1. Introduction
The weather of the Baja California Peninsula in northwestern Mexico is mild and dry most of the year. The peninsula’s geographical position places it in the path of humid air masses associated with tropical cyclones moving northward in the eastern Pacific Ocean from July through September. Added features that create a unique meteorological situation include mountain ranges along the peninsula, warm water in the Gulf of California, and the cold California Current in the Pacific Ocean. These features interact with the flow of humid air masses inducing conditions that play a role in localized, convective systems during the passing of tropical cyclones. These systems occur late in the summer over the southern part of the peninsula and can generate heavy precipitation, strong winds, and lightning. They can also result in significant property damage to the local population.
The peninsula occupies a southwestern extension within the Sonora Desert of North America (Warner 2004). Figure 1 shows the geographical area covering Baja California and adjacent areas of the gulf and the Pacific. The southern peninsula is approximately 700 km in length, is variably narrow in certain areas at 45–200 km, and has a coastal perimeter of more than 2000 km. Among the salient topographic features are the southern (23.0°–24.0°N) and central (24.8°–27.6°N) mountains with maximum elevations above 600 and 900 m, respectively. The ranges are extremely rugged and steeply inclined in many localities. The coastal plain along the west coast is below 300 m and most settlements are located on this plain and along the gulf coast. Figure 1 also shows the location of the network of meteorological stations discussed in this paper.
For this study, general characteristics of late summer are derived from long-term information for the month of September. Table 1 contains climatological data available from the Servicio Meteorológico Nacional (SMN 2004) in Mexico and the data are limited to the southern half of the peninsula (22.9°–28.1°N). Maximum temperatures at sites along the gulf coast exceed 34°C and differences between maxima and minima are less than 12°C. Larger differences (>15°C) occur at inland stations (Ciudad Constitución and Gustavo Díaz Ordaz) and cooler conditions are present at the Pacific station (Isla de Cedros).
September brings a share of the annual rainfall (Mosiño and García 1974). Table 1 shows that accumulations depend on latitude and a maximum (>45 mm) is located south of 25°N. Latorre and Penilla (1988) used the network of rain gauges to document that, in the majority of cases, convective events occur within 1 day and they individually provide less than 5 mm of precipitation. In contrast, there are certain events when more than 50 mm fall and these are associated with approaching tropical cyclones. In their study, from 1960 to 1979, 30 tropical cyclones approached the peninsula within a radius of 250 km and 7 that made landfall brought heavy rainfall and strong winds lasting several days.
In general, tropical cyclones in the eastern Pacific tend to follow long, northwestward tracks until adverse conditions are found north of 20°N (Elsberry 1995). Most systems develop off the western coast of Mexico and become an important source of water resources during the warm season. Englehart and Douglas (2001) investigated 1949–97 systems that provided significant rainfall to a group of stations in the peninsula and found that tracks close to the southern peninsula (<371 km) resulted in accumulations above 75 mm per event. Additionally, cyclones developing farther away (371–550 km) are still able to provide rainfall above 30 mm.
There are few recent studies documenting weather conditions associated with distant tropical cyclones from the Baja California Peninsula. This paper examines an event where the interaction of humid flow and topography results in significant precipitation events. In this case, Tropical Cyclone Linda (2003) is assumed as representative of storms at moderate distance from Baja California where localized convective systems were sufficiently documented to display significant precipitation and contributing topographic influences.
This study describes the following:
the role of tropical cyclone circulation in changing low- to midlevel moisture,
the location, life cycle, and impact of associated convective systems over land, and
the regional weather conditions during the development of convective systems.
Further, the study documents the occurrence of these systems by using the regional network of surface and upper-air data stations, along with satellite and gridded products available in real time. Section 2 provides an overview of data sources. The development of Linda, its environmental conditions, and the occurrence of convection are presented in section 3. Finally, section 4 provides a summary and concluding remarks.
2. Data sources
Digital imagery from the Geostationary Operational Environmental Satellite-10 (GOES-10) is used to document the distribution of humidity and cloud cover. This includes imagery from the water vapor, visible, and infrared channels. Additional information used to explore the three-dimensional structure of the large-scale flow is derived from operational analyses of the Eta Model issued by the National Centers for Environmental Prediction (NCEP) at 40-km resolution. To examine in situ characteristics of the environment over northwestern Mexico (including Isla Socorro) and the southwestern United States, the standard 1200 UTC upper-air observations were analyzed. As a reference, and for the period under review, the conversion from UTC to local time (LT) is −6 h.
Automatic surface stations operated by the SMN and the Secretaría de Marina (SEMAR) are used, and these sites are shown in Fig. 1. Station data are reported at 10-min intervals for SMN and 15-min intervals for SEMAR sites. Observations include sustained winds, air temperature, relative humidity, and precipitation. Raw data from SMN stations are transmitted to management offices every 3 h, which qualifies them as a source of near-real-time observations (Rosengaus 2001). This dataset has also been used in research studies of weather phenomena throughout Baja California, including the analysis of tropical cyclones at landfall over the southern peninsula (Farfán 2004; Farfán and Cortez 2005) and the documentation of Santa Ana conditions to the northwest (Trasviña et al. 2003).
Data coverage is augmented with meteorological reports from airports issued by the Servicios a la Navegación en el Espacio Aéreo Mexicano (SENEAM). To determine spatial distribution and intensity of rainfall episodes, a network of 120 rain gauges is used, which is managed by the Comisión Nacional del Agua (CNA). Observations are available as 24-h totals and the majority of stations are located over relatively low-level terrain. In fact, 69% of the sites are located between 10- and 200-m elevation and the highest station is at 840 m, over the southern mountains. Therefore, the network cannot provide estimates of rainfall at the higher mountain elevations.
3. Observations
a. Overview of moisture (water vapor imagery)
Figure 2 shows the distribution and evolution of mid- to upper-level moisture over northwestern Mexico with the water vapor (6.7 μm) imagery at 1200 UTC. This time coincides with minimum intensity of deep convection that may otherwise contaminate the ready detection of dry and moist areas. Images are shown for 12–20 September 2003, and include precipitable water derived from upper-air stations. Dry conditions were present in Baja California, mainland Mexico, and the southwestern United States on 12 and 13 September (Figs. 2a and 2b, respectively). Increasing moisture was detected during the following 4 days (Figs. 2c–f), with a maximum occurring over the southern part of the peninsula and the Gulf of California. The location and structure of this maximum is related to the development of a tropical cyclone over the eastern Pacific.
According to U.S. National Hurricane Center (NHC) records, Tropical Cyclone Linda developed during 13–17 September (Beven 2003). The initial depression was located southwest of Manzanillo (MAN in Fig. 2b) and reached hurricane intensity at 1200 UTC 15 September with its center positioned 480 km south of Cabo San Lucas (Fig. 2d). At that time, the La Paz sounding estimates precipitable water of 46 mm and they increased to a maximum of 53 mm on 16–17 September. This is in contrast to 36–39 mm before Linda’s approach (12–14 September). While weakening, Linda moved westward and this displacement allowed dry conditions to return to the southern part of the peninsula and adjacent gulf (Figs. 2g–i).
b. Vertical structure of the flow (upper-air soundings)
Analysis of temporal changes in the vertical structure of the flow is derived from La Paz and Guaymas in situ data shown in Fig. 3. The evolution of horizontal winds in La Paz (Fig. 3a) indicates a significant easterly component in the 500–850-mb layer from 13 to 19 September. Specifically, southeasterly winds occurred on 16 and 17 September in conjunction with a well-defined relative humidity maximum at midlevel and near the surface. On 16 September, there are relative humidity maxima at the 567-mb (90%) and 910-mb (85%) levels. These conditions occurred while the eastern edge of Linda was approaching the southern part of the peninsula and were consistent with increased moisture detected by GOES-10 (Fig. 2e). As the remnants of Linda moved away from Baja California (18–20 September), the vertical extent of the dense moisture layer decreased and was replaced by dry, westerly flow that developed at the upper levels.
The vertical extent of the moisture observed at Guaymas is shown in Fig. 3b. From 12 through 14 September, low relative humidity (<25%) occurred at levels above 700 mb with moderate conditions (25%–50%) near the surface; therefore, resulting in low precipitable water of 27–33 mm (Fig. 2). Winds had a significant component from the north in the layers above 700 mb and below 850 mb. On 15–16 September, flow in the layer below 500 mb acquires significant components from the south and east, suggesting advection of moist air that originated in the southern Gulf of California. Estimates of precipitable water rose to the range of 42–46 mm and some moistening occurred near the surface. However, these amounts are lower than the humidity findings from the La Paz soundings, as shown in Figs. 2 and 3a.
c. Large-scale circulation (Eta Model analyses)
The large-scale structure of the flow and moisture are derived from Eta Model analyses for 13–19 September. This covers the early stages of the tropical cyclone and its movement toward Baja California. Figure 4 shows winds and geopotential heights at 500 mb, along with averaged relative humidity in the 500–700-mb layer as an indicator of moisture content. At 1200 UTC 13 September (Fig. 4a), there is anticyclonic circulation off the Pacific coast that provides northwesterly winds to the southwestern United States and northern Mexico, as well as northeasterlies over the southern gulf and peninsula. The air was relatively dry (relative humidity < 50%) in these areas, while moist air (>50%) was confined to central and southern Mexico.
This pattern is modified during the following days. On 16 September (Fig. 4b) the flow over Baja California came from the south with the advection of humid air from the eastern flank of Tropical Storm Linda. In contrast, dry and westerly flow remained over the southwestern United States, which is similar to the distribution of moisture from satellite data (Fig. 2e). The mass of moist air remained over the gulf for several days (Fig. 4c). From 18 to 20 September, Tropical Cyclone Marty was located southwest of Manzanillo and provided additional moisture (Figs. 2h and 2i). Marty later made landfall near Cabo San Lucas (Farfán and Cortez 2005).
Information on the three-dimensional structure of the flow comes from fields at the 200-mb level and the lowest 30 mb above the surface at 1200 UTC 16 September (Fig. 5). The distribution of horizontal winds at 200 mb (Fig. 5a) shows the presence of moderate (10–30 m s−1) westerly winds over northern Mexico, including the Baja California Peninsula. This pattern reflects favorable conditions for upper-level outflow upon development of deep convection. Low-level relative humidity (Fig. 5b) was above 70% over a large area in the southern part of the peninsula, with westerly to southwesterly winds along the Pacific coast. A vertical cross section is constructed through the southern part of the peninsula and adjacent gulf (Fig. 5c) that shows the presence of humid (>80%) flow up to the 925-mb level over the mountains. A similar pattern was present in the 17 September fields (data not shown).
d. Mesoscale convection (visible imagery and rain gauge network)
To explore characteristics of convection during the increase of moisture, a discussion of developments detected from GOES-10 is provided. The visible imagery is available at 15-min intervals and is used to examine the structure and motion of organized cloud clusters. Figure 6 shows a sequence of images at 2000 UTC (1400 LT) and 2300 UTC (1700 LT) on 16 and 17 September, when mesoscale convective systems (MCSs) developed over the southern peninsula. MCSs occurred primarily north and south of La Paz, over the mountains, and a few isolated clusters were observed north of Loreto (26°N). No significant MCSs occurred prior to 16–17 September (12–15 September) or after (18–20 September) during this period.
Animation of the 16 September imagery was used to identify the evolution of key features shown in Fig. 6a. Features include deep convection over Linda’s northeastern quadrant (feature 1) and a cirrus cloud layer from the upper-level outflow (feature 2). Initial formation of cells over land occurs at 1845 UTC (Fig. 8a) near the Pacific coast and west of the southern mountains. At 2000 UTC, the core of convection was concentrated over the northern edge of the mountains (feature 3). The core expanded northwestward over La Paz from 2100 to 2230 UTC and was centered east of Ciudad Constitución at 2300 UTC (Fig. 6b, feature 5). Note that the area of initial convection remained active (feature 4) and seemed to have no motion. Infrared imagery (not shown) was used to identify anvil clouds over the gulf (feature 6); their position to the east is explained by the presence of westerly winds at upper levels (Fig. 5a). By sunset (0100 UTC 17 September), the MCSs had weaken and dissipated.
In summary short-lived, afternoon MCSs developed over the western slopes of the southern mountains, across the low-terrain near La Paz, and over part of the central mountains on 16 September. The location of initial cells suggests the intrusion of moist air from the Pacific and upslope flow at low elevations. Mean storm motion is consistent with southeasterly winds at middle levels from the morning sounding (Fig. 3a). Because the moist air remained in the area, additional systems developed the next day and convection developed to include the gulf. Cells formed near the Pacific coast (Fig. 8b) and around the southern mountains. At 2000 UTC, features 1 and 2 in Fig. 6c show localized MCSs over land and, as in the previous day, they grow to cover low-level terrain. The La Paz airport reported a thunderstorm at 2200 UTC (1600 LT) to the south-southeast, near the southern mountains. At 2300 UTC (Fig. 6d), a system was still active on the Pacific coast north of Cabo San Lucas (feature 3) and there are signatures of anvil clouds over the adjacent gulf coast (features 4 and 5).
Rain gauge data are shown in Fig. 7. Rainfall on 16 September (Fig. 7a) reveals distinct areas west of the mountain ridges with heavy precipitation, a maximum accumulation of 42 mm. This maximum is associated with the MCSs identified from satellite imagery (Figs. 6a and 6b). A similar situation occurred on 17 September (Fig. 7b) when up to 85 mm is recorded by a station north of Cabo San Lucas. This is related to feature 3 in Fig. 6d. In contrast, only one report of measurable rainfall (17.5 mm) comes from a station east of the mountains and north of San José del Cabo. As expected, most stations on the low-lying plains reported limited amounts or no precipitation at all.
To understand thermodynamic conditions in the atmosphere that supported the development of the convective outbreaks, an inspection of instability parameters from soundings at La Paz was performed for 15–18 September; results are summarized in Table 2. These data represent conditions approximately 7–12 h prior to the development of afternoon convection. The elements that distinguish convectively active days are the simultaneous occurrence of 1) high levels of precipitable water (>50 mm), 2) high CAPE (>330 J kg−1), 3) an unstable atmosphere [lifted index (LI) < –1.5°C], and 4) a nearly saturated report (>80% relative humidity) at 925 mb. Because these criteria were satisfied on 16 and 17 September, 29.8 and 11.5 mm were recorded, respectively, at a surface station collocated with the upper-air site (CNA 2003). More rainfall was recorded on 16 September and this is explained, in part, by the proximity of storms to the station and the longer duration of MCSs. This is also consistent with the absence of MCSs in the satellite imagery, no precipitation at the station on 15 and 18 September, and atmospheric conditions that did not meet the criteria listed earlier.
e. Analysis of weather conditions (surface data)
Despite the limitations of the network of stations, it is worth examining the data for the active period, 16–17 September. In Fig. 8, regional weather data (temperature, dewpoint, and wind speed and direction) are displayed in the satellite imagery for 1900 UTC (1300 LT). This is representative of conditions during the early development of MCSs.
Figure 8a shows humid conditions prevailing at La Paz and Cabo San Lucas, both with dewpoints of 26°C, in contrast with a lower dewpoint (24°C) 24 h earlier (1900 UTC 15 September). Drier conditions (dewpoint 23°C) occurred inland at San José del Cabo and Ciudad Constitución. Winds near La Paz are from the north-northwest, suggesting an onshore sea breeze directed toward the southern mountains, while winds at Cabo San Lucas are from the south. Convergence over the western slopes is likely, supported by the line of convective cells shown in Fig. 8a. Conditions during the early development of MCSs on 17 September are shown in Fig. 8b, with upslope humid flow over the mountains and the formation of cells around them.
Time series from the (SEMAR) automatic station north of La Paz are useful in analyzing changes that occurred prior to and during the period of interest (Fig. 9). Rainfall equivalent to 7.1 mm was recorded from 2100 to 2300 UTC on 16 September (Fig. 9a) and this was preceded by 2–5 m s−1 winds from the north for 3 h. A significant decrease of 7°C in temperature is likely the result of cool air advection by the storms’ low-level outflow. For MCSs on 17 September (Fig. 9b), the temperature drop is 6.2°C from 2045 to 2245 UTC and northwesterly flow occurred at the time convective cells were detected in the satellite imagery (Fig. 8b). This flow is associated with a sea breeze and contrasted with early morning and evening winds from the south. The sea breeze provides a favorable condition for advection of moist air from the gulf in the early afternoon, which may be subjected to lifting over the southern mountains. According to Robles (1998), the sea breeze is part of a normal circulation starting just before noon during the summer, July through September, and reaches maximum intensity from 1300 to 1500 LT.
4. Summary and concluding remarks
The goal of this study was to characterize convective systems over the southern part of the Baja California Peninsula during Tropical Cyclone Linda in the eastern Pacific Ocean. An examination of satellite imagery, upper-air soundings, and gridded analyses showed changes in low- to midlevel moisture. Linda developed and affected the peninsula from 13 to 17 September 2003. While centered southwest of Cabo San Lucas, convective outbreaks occurred over the mountains on 16 and 17 September. Available data indicate that enhanced moisture, from Linda’s eastern flank, was advected by the large-scale flow into the peninsula and the Gulf of California.
The following method was applied to analyze this case: 1) development of the tropical cyclone and associated moisture were detected with GOES-10 water vapor imagery, 2) the structure of flow and humidity at low to midlevels was identified from regional observations and (Eta Model) operational analyses, 3) data from La Paz morning soundings were used to determine parameters that characterized the local environment (precipitable water > 50 mm, CAPE > 330 J kg−1, lifted index <–1.5°C, and relative humidity > 80% at 925 mb), and 4) initiation of convective cells occurred over the mountains in the early afternoon, which were monitored by high-resolution (1 km and 15 min) visible images. The mean motion of the convective storms was consistent with winds at middle levels.
Development of MCSs occurred in conjunction with heavy precipitation and was active for several hours on two consecutive days. This information was derived from the existing network of surface stations, including automatic sites, airport sites, and rain gauges. Even though spatial coverage was limited, data analysis suggests that interaction of low-level flow with topography resulted in episodes of localized, afternoon convection. That is, the formation of MCSs was associated with areas of high moisture content and convergence along the western slopes of the southern mountains. The scale of MCSs (30–50 km) and duration (6 h) were less than those known for similar systems over the Sierra Madre Occidental (e.g., Farfán and Zehnder 1994). This is partly explained by the limited width (<100 km) of the mountain ranges in the Baja California Peninsula. The systems provided daily totals of precipitation ranging from 25 to 85 mm. This fact is consistent with documented rainfall amounts from tropical cyclones developing at great distances from the peninsula (Englehart and Douglas 2001).
Time series from the automatic station near La Paz were useful in determining changes associated with the development of the MCSs. This includes rainfall rates and cooling from the storm’s outflow. The series were also useful for observing features in the diurnal cycle of airflow that are expected to enhance moist air convergence. Specifically, a sea breeze was associated with a significant wind component from the north and remained active for 3–5 h in the early afternoon.
Results are derived from a single case study and may not occur in other instances of convective outbreaks. To generalize these results, the author will be analyzing several years of observations to identify similar cases in this area. Among the specific aspects to investigate are 1) a classification based on tropical cyclone track, intensity and size, and distance from the peninsula; 2) characteristics of location, intensity, and structure of the MCSs; and 3) a set of sounding parameters known to occur prior to the development of MCSs. These are critical factors to be considered when assessing the behavior of MCSs associated with enhanced moisture during tropical cyclone development. The broader study should also lead to useful recommendations for operational forecasters.
Availability of additional stations and observations will provide better documentation of tropical cyclone–topography interactions over the peninsular mountains. Data collected from the field phase of the North American Monsoon Experiment (NAME Project Office 2004) offer an enhanced set of observations and research products covering the Baja California Peninsula, Gulf of California, and mainland Mexico. This dataset will also document characteristics of the large-scale flow during the approach of Pacific storms. Additionally, the application of numerical simulations with mesoscale models will be a tool helpful in identifying the specific mechanisms active during the onset and organization of convective cells over the mountains. The inclusion of high-resolution grids should provide a detailed representation of low-level flow through the diurnal cycle and its role during MCS development.
Acknowledgments
Support for this study was provided by CONACYT (Grant SEP-2003-C02-42942) and CICESE. The author thanks Miguel Cortez (SMN) for providing automatic station and rainfall data, Capt. Juan M. Aguilar (SEMAR, Dirección de Meteorología Marina) for automatic station data, and Joaquín H. Rodríguez (SENEAM, Dirección de Meteorología y Telecomunicaciones Aeronáuticas) for airport reports. The author wishes to thank the anonymous reviewers for comments that improved the manuscript. Ira Fogel edited the final manuscript and provided additional comments.
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Elevation of southern Baja California and meteorological observation network. Topographic contour intervals (m) are in the vertical bar. Selected communities are indicated.
Citation: Weather and Forecasting 20, 5; 10.1175/WAF879.1
GOES-10 water vapor imagery at 1200 UTC 12–20 Sep 2003. Precipitable water (mm) derived from the network of upper-air soundings is indicated. Circles indicate (black or white 0) the best-track center of Tropical Cyclones Linda (15–17 Sep) and Marty (20 Sep).
Citation: Weather and Forecasting 20, 5; 10.1175/WAF879.1
Time series of winds and relative humidity at (a) La Paz and (b) Guaymas. Soundings taken at 1200 UTC 12–20 Sep 2003. Relative humidity (%, isopleths) is shaded and changes are indicated by the vertical bar at right border. Vertical axis represents pressure level (mb), horizontal axis shows time (UTC); full wind barbs are 5.0 m s−1 and half barbs 2.5 m s−1.
Citation: Weather and Forecasting 20, 5; 10.1175/WAF879.1
Winds and geopotential heights (dashed lines, dam) from the Eta Model analysis at 500 mb and 1200 UTC for (a) 13, (b) 16, and (c) 19 Sep 2003. Mean relative humidity (solid lines, % isopleths) for the 500–700-mb layer is shaded by levels above 25% (light), 50% (medium), and 75% (dark). Black dots (•) are best-track centers of Tropical Cyclones Linda and Marty. Full wind barbs represent 5.0 m s−1 and half barbs 2.5 m s−1.
Citation: Weather and Forecasting 20, 5; 10.1175/WAF879.1
Winds from the Eta Model analysis for 1200 UTC 16 Sep 2003 for (a) 200 mb with shading representing isotachs (m s−1), (b) the lowest 30 mb with averaged relative humidity (%, isopleths) shaded, and (c) cross section along the southern peninsula for the 1000–700-mb layer with relative humidity (%, isopleths) shaded. Edge locations (A and B) for cross section in (c) are indicated in (b) and the magnitude of the zonal wind component (m s−1) is represented with thick solid (positive) and dashed (negative) lines. Full wind barbs represent 5.0 m s−1 and half barbs 2.5 m s−1. Black dot (•) indicates best-track center of Tropical Cyclone Linda.
Citation: Weather and Forecasting 20, 5; 10.1175/WAF879.1
GOES-10 visible imagery at (a) 2000 UTC 16 Sep, (b) 2300 UTC 16 Sep, (c) 2000 UTC 17 Sep, and (d) 2300 UTC 17 Sep 2003. Numbers and plus signs indicate reference locations discussed in the text.
Citation: Weather and Forecasting 20, 5; 10.1175/WAF879.1
Rainfall (mm) recorded by network of rain gauges for two 24-h intervals: (a) 1500 UTC 16 Sep–1500 UTC 17 Sep 2003 and (b) 1500 UTC 17 Sep–1500 UTC 18 Sep 2003. Selected maxima are denoted with values in parentheses. Elevation contours (solid lines) are 300, 600, and 900 m.
Citation: Weather and Forecasting 20, 5; 10.1175/WAF879.1
GOES-10 visible imagery at 1845 UTC (a) 16 Sep and (b) 17 Sep 2003. Surface observations at 1900 UTC are temperature (top, °C), dewpoint (bottom, °C), and wind direction and speed (barbs, m s−1). Full wind barbs represent 5.0 m s−1 and half barbs 2.5 m s−1.
Citation: Weather and Forecasting 20, 5; 10.1175/WAF879.1
Time series from two 24-h automatic observations near La Paz: (a) 1200 UTC 16 Sep–1200 UTC 17 Sep and (b) 1200 UTC 17 Sep–1200 UTC 18 Sep 2003. Sustained winds (barbs, m s−1), temperature (°C, solid line), and dewpoint (°C, dashed line) are indicated. Full wind barbs represent 5.0 m s−1 and half barbs 2.5 m s−1. Asterisks represent starting and ending times of sea breeze. Dot pattern in top panel denotes precipitation intensity at 15-min intervals (1 dot, 0.25 mm; 2 dots, 1.02 mm; 3 dots, 1.52 mm; 4 dots, 2.54 mm). Straight dash symbols represent precipitation from a rain gauge collocated with the upper-air site.
Citation: Weather and Forecasting 20, 5; 10.1175/WAF879.1
Mean meteorological parameters for the month of Sep 1961–90 and selected stations in Baja California.
Parameters derived from the 1200 UTC upper-air sounding at La Paz from 15 to 18 Sep 2003.
