SR/SSD 97-40

09-01-97

Introduction

Puerto Rico is an island territory of the United States located in the Caribbean Sea bounded by 65.6 W to 67.25 W and from 17.9 N to 18.5 N. The Commonwealth of Puerto Rico contains a brick-shaped main island, measuring 180 x 65 km, and five nearby smaller islands. Although only 8,897 km² in area, smaller than Connecticut, Puerto Rico is topographically diverse. Much of the interior of the main island is a mountain range, with its spine running east-west along its length. The highest peak in this range, Cerro de Punta, is 1,338 m (4,389 ft) above sea level. Foothills comprise the area surrounding this central range, and give way to a coastal plain in the northern and southern parts of the island. In the east and the west, foothill valleys extend finger-like to the sea. There exists a divide between the central range, or Cordillera Central, and a small but steep range to the northeast. This isolated range, Sierra de Luquillo, contains the peak El Yunque, which acts as an important convection point for rainfall. Figure 1 is a smoothed contour map of elevation.

Puerto Rico's varying topography over a small area in the Caribbean Sea leads to stark land, sea and air interactions. Easterly trade winds in the tropics prevail over Puerto Rico during much of the year. During the summer, San Juan, which lies on a narrow peninsula on a northern coastal plain, experiences a wind from the east, northeast, or southeast about sixty percent of the time (Pico 1974).

Convergence of the sea breeze with the prevailing easterlies often leads to rainfall as the air is forced rapidly upward. In addition, topography induces orographic rain. On the windward side of mountains, moist air is forced up the slope where it cools and condenses, leading to precipitation. These peaks also provide convection points of instability. Insolation heats the peaks much faster than the valleys below which are shaded by the mountains (Pico 1974). Resulting towering cumulus and cumulonimbus clouds are common over the interior during most days, but particularly in the summer.

Surface pressure waves (easterly waves) that originate off the west coast of Africa and propagate in the tropical trade belt also bring rainfall to Puerto Rico. These are primarily summertime phenomena, and number between fifty and seventy annually. Not all waves bring rain to Puerto Rico, and some may in fact inhibit mesoscale diurnal rainfall effects. Other waves develop into depressions, tropical storms, and hurricanes, which bring copious amounts of rain.

The tropical upper-tropospheric trough (TUTT) is a climatological feature that exists over the tropical Atlantic during the summertime. Cold core, upper-level cyclones (Kousky and Gan 1981, Kelly and Mock 1982) often originate along the TUTT axis and can influence rainfall over Puerto Rico (Frank 1970). These upper-level lows with their direct thermal circulation and absence of dissipative mechanisms can last for several days to weeks. Precipitation amounts associated with these lows are proportional to their vertical extent, which on occasion reach down to the surface (Frank 1970). During the wintertime, convective rainfall is often associated with frontal systems from the north.

Tropical convection is notoriously difficult to forecast. However, the value of such forecasts is large because of the potential for flooding and mudslides. This is particularly important in the tropics where rainfall can be locally intense. A conditionally unstable atmosphere and an abundance of low-level heat the moisture combine with forcing mechanisms, such as sea breeze fronts, to explain the contingency of tropical rainfall. As a step toward developing quantitative precipitation guidance for forecasting rainfall events in Puerto Rico, we provide a monthly rainfall climatology.

Data and Methodology

Data for this study come from the National Weather Service Cooperative precipitation network and was made available through CD-ROM prepared by the National Climatic Data Center in Asheville, NC. Daily rainfall totals for stations over the main island were available for the period 1980-89. In order to make the climatology more representative of expected values, we excluded rainfall from tropical storms and hurricanes. Daily values were summed over all days during a particular month for each station for which data were available for more than 15% of the entire period.

Objective analysis is the procedure of transforming data from observations at irregularly spaced points into data at points of a regularly arranged grid. Here the regular grid is the intersection of latitudes and longitudes at 1/16 intervals. We use 12 grid points in the meridional direction and 29 in the zonal direction to cover the island. The lower-left (or southwest) grid point is 17.8750N latitude and 67.3125W longitude. In the Barnes' objective analysis scheme (Barnes 1964) the value at grid point i,j is given by a weighted average

(1)

where f_k is the kth observation and W_i,j,k are the weights. The weights are given as

(2)

where R is the radius of influence and d is the Euclidean distance between the kth observation and the grid point i,j. The number of stations (M) included in the weighted average is determined by the radius of influence R. For each grid point all stations with a distance less the R are used in the summations for both the numerator and denominator of the weighted average (eq. 1). Initially R is set at 20 km, but if M is less than 3, then R is incremented by 3 km.

Using three passes through the weighted average scheme, whereby the errors at each station--calculated as the average of the four nearest grid points--are corrected with the same weighted average formula, gives final errors of less than 1 km. The gridded analyses are contoured using NCAR graphics.

Results

To understand the seasonal distribution of rainfall, we must construct climatologies on a time scale less than a year. Puerto Rico has a tropical climate, so seasonal changes in temperature and precipitation are much more subtle than in the mid-latitudes. Therefore, month to month variations in rainfall are important to the capacity of the island to retain soil moisture without reaching flood levels.

The mean monthly rainfall maps are shown in Figs 2 and 3. Figure 2 contains the months January through June, and Fig 3 displays July through December. The most notable feature is the persistence of rainfall over the Sierra de Luquillo on the eastern part of the island. This mountain range contains the Caribbean National Rain Forest, and receives copious rain throughout the year. This is especially true during the summer when solar insolation enhances convection. The annual persistence of rainfall over the Sierra de Luquillo, however, can be explained by orographic condensation as the easterly trade winds quickly ascend the steep slopes of eastern Puerto Rico, dropping rain on the forest below.

The winter months of December to March remain dry over much of the island. Beginning in April, the Cordillera Central begins to receive more rainfall, and the arid regime becomes confined to the southern coast. The Cordillera Central and Sierra de Luquillo shield the southern coast from the prevailing easterlies. Not only do these mountains inhibit sea breeze convergence with the easterlies, but they also relegate the southern coast to the lee side of these ranges where trade-wind-lifted orographic rain rarely occurs.

In June and July, the central third of the island dries out, only to give way to copious rainfall in September and October. In early summer, the rainfall maxima are found over the extreme western and eastern parts of the island, the latter associated with the persistent orographic effect over the Sierra de Luquillo. The summertime maximum near Mayaguez at the western end of the island is due to the combined effect of a standing internal gravity wave on the lee edge of the Cordillera Central and sea breeze convergence with the easterly trade winds.

October is wet over nearly all of the island, with monthly rainfall means of less than 5.0 in confined to a very small region of the south-central coast. By November, the influence of passing easterly waves subsides, and the Cordillera Central receives much less rainfall than in preceding months. By December, half of the island receives less than 4.0 in on average, and by January only isolated mountainous regions on the island's eastern end exceed a mean monthly value of 4.0 in.

The 95th percentile of annual precipitation over Puerto Rico is shown in Fig 4. In 19 years out of 20, any given place on the map will have at least the amount of rain shown. Easily seen are persistent features of the Loquillo mountains orographic lifting and the relative aridity of the southern coast (Galinanes 1977). While annual maps reveal the persistent features of rainfall, it is the monthly climatologies that describe the seasonal variance important to day-to-day forecasting.

Conclusions

Monthly rainfall climatologies are an important first step toward quantitative precipitation forecasting (QPF) in the tropics. Mid-latitude weather systems penetrate into the Caribbean only a few months of the year. Rainfall in Puerto Rico is primarily a function of local effects, passing easterly waves, and tropical cyclones. Monthly climatologies reveal how these effects act to vary rainfall throughout the year. Under the continued assistance of the Cooperative Institute for Tropical Meteorology (CITM) at Florida State University, our Puerto Rico rainfall studies will progress from monthly climatologies to QPF assistance, analysis of extreme rainfall events, and further designation of rainfall regions on the island. Throughout, we are maintaining close contact with forecasters at the NWSFO in San Juan. Additional results from our research may be found on-line at http://www.met.fsu.edu/Grads/carter/prrs/.

References

Barnes, S. L., 1964: A technique for maximizing details in numerical weather map analysis, J. Applied Meteorol., 3, 396-409.

Frank, N., 1970: On the Nature of Upper Tropospheric Cold Core Cyclones Over the Tropical Atlantic, Florida State University PhD Thesis.

Galinanes, Maria Teresa B. De, Editor, 1977: Geovision de Puerto Rico, Universidad de Puerto Rico.

Kelley, W. E. Jr and D. R. Mock, 1982: A Diagnostic Study of Upper Tropospheric Cold Core Lows Over the Western North Pacific, Mon. Wea. Rev., 110, 471-480.

Kousky, V. E., and M. A. Gan, 1981: Upper Tropospheric Cyclonic Tropospheric Cyclonic Vortices in the Tropical South Atlantic, Tellus, 33, 538-551.

Pico, R., 1974: The Geography of Puerto Rico, Aldine Publishing Company, Chicago, 26-33.