Precipitation II: Spatial and temporal variability

Transcription

1 Precipitation II: Spatial and temporal variability

2 Probability density function (PDF) of precipitation The PDF of daily precipitation at a given location typically looks like a gamma function with an extended tail towards the large precipitation events. Looking in turn at a CDF, the majority of annual total precipitation tends to be accounted for by a relatively small number of large events. With such a gamma distribution, The mean precipitation is larger than the median precipitation.

3 Some precipitation extremes Cherrapunji, India, average 1150 cm yr -1, record of 2626 cm yr -1 Austin, Texas, 43 inches of rain in a day (July 1971) 24-hour snowfall record: Silver Lake, CO, 75.8 inches in 24 hours April Georgetown, CO, 63 inches in 24 hours, December 4, 1913 Seasonal snowfall: Mat. Baker, WA, 1140 inches (95 feet), Aridity: Death Valley, CO, average of about 50 mm yr -1 Atccama: About 1 mm yr -1.

4 Annual precipitation for the contiguous U.S. Over the west, precipitation is greatest over high elevations due to orographic uplift. Inter-montane areas in rain shadows are quite dry. The east is well-watered; high precipitation over the southeast is favored by proximity to the Gulf of Mexico

5 The highest precipitation is in the high elevation areas, and much of this is stored seasonally in the winter snowpack. Water resource management In Colorado is critically tied to snowpack water storage.

6 Colorado Precipitation Precip. (Inches) Temp. ( o F) Courtesy Drew Slater

7 Monthly Precipitation in Boulder, Colorado Average annual precipitation in Boulder, Colorado, is just a touch under 20 inches (50 cm), which is actually rather high for the plains towns along the Front Range. Precipitation peaks in May, due in considerable part to convective storms. Precipitation is lowest in January. However, as seen in the figure at right, monthly precipitation is highly variable; for most months, the standard deviation is almost as large as the mean.

8 Precipitation over the Pacific NW Orographic effects are especially pronounced over the Pacific NW. The coastal mountains represent the first barriers to moist airmasses streaming in from the Pacific Ocean. The Olympic Peninsula qualifies as a temperate rain forest.

9 Death Valley Death Valley, situated within the Mojave desert, is the lowest, driest, and hottest location in North America. The lowest point is Badwader, which is 282 feet (86 m) below sea level. Death Valley lies within the rain shadow of the Sierra Nevada range. The figure at right shows precipitation for the city of Death Valley compared to the U.S. average. The average annual precipitation at the Greenland Ranch Station of Death Valley is 1.58 inches (40 mm).

10 Indian monsoon A monsoon refers to a seasonal shift in winds which typically are at least in part driven by seasonal changes in land-ocean temperature contrasts. While monsoon phenomena occur on many parts of the world (including the southwestern U.S), the best known on these is the Indian monsoon. In summer, low pressure develops over land, resulting in an inflow of moist air which promotes precipitation; in winter, there is an outflow from the colder land, promoting dry conditions.

11 Indian monsoon (cont.) The figure at left shows the annual cycle of precipitation at Cherrapunji, India, linked to the seasonal monsoon circulation. The average of over 300 cm in July contrasts with January, then there is almost no precipitation. As mentioned earlier, the average annual precipitation is 1150 cm yr -1, with a record of 2626 cm yr -1. Remember that Boulder Colorado gets about 50 cm per year. raphics/indian_monsoon_rain.jpg

12 Annual precipitation over the Sahel of Africa Joint Institute for the Study of the Atmosphere and Ocean) The Sahel forms a border between the Sahara desert to the north, at about 18 N, and the savannah and tropical forest to the south, at about 15 N. During the summer, when the ITCZ moves northward, the region is affected by the south-westerly monsoon that brings rain. The ITCZ then shifts south, and the predominant winds become northerly (from the desert) resulting in aridity. This region became significantly more arid during the second half of the 20th century, especially after the 60s. The situation reached its peak during the 70s and 80s and in recent years, the trend seems to have reversed (taken from

13 Precipitation recycling What fraction of precipitation that falls within a watershed is due to water that evapotranspires from that watershed and then falls back within the watershed? From the formulation of Brubaker et al. (2003): P L /P = 1/(1+ 2.F + /ET.A) To get a high recycling ratio (P/P L ) you want a large evaporation rate and a small advective moisture term. P = Total precipitation P L = Precipitation of local origin E T = Evapotranspiration A = Area of watershed F + = Vertically integrated vapor flux directed into the watershed (advective moisture term) P E T F+ The ratio is very scale dependant. At the global scale, ALL precipitation is recycled! F+ Dingman 2002 Figure 2-3

14 Precipitation recycling (cont.) A few estimates for the annual ratio from Brubaker et al (1993) Amazon 25% Eurasia 10% Sahel 35% Mississippi Basin 24% Key points: For most regions, the bulk of precipitation is imported in that the water vapor associated with the precipitation comes from outside of the region. However, recycling is important for some areas, such as the Amazon and the Sahel of Africa. The concern is that deforestation in the Amazon will significantly affect the hydrologic cycle there.

15 El-Nino Southern Oscillation (ENSO) ENSO is a coupled atmosphere-ocean phenomenon. El Nino (the warm phase of ENSO) is characterized by unusually warm ocean temperatures in the equatorial Pacific; La-Nina (the cold phase of ENSO) is characterized by unusually cold ocean temperatures in the Equatorial Pacific.

16 Impacts of ESNO on temperature and precipitation El-Nino La-Nina

17 Anomalous precipitation associated with the 1997/1998 El-Nino

18 El-Nino and tropical cyclones Hurricanes do not like wind shear a rapid increase in wind speed with height. Atlantic hurricanes tend to be suppressed during El- Nino conditions, as El-Nino tends to be associated with increased wind shear.

19 Annual frequency of hurricanes and strong hurricanes (intensity of 3-5 on the Saffir-Simpson scale and wind speed exceeding 178 km/h) in the Atlantic, Figure from NOAA

20 The Pacific North American (PNA) teleconnection pattern The positive PNA phase pattern has above-average pressure heights in the vicinity of Hawaii and the intermountain region of North America, and below-average heights located south of the Aleutian Islands and over the southeastern United States, associated with above-average temperatures over western Canada and the extreme western United States, and belowaverage temperatures across the south-central and southeastern U.S. The PNA tends to have little impact on surface temperature variability over North America during summer. Precipitation anomalies include above-average totals in the Gulf of Alaska extending into the Pacific Northwestern United States, and below-average totals over the upper Midwestern United States. The negative phase has broadly opposing anomalies in pressure heights, temperature and precipitation.

21 The North Atlantic Oscillation The positive (left) and negative (right) modes of the NAO. In the positive mode, both the Icelandic Low (marked L) and the Azores High (marked H) are strong, and the Atlantic storm track brings heat and moisture into northern Europe and the Arctic via the North Atlantic. In the negative NAO mode, the Icelandic Low and Azores High are both weak, and the storm track is shunted south.

22 The Arctic Oscillation (AO) or Northern Annular Mode (NAM) The Arctic Oscillation refers to a transfer of atmospheric mass between Arctic latitudes and middle latitudes. When it is positive mode, surface pressure is unusually low over the Arctic and unusually high over middle latitudes. The NAO can be viewed as the Atlantic-side part of the AO.