the run but with more small-scale details. The second 20-km model experiment ( SiB) uses model physics configuration but laterally forced by the 50-km SiB simulation. This experiment provides the local impact of physics under the SiB-simulated synoptic-scale circulation. The comparison between the and the SiB experiments represents the effect of large-scale forcing under the same model physics. The third 20-km model experiment (SiB ) uses the SiB model physics configuration but forced with the 50-km simulation. The comparison between SiB and indicates the effect of the change in physics over the AZNM area under the same large-scale forcing. As expected, the mean precipitation pattern of the experiment resembles the precipitation pattern of the 50-km experiment (Fig. 6h), but has more detailed structure and the precipitation maxima are higher. Over the high mountains the rainfall amounts increase. Since the high-resolution model has a more realistic representation of terrain and land surface characteristics, it allows a more detailed simulation of precipitation (Mo et al. 2000). Some stations at high elevations reported high monthly mean rainfall for July. For example: Alpine (33.5 N, W) reported 7.8 mm day 1, Beaver Creek (34.4 N, W) reported 5.54 mm day 1, Wikieup (34 N, W) reported 1.89 mm day 1, Dilia SSE (35.1 N, W) reported 3.19 mm day 1, and Conchas Dam (35.2 N, W) reported 5.53 mm day 1 rainfall. Thus, the 20-km simulation is quite reasonable. We will first examine the effect of lateral boundary forcing. The comparison of and SiB shows that patterns of precipitation are similar but the magnitude of P is much lower for the SiB (Figs. 6a and 6b). This suggests that P in AZNM is largely controlled by the synoptic-scale boundary forcing. As mentioned in section 3, the SiB experiment has fewer surges from the Gulf of California (Figs. 5a and 5b), which results in fewer heavy precipitation events for the SiB experiment and less P. The also has more E than the SiB experiment (Figs. 6e and 6f) even though the model physics is the same. This difference is due to large precipitation in the experiment, which makes the soil wetter over AZNM and consequently increases E. Thus, the increase in P is not the result of increase in local E in these experiments. Turning to the local effect of E, we compare the and SiB experiments. The pattern of mean precipitation of the and SiB are similar. Both capture roughly the same rainfall amount, showing large precipitation over the area (33 34 N, W) and another maximum located at the northeastern boundary. The rainfall amounts from the SiB are very similar to those of the. Although the SiB experiment uses the SiB physics, both P and E over the AZNM area are comparable to those from the. Recall that the SiB physics produces less rain in the 50-km experiment in comparison to the 50-km experiment. These experiments again indicate that lateral boundary forcing, rather than the model physics, is the major factor in determining the amount of P over the AZNM. These experiments confirm the following dynamical effect: The 50-km model physics extracts more moisture from the soil and increases E. This increase of E is compensated by the decrease in the sensible heat flux, accordingly lowering the land surface temperature. These changes are significant enough to change the synoptic-scale wind and moisture field, and consequently change the precipitation over the AZNM. The local E has less impact on P, and the increase of E is due to the moistening of land due to increased P. 5. Implication to interannual and day-to-day variability of precipitation over AZNM The model-based mechanism of the change in P discussed above can be applied to understand the observed interannual variability of AZNM precipitation. For this purpose, we look at the observational relation between the intensity of heat low over AZNM, moisture surge events, and precipitation from two contrasting wet and dry monsoon years. Since low-resolution reanalysis data cannot resolve the low-level jet from the Gulf of California (Schmitz and Mullen 1996), which is essential

8 1FEBRUARY 2003 KANAMITSU AND MO 503 FIG. 6. Mean precipitation for (a) the, (b) SiB, and (c) SiB experiments. Contour intervals are 1, 2, 4, 6, 8, 20, and 30 mm day 1. Values greater than 1 (6) mm day 1 are shaded blue (green). (d) Surface height for the 20-km RSM. Contour interval is 200 m. Values greater than 2200 m are shaded blue. (e) Mean E for the experiment. Contour interval 1 mm day 1. Values greater than 1 mm day 1 are shaded blue. (f) Same as (e) but for the SiB experiment. (g) Same as (e) but for the SiB experiment. (h) Same as (d) but for the 50-km RSM.

9 504 JOURNAL OF CLIMATE VOLUME 16 FIG. 7. (a) Mean precipitation difference between Jul Aug 1999 and Jul Aug Contour interval is 1 mm day 1. Positive values are shaded and zero contours are omitted from the RSM simulations. (b) Same as (a) but for E. Contour interval is 0.4 mm day 1. (c) Same as (a) but for temperature 2 m above the surface. Contour interval is 0.8 C. (d) Same as (a) but for observed precipitation difference. (d) Same as (a) but for height at 850 hpa. Contour interval is 2 m. (e) Same as (a) but for vertically integrated meridional moisture flux. Contour interval 20 kg m 1 s 1 and contours 20 kg m 1 s 1 and 20 kg m 1 s 1 are added. for analysis of surge events, we used the downscaled reanalysis obtained by applying RSM. For the comparison of contrasting years, we chose two summer months of 1999 and July and August 1999 were wetter than normal months. Rainfall occurred almost every afternoon. There were 39 days when rainfall over AZNM was over 2 mm day 1. In comparison to 1999, July and August 1995 were drier. Rainfall exceeded 2 mm day 1 on only 6 days. The observed precipitation difference between 1999 and 1995 (Higgins et al. 2000) indicates that the Southwest was wetter (Fig. 7d). The reanalysis for July and August of these two years was downscaled using the 50-km RSM with the configuration. Although the downscaled analyses are model-physics dependent, our results will be less dependent on model physics since our interest is the comparison of the analyses of two contrasting years, and these are downscaled with the same model that shares the same physics. As shown in Fig. 7a, the downscaled analysis successfully captured most of the observed precipitation differences between 1999 and 1995 (Fig. 7d), particularly over the western United States. Both show wetness over AZNM, Colorado, and Kansas, and dryness over the central United States. Because the model is able to capture the difference over the western and central United States, the associated changes in atmospheric and land surface conditions may be used to represent the contrasting conditions for wet and dry months over AZNM. The difference in P between 1999 and 1995 indicates that the wet (dry) areas are associated with the increase

10 1FEBRUARY 2003 KANAMITSU AND MO 505 (decrease) in E and cooler (warm) temperature (Figs. 7b and 7c). The 850-hPa height difference shows positive height anomalies over the western region with a maximum centered over the state line of Arizona New Mexico and negative anomalies to the east. It is associated with the stronger low-level jet from the Gulf of California and the weaker low-level jet from the Gulf of Mexico to the central United States as indicated by the difference of meridional moisture transport (Fig. 7f). This comparison shows that the wet monsoon years are associated with larger E, cooler temperature, positive 850-hPa height anomalies and stronger low-level moisture flux into the AZNM area from the Gulf of California allowing more frequent moisture surge events. The situation reverses for the dry monsoon years. The relationship between P and circulation anomalies over AZNM has also been discussed by Higgins et al. (1998), Tang and Reiter (1984), Carleton et al. (1990), and many others. They also found that the wet season is associated with a heat low and an upper-level monsoon anticyclone over the Southwest. We also examined the linkage between the surface southerly wind and the strengthening of the heat low over the Southwest and P over AZNM in the daily timescale. We utilized the satellite-derived wind from scatterometer data. Since moisture flux dominates at low levels, the meridional wind from the Gulf of California can infer the moisture flux into Arizona without moisture information. The scatterometer measures the ocean surface wind speed and direction from scattering of the microwave at the ocean surface. The derived winds are interpolated to grid twice a day by Tang and Liu (1996). The data were transferred from their Jet Propulsion Laboratory web site for this study. These winds are not used in reanalysis and serve as an independent confirmation of our proposed mechanism. The 850-hPa height anomalies were obtained from the reanalysis and anomalies were calculated as departures from the mean from 1979 to The scatterometer data were available from 2000 on and therefore an example was chosen for summer July September 2001 was drier than 1999, but there were many rainy periods in July and August as indicated by daily rainfall averaged over AZNM (Fig. 8a). The meridional surface wind at 30 N, W (in the Gulf of California), observed by the scatterometer, and the 850-hPa height anomaly averaged over the AZNM area indicate a good correspondence between positive height anomaly, strong southerly flow from the Gulf of California, and rainfall episodes. For the three rainfall episodes from 1 16 July, 25 July 14 August, and September, rainfall was associated with a strong positive height anomaly over AZNM and stronger northward wind from the Gulf of California to Arizona (Fig. 8). Thus, the relation between the low-level jet intensity of the heat low and precipitation over AZNM roughly holds even for daily timescales (and most likely for diurnal timescales). FIG. 8. (a) Daily observed precipitation averaged over AZNM (32 36 N, W), for The unit is mm day 1. (b) Surface wind at 30 N, W from satellite measurements. The unit is m s 1. (c) 850-hPa height anomaly averaged over (32 36 N, W). The unit is m. 6. Conclusions and discussion Precipitation is determined by surface evaporation, moisture convergence, and convective activity. Over land, these are tied together by soil moisture. Because of the interrelationship between these parameters, the mechanism that controls precipitation cannot be described in a straightforward manner. In this study, model experiments and observational diagnostics are used to describe the relationship between soil moisture and precipitation over Arizona and New Mexico during summer. The method applied in this study involves the employment of a fine-resolution regional model imbedded within the coarse-resolution regional model simulation. This setup effectively turns off the synoptic-scale dynamical response to change in land surface conditions that exist in the coarse-resolution model, thus making it possible to examine each process separately. This study was initiated by two control experiments

11 506 JOURNAL OF CLIMATE VOLUME 16 performed using different land surface physics packages. The NCEP 50-km resolution regional spectral model covering the contiguous United States was used to simulate precipitation over AZNM in these two control runs. The model was forced by R-2 data at the lateral boundaries, but soil moisture and temperature were predicted. One-month-long simulations were made with two different versions of the NCEP global model physics. One of the runs produced more rain over AZNM and was found to be much superior to the other. Simple diagnostics of these two simulations suggested the following mechanism that increased P in one of the experiments. The longwave and shortwave radiation fluxes at the surface, as well as the surface albedo, are comparable between the two runs, but evaporation (E) over AZNM is much larger in the experiment. Increased E in the run is compensated for by a decrease in sensible heat, which resulted in colder land surface. Associated with this lowering of temperature, the surface pressure increases. This causes the weakening of the heat low circulation that dominates this area, alters the surface circulation, and increases the low-level moisture flux into the AZNM from the Gulf of California. This increase of moisture flux appears in the form of increased moisture surges, and consequently increases P. This scenario was further examined by turning off the large-scale response to change in the land surface condition using a double-nested 20-km resolution regional model integration covering only the AZNM area but forced by the 50-model simulated synoptic-scale fields. These experiments showed that the precipitation over AZNM is controlled by the large-scale forcing and the local evaporation does not play a controlling role. This is a demonstration of land soil moisture directly modifying the wind field through changes in surface temperature, subsequently changing the large-scale rainfall pattern. This scenario is also confirmed by comparing RSM downscaled reanalysis precipitation over AZNM between the two contrasting wet and dry years of 1999 and The strong relationship between the occurrence of moisture surge from the Gulf of California and P over AZNM is also found to occur on a day-to-day timescale using scatterometer winds. From these modeling and observational studies, we propose the following mechanism that controls the variation of precipitation over AZNM. During the summertime, a heat low develops over the western United States; its circulation is confined to low levels. The strength of the heat low modulates the low-level jets from the Gulf of Mexico to the central United States and from the Gulf of California to AZNM. When the heat low weakens, the low-level jet from the Gulf of Mexico weakens while the jet from the Gulf of California is less suppressed, thus providing a better environment for the low-level jet from the Gulf of California to penetrate into the AZNM area, producing more precipitation. Although the heat low is essential for the Southwest monsoon, there seem to be an optimal strength and location of the heat low that produces more frequent precipitation over AZNM. Heating at the land surface generates the heat low in the Southwest and its intensity is determined by surface temperature. Since land surface temperature is strongly influenced by soil moisture, the large-scale distribution of soil wetness over the western United States plays a crucial role in determining the interannual variation of the strength of the southwestern monsoon. The semiarid nature of AZNM enhances the soil moisture dynamics interactions through the change in the heat low circulation. This type of interaction may be strong over semiarid regions of the Sahara and other fringes of desert areas, but may not be so over other more humid areas. The mechanism proposed here also suggests the source of moisture for the AZNM precipitation (Hales 1974; Adams and Comrie 1997). When the heat low over the Southwest is weak, the Gulf of California is the primary moisture source. In this situation, moisture from the Gulf of Mexico crosses over the Mexican highlands and merges into the low-level jet from the Gulf of California acting as an additional source. When the heat low is strong, the moisture source from the Gulf of California is suppressed and the flux from the Gulf of Mexico becomes more dominant. Thus, the moisture source for AZNM precipitation varies with the intensity of the heat low. The implication of this mechanism to the prediction of AZNM rain is interesting. The southwest precipitation depends on the strength of the heat low, or equivalently the surface temperature. Therefore, the soil moisture condition during the preceding season and the snow accumulation over the Rockies may provide a useful predictor for the strength of the Southwest monsoon during the summer season. The double-nested experiment inadvertently shows that the effect of regional model physics is a strong function of the model domain. If the model domain is small, the change in the model physics does not significantly affect the simulation, while in larger domain simulations, the model physics significantly affects the simulation. This should be taken into account when the downscaling is performed with a model having different physics from the model supplying the boundary condition. Finally, the result of this study does not imply that local evaporation is not important. It will certainly change the local circulation and precipitation. These topics are not independent of this paper and are left for future research. Acknowledgments. Dr. Song-You Hong coded most of the configuration, and his effort is indispensable to this paper. Dr. Henry Juang assisted in performing some of the simulation experiments. Diane Boomer helped in improving the wording. We would also like

12 1FEBRUARY 2003 KANAMITSU AND MO 507 FIG. A1. Mean observed and simulated precipitation for Jul 1990 and Aug (a) Observation for Jul 1995, (b) simulation by for Jul 1995, (c) simulation by SiB for Jul 1995, (d) observation for Aug 1990, (e) simulation by for Aug 1990, and (f) simulation by SiB for Aug to thank Dr. John Roads for providing very useful comments. This project is supported by GEWEX Grant GC and by Cooperative Agreement NOAA- NA17RJ1231. The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA. APPENDIX Additional Results Additional experiments were performed to show that the differences between the and SiB control experiments are not due to the particular month chosen (July 1999) or the domain location and size. For this purpose, we performed the and SiB experiments for a dry month (July 1995) and a wet month (August 1990), and shifting the regional domain southward by 5 from the 1999 case (15 55 N, W). Results are given in Fig. A1. The experiments capture the overall mean precipitation features well. It shows the relative dryness of July 1995 over AZNM and wetness over the central United States. The experiments also capture the wetness of August The deficiency is that there is too much rain over the Rockies. The SiB cases for 1995 and 1990 show overall dryness and very little rain over AZNM. These experiments demonstrated that the differences between the and SiB are not limited to the July 1999 case and do not depend on the domain location. As a more extreme example of domain size, we performed a global simulation with observed SSTs for July The experiment was conducted with a global version of the regional model with SiB and physics. The initial condition was taken from R-2 1 July 1999 data. The only external forcing given to the model was the SST; the soil moisture and temperature are predicted.

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