Precipitation I: Processes and measurement

Transcription

1 Precipitation I: Processes and measurement

2 For precipitation to occur, you must have water vapor Saturation vapor (e s ) pressure over water The amount of water vapor that the atmosphere can carry (the saturation vapor pressure, e s ) is a function of temperature. The saturation vapor pressure does not increase linearly with temperature but rather exponentially through the Clausius- Clapeyron equation Magnus-Tetens formula a good approximation: e s (T)= exp[( t)/(t )] Having water vapor in the atmosphere is a necessary but insufficient condition for precipitation. There must also be a mechanism to promote uplift, cooling and condensation.

3 Precipitation requirements and processes Condensation occurs when the temperature is the dewpoint T d. Without small particles in the air (e.g., dust, soot, sea salt, clay, sulfate, phytoplankton) to act as cloud condensation nuclei (CCN), the air can become supersaturated. The number of cloud condensation nuclei in the air ranges between around 100 to 1000 per cm 3. Typical CCN = 0.2 μm Typical cloud droplets are μm Rain drops are around 2000 μm - 2 mm Downward velocity must exceed uplift velocity for precipitation to fall. Uplift velocities in convective storms can be large.

4 Relative sizes of CCN, cloud droplets and rain droplets

5 Precipitation requirements and processes (cont). Uplift and cooling to the dew point Condensation Growth of droplets (two basic processes) 1) Bergeron-Findeisen theory (top figure): If ice and supercooled water droplets exist together in a cloud, the ice crystals grow at the expense of water droplets, the reason being that the saturation vapor pressure over ice is lower than over water. Solid precipitation that falls may (or may not) melt at a lower, warmer atmospheric level to become rain. 2) Coalescence (bottom figure): Occurs in warm (liquid) clouds. Falling droplets have terminal velocities directly related to their diameter, such that the larger falling drops overtake and absorb smaller drops, the smaller drops can also be swept into the wake of larger drops and be absorbed by them. Key processes that provide uplift, cooling and condensation are convection, frontal uplift and forced ascent over orographic barriers.

6 Bergeron-Findeisen theory (cont.) Arapahoe Basin May The process of vapor diffusion is most efficient at around -12 to -15 o C, corresponding to the temperature of largest difference between water and ice saturation vapor pressures. Different combinations of temperature and supersaturation determine the type of snowflake that forms.

7 Precipitation requirements and processes (cont.) Import of water vapor is important to maintain precipitation Consider a thunderstorm For a column within the thunderstorm of unit area (m 2 ), the volume = 10,000 m 3 Typical average water mass is 0.5 g m -3 = 5000 g water in column = 5000 cm 3 /10,000 cm 2 = 0.5 cm precipitation, which is not much Thus to maintain precipitation there must be an influx (entrainment) of vapor-rich air to replace the water that falls from precipitation.

8 Precipitation quality Precipitation is not pure, but rather contains a number of ions. The straight lines indicate concentration ranges for continental rain, while the wavy lines indicate concentrations for marine rain. Ranges found in high pollution areas are indicated by dashed lines. Dingman 2002 Figure 4-53

9 Convective precipitation Convective precipitation tends to be localized. The typical convective storm occurs when local surface heating makes air parcels warmer (less dense) than their environment, such that they rise and condense. Keys to convective storm development are low atmospheric stability and ample atmospheric water vapor. The stronger the decrease in temperature with height (the more negative the environmental lapse rate), and the more water vapor that is available, the more favorable the conditions for convective storms.

10 Lapse rates and stability Environmental lapse rate: The vertical rate of change of temperature with height that we would measure with a thermometer Is usually negative (temperature decreases with height), the reason being that the atmosphere is heated primarily from the surface and rising air cools. An increase in temperature with height is called an inversion. Typical values are -6.5 o C km -1 Dry adiabiatic lapse rate (DALR): The rate of cooling of a rising parcel (a given mass) of air due to expansion and doing work against the environment. The DALR is -9.8 o C km -1 Saturated (wet) adiabatic lapse rate: The lapse rate after condensation occurs. Condensation releases latent heat, such that the saturated adiabatic lapse rate is les than the DALR. Value varies; a typical value is -5 o C km -1 A rising air parcel cools and expands. If the temperature of the parcel is greater than (less than) the temperature of the surrounding environment, it is less (more) dense than the environmental air and will continue to rise (it will fall). If condensation occurs, then the latent heat release makes the parcel warmer (hence less dense) than it would otherwise be, making it more likely that it will stay warmer than the surrounding environment and continue to rise. Condensation may result in precipitation. Key: If the environmental lapse rate exceeds the DALR, the situation is unstable and the parcel will rise.

11 Tropical cyclones NOAA composite satellite image of Hurricane Katrina Dingman 2002 Figure 4-4 Tropical cyclones feed on evaporation and latent heat release. They form between 5-20 o latitude in each hemisphere where the sea surface temperature is at least 27 o C. The grow through a process called CISK (Conditional Instability of the Second Kind) through which rising air, condensation and latent heat release results in a drop in surface pressure, causing inflow (converging winds) and more evaporation, further latent heat release, and a further drop in surface pressure. Rainfall totals can be impressive.

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13 48 hour precipitation from hurricane (and tropical storm) Irene National Weather Service

14 Frontal precipitation Dingman 2002, Figure 4-2 Frontal precipitation is associated with travelling extratropical cyclones (low pressure systems). They tend to be strongest in winter. Precipitation is associated with uplift, cooling and condensation in warm fronts, cold fronts and occluded fronts. Extratropical cyclones should not be confused with tropical cyclones (hurricanes, typhoons).

15 Surface weather analysis from the Canadian Meteorological Center for Nov. 11, 2010 showing locations of cyclones and their associated fronts, as well as anticyclones

16 Traveling cyclones and anticyclones From here (screen shot above): Click on anim for either the 00z run of the 12z run This starts an animation from a numerical weather prediction model (Canadian Climate Center) that shows traveling eddies. You loop through the forecasts through 144 hours (six days). Look especially at the upper right-hand panel, showing sea level pressure (SLP) and the hpa thickness. Keep looking to get a feel for how the eddies behave.

17 Vertical structure of cyclones and anticyclones The low (cyclone) at the surface is located to the east of the shortwave trough at higher levels. The is necessary to place the surface low under an area of mass divergence aloft, evacuating air. In turn, this is compensated by convergence near the surface, with rising motion in between (fostering cloud formation and precipitation). If the upper level divergence exceeds the lower level convergence, surface pressure falls. Similarly, the anticyclone at the surface is located to the east of the ridge in the shortwave at higher levels, necessary to place the surface high under an area of mass convergence. There is compensating divergence at low levels, with sinking motion in between, fostering clear skies and fine weather.

18 Temperature (thickness) advection Temperature advection is the primary process that cyclones and anticyclones intensify. The temperature advection amplifies the shortwave, making the upper-level patterns of divergence and convergence stronger. Initially (a), streamlines and isotherms parallel each other (the atmosphere is barotropic). In (b), the shortwave has cause the streamlines to cross the isotherms west and east of the trough (the atmosphere there is now baroclinic). In the baroclinic region west (east) of the trough, cold (warm) advection is occurring. Along with amplifying the wave, cold-air advection west of the trough will produce sinking motion as the cold air descends to the surface behind the cold front, while warm-air advection east of the trough will produce rising motion near the center of the low. In (c) the temperature advection is cut off and the cyclone occludes.

19 Patterns of extratropical cyclone activity Extratratropical cyclone frequency (left) and tracks (right) based on a detection and tracking algorithm applied to data from the NCEP/NCAR reanalysis for the winter of 1989/1990. (

20 Sea level pressure pattern for October 24, 1997, showing a memorable upslope storm that affected Boulder CO With a cyclone centered over the Oklahoma panhandle, winds over the eastern plans of Colorado have a component from east to west, moving upslope, promoting cooling, condensation and precipitation. The system draws in water vapor from the Gulf of Mexico.

21 Orographic precipitation and chinooks Because of the latent heat release during ascent, the air at the top of the mountain barrier is warmer that it would have been without the latent heat release. As it descends on the leeward side, it warms at the adiabiatic lapse rate, and arrives at a higher temperature than it had before the ascent process. These warm, leeside conditions are often associated with strong gusty winds called chinooks. Orographic precipitation results from ascent of air over a mountain barrier, resulting in adiabiatic cooling, condensation and precipitation on the upwind side of the barrier. The leeward side experiences a rain shadow. This is in large part why the west slope of the Front Range of Colorado receives more precipitation than the plains to the east. However, the plains can receive considerable precipitation from upslope storms when a low pressure system lies to the south. These are most common in winter. Some notable temperature changes linked to chinooks: Loma, MT: -56 o F to 49 o F in 24 hours Spearfish SD: -4 o F to 45 o F in 2 minutes!

22 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

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24 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.

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26 Precipitation gauges There are many types of gauges, which vary in their catch efficiency. Shielded gauges, like the one above, tend to perform better than unshielded gauges. The liquid water equivalent of snowfall is especially hard to measure.

27 Wyoming snow gauge Wyoming snow gauges are designed to provide accurate measurements of snowfall water equivalent. The one pictured above is located on the drainage divide of Imnaviat Creek, near Toolik Lake, on the North Slope of Alaska. Photo by M. Serreze.

28 Typical SNOTEL sites (summer) SNOTEL (SNOwpack TELemetry) is an automated system to measure snowpack water equivalent. There are over 600 SNOTEL sites across the western U.S. Snow pillows measure the weight of the overlying snowpack. SNOTEL sites complement snowcourses, where snow water equivalent is measured manually.

29 Snotel Sites From Serreze et al., 2001

30 Areal or gridded estimates of precipitation A problem often faced in hydrology is using point measurements of precipitation to come up with a regional average (such as for a watershed) or gridded field of precipitation. This generally requires interpolation. While there are many types of interpolation, ranging from a simple average of all stations within a given region to inverse distance weighting to optimal interpolation, they pretty much all boil down to the following: P interp = Σ (P i.w i )/Σ w i Where P interp is the regional or grid point value we wish to interpolate to, P i are measurements at each precipitation gauge (as in the watershed shown at right) and w i are the interpolation weights. Simply phrased, the interpolated value is the sum of the station values times the weights divided by the sum of the weights. The key is how one determines the weights. Dingman Figure 4-20 A practical note: If there is a dense station network, all interpolation techniques work well. If the network is sparse, none of them work well.

31 Station Density: Contiguous U.S The station density from the U.S. cooperative network looks rather dense, but at the scale of medium of small watersheds is can be quite sparse. The country with the densest station network is Israel, followed by the UK.

32 The Arctic: A very sparse network 3 From Serreze and Barry, 2005 The figure at left shows monitoring stations north of 40 o N with at least ten years of record for the period Note the very sparse network in high northern latitudes. The situation over the Antarctic continent is pathetic. Coverage is also very sparse over large areas of the world s ocean.

33 Getting the weights: Examples One can develop a function based on the distance decay of the correlation in precipitation. (see the left hand figure). While attractive, the problem is that the correlation length scale of precipitation tends to be short, especially in mountainous terrain. One can also make use of relationships between precipitation and elevation (hypsometric method). A problem is that in mountainous terrain, relationships between precipitation and elevation can be complex. Dingman 2002 Figure 4-27 Dingman 2002 Figure 4-28

34 An alternative: The aerological approach W/ t = time change in precipitable water (column water vapor) W/ t = ET - P - Q ET = Evapotranspiration P = Precipitation Q = Vertically-integrated vapor flux divergence W/ t Q Rearrange: P- ET = - Q - W/ t P ET Key: While P and ET may be hard to measure over large areas, we can get the net precipitation (P-ET) using atmospheric winds and humidities.

35 Annual P-ET from the aerological approach Aerological estimates of mean annual precipitation minus evapotranspiration (P-ET) based on NCEP/NCAR data for the period (mm) for the region north of 60 o N. Contours are at every 100 mm up to 500 m (negative values dashed) and at every 200 mm for amounts of 600 mm and higher [from Serreze and Barry, 2005]. Annual P-ET is positive everywhere except locally in the Norwegian Sea and southern Barents Sea where evaporation rates are high from autumn through spring (because of cold, dry air blowing over a fairly warm open ocean).