Hydrometeorology Flashcards
Describe Evaporation / Transpiration:
Evaporation is the movement of water from a liquid to a vapor state, and is the opposite of condensation. Transpiration is the process whereby soil moisture is taken up by a plant’s root system to drive photosynthesis. The combined effect of evaporation and transpiration is often called evapotranspiration, or ET, and is generally constitutes the largest removal of water from the soil water system.
Describe Condensation:
Condensation is the movement of water from a vapor to a liquid state, and is the opposite of evaporation.
Describe Precipitation:
Precipitation is generally described as water falling to the surface of the Earth, either in the form of liquid or frozen water; therefore, it is also known as hydrometeors (hence, meteorology!).
Describe Runoff/Infiltration:
Runoff is the portion of rainfall that does not infiltrate into the soil. As water infiltrates, some water will flow just below the surface. This is called interflow or through-flow.Infiltration is defined as the downward movement of water through the soil surface into the soil profile.
Describe Storage
Storage is the general amount of water in a particular location and it can be calculated using an accounting budget approach. (Infow) – (Outflow) = (Change in Storage)
Describe Groundwater Discharge
Groundwater discharge occurs when water seeps from aquifers into rivers, streams, and lakes.
Where and in what percentages is out ground water stored?
• The oceans store over 97% of the Earth’s water supply in the form of
saltwater.
• Polar icecaps and glaciers account for slightly more than 2% of the Earth’s
water, and comprise the largest percentage of freshwater on the planet.
• Surface water storage in freshwater lakes, ponds, rivers, and streams
account for less than 0.01% of the total water on Earth.
• Groundwater is normally stored within aquifers, which are subsurface
regions comprised of unconsolidated rock and soil particles. Less than
1% of the Earth’s total water is stored as groundwater or soil moisture.
How does water enter and leave out atmosphere?
Water reaches the atmosphere through transpiration, evaporation, and sublimation, meaning that water vapor is the primary form by which water enters the atmospheric system. Water leaves the atmosphere almost solely through precipitation, either solid (snow, hail, etc.) or liquid (rain, dew).
How does surface water become ground water and how does surface water enter the atmosphere?
Infiltration to become groundwater. Evaporation to enter the atmosophere.
What process inhibits water from draining from soil even when there may still be water present?
Capillary tension.
Define wilting point.
There is a point where the tension of the water to the soil particle becomes so tight that the water cannot be used by plant roots. This is called the wilting point.
Name the soil textures. (Hint 3)
Clay, sand and loam.
What is a confined aquifer?
. In confined aquifers the groundwater is restricted by a nonporous or very low porous layer termed an aquiclude and is not in contact with the atmosphere.
What is an unconfined aquifer?
In unconfined aquifers, the groundwater is in contact with the atmosphere through the pores of the overlaying soil. The top of the groundwater is termed the water table.
Define recharge.
Recharge is the introduction of surface water to the groundwater system.
Define withdrawl.
. Withdrawal is the artificial extraction of groundwater through a well or network of wells. When groundwater withdrawal rates are greater than the recharge of water into the ground, a lowering of the local water table occurs.
Name the two ways that a cloud droplet can form.
homogenous nucleation and heterogeneous nucleation.
Define homogenous nucleation.
Homogeneous nucleation occurs when water droplets form by the chance collision and bonding of water vapor molecules under supersaturated conditions. In other words, water vapor molecules bond to other water vapor molecules with no condensation nuclei involved. This is possible under the absence of atmospheric aerosols and the droplets are inherently small with a high degree of curvature.
Define heterogeneous nucleation.
Heterogeneous nucleation occurs when water droplets form on external hygroscopic (a.k.a., water attracting known as condensation nuclei) particles. Heterogeneous nucleation can occur under saturated or minimally supersaturated conditions. Haze forms under unsaturated conditions.
Explain Condensation Nuclei.
The hygroscopic particles over which droplets are formed are called condensation nuclei. When condensation occurs, the condensation nuclei dissolve to form a solution. The resulting solution further reduces the saturation necessary for condensation to occur. This is because the surface area of the droplet becomes populated by solute, not water.
What is required to get supper cooled water to freeze at temps near 0?
Saturation can occur at below-freezing temperatures, but this does not necessarily lead to the formation of ice crystals. In order for water to freeze at temperatures just below 0°C, an ice nucleus is required. The ice nucleus performs a similar role to condensation nuclei. Ice nuclei are far rarer than condensation nuclei. They must have a six-sided structure, just like ice. Ice crystals themselves act as efficient ice nuclei.
How do cloud droplets form, grow in above freezing conditions?
In above-freezing conditions, cloud droplet growth through condensation is the dominant formation processes. Condensation occurs when air is lifted adiabatically past the lifted condensation level (LCL). Above the LCL, most of the water is drawn to condensation nuclei. In general, there is relatively little water but a lot of nuclei. This creates a large number of smaller particles competing for a limited amount of water. Growth through condensation by itself cannot lead to large water droplets; therefore, other processes must come into play.
In warm clouds, what process leads to precipitation?
collision-coalescence
Explain the collision-coalescence process.
The collision-coalescence process depends on the differing fall speeds of different sized droplets. The process begins with droplets falling through a cloud where large droplets fall faster than smaller droplets, and eventually overtake smaller droplets. As the droplets collide they form bigger droplets, which fall even faster.
As a collector drop falls, it only collides with some of the drops in its path. The Likelihood of a collision depends on the size of the collector drop and the size of the drops in its path. The larger the collector drop, the lower the collision efficiency, and vice versa. Efficiencies are low for droplets near the same size. When the droplets have the same terminal velocity, it is difficult for the particles to catch up to each other and collide. Surface tension causes droplets to “bounce” off of one another.
Turbulence can cause collision efficiencies over 100%. This is because falling drops entrains particles from outside of the fall path. The centrifugal force of a spinning cloud tends to sort droplets by size with smaller droplets toward the center and larger droplets on the outside. The larger droplets get slung away from the center which allows it to encounter smaller droplets along as it moves.
When a collector and a smaller drop collide, one of two things can happen: The two drops can bounce apart due to surface tension or the two drops can stick together, forming a single larger
droplet (a.k.a., coalescence). Coalescence is the process of two or more drops combining upon collision. Coalescence efficiency is the percentage of drops that coalesce upon collision. Most collisions result in coalescence and coalescence efficiencies are assumed to be near 100%.
Name the temperature layers and corresponding forms of water present in cool/cold clouds.
-4 C: Water droplets make up the lower portion.
Define a cool cloud and a cold cloud.
Cool clouds are characterized as having temperatures both above and below freezing in the region of precipitation generation.
Cold clouds are characterized as having subfreezing temperatures throughout their entire structure. A cold cloud can be composed of both super-cooled water droplets and ice crystals.
How do ice crystals from in cool/cold clouds? (Hint 2 ways)
homogeneous or heterogeneous ice nucleation
Name the ways that ice crystals grow in cool/cold clouds. (Hint 3)
- Diffusion-deposition (Bergeron process).
- Riming (accretion).
- Aggregation.
Define Diffusion-Deposition (Bergeron) Process.
In the diffusion-deposition process, the coexistence of ice and supercooled water is essential to precipitation development. Saturation vapor pressure over ice is less than that over supercooled water at the same temperature. The water vapor necessary to keep a supercooled water droplet from evaporating is more than enough to maintain an ice crystal. Water vapor is preferentially deposited onto ice nuclei, causing the super-cooled water to evaporate due to lower humidity levels
Define Riming.
When ice crystals fall through clouds, supercooled water droplets freeze onto them. This process is called riming, or accretion. The process leads to rapid crystal growth, increased mass, and increased terminal velocity.
Define Aggregation.
Aggregation occurs when two or more ice crystals join to form a single, larger crystal. This is an important process in the development of frozen precipitation. It occurs most easily when the crystals have a thin layer of liquid water on them; therefore, the process is most efficient at temperature just above 0°C. The liquid water acts as a sort of “glue” to make them stick together. This is why large snowflakes normally occur during warm, early season snow events.
What are the advantages of using radar based QPE and what relationship does the radar use to estimate rainfall?
Radar is a remote sensing QPE tool with excellent spatial and temporal resolution. Despite the regional and seasonal inconsistencies in radar-derived precipitation estimates, radar guidance is generally considered superior to satellite guidance of QPE. The is primarily due to the superior spatial and temporal resolution and overall better quantitative guidance.
Radar reflectivity (Z), expressed in units of dBZ, is used to compute rainfall rates (R) in mm/h using a reflectivity to rainfall rate relationship, known as a Z-R relationship.
What factors influence the Z-R relationship? (Hint 4)
· Droplet size.
· Droplet size distribution (DSD).
· Phase of the droplets (liquid or solid).
· Droplet shape.
Name some limitations of using radar to estimate QPE. (Hint 8)
Radar coverage may be inconsistent from place to place and from storm to storm.
Radar assumes a random droplet distribution, which is not always true. Radar is more sensitive to the horizontal diameter of hydrometeors than it is to the concentration of hydrometeors; therefore, a small number of large hydrometeors can result in the same reflectivity value as a large number of smaller drops. As a result, more than one Z-R relationship may be necessary to accurately measure precipitation accumulations.
Ice has a vastly different reflectivity profile than liquid water due to the crystalline nature of the hydrometeors. Pure snowflakes and crystals violate the assumption of liquid hydrometeors that go into the Z-R equation. This means that frozen precipitation may require a Z-S, or reflectivity-snowfall rate relationships.
As snowflakes begin to melt, a coating of water can make them appear as very large raindrops to the radar. This can lead to high reflectivity and overestimated rainfall rates where the radar is sampling the melting layer. Hail results in anomalously high derived-rainfall rates as well.
Radar may inaccurately believe that non-hydrometeors like insects or ground targets are hydrometeor targets.
Terrain blocking limits the sampling distance.
Because stratiform clouds are generally not as deep as convective clouds, radar may overshoot stratiform clouds at closer distances to the radar than for convective clouds. Sampling of stratiform precipitation is almost always poor beyond 100 km from the radar. Shallow convective clouds may be under sampled as well do to over shooting. Convective cloud tends to be deeper so deep convection may be sampled at distances of 150 km or less from the radar.
Since the radar beam gets higher in the atmosphere with distance from the radar, even a low tilt angle like 0.5° is ~1.5 km (5,000 ft) above the ground at 100 km from the radar, and 5.2 km (17,000 ft) high at 230 km.
Higher elevation samples may not be as representative of surface rainfall, especially if the precipitation is falling through a dry boundary layer.
What are the advantages of using satellite derived QPE? (Hint 2)
Satellite estimates of precipitation are more regionally consistent than radar estimates.
Satellite estimation of precipitation is potentially useful in areas with poor coverage from radars and rain gauges.
When does satellite QPE preform best and what can be done to improve estimates?
Satellite-derived rainfall products perform best in the tropics and in the middle latitudes during the warm season. Verifies best when wind shear is minimal and precipitation is dominated by convection.
There is increasing use of additional sensor capabilities, such as microwave satellite sensing and lightning detection, to improve satellite QPE.
Terrain enhancement helps with spatial resolution of precipitation. NOAA product known as the hydro-estimator, based on geostationary satellite observations, applies a terrain factor to the precipitation estimate using 700 mb winds.
Other improvements to precipitation rates may be achieved by applying factors related to cloud, moisture, and stability characteristics.
What are limitations of satellite derived QPE? (hint 2)
Satellite is a remote sensing QPE tool with much coarser resolution spatially and temporally than radar. This is especially true in terms of temporal resolution with polar orbiting satellites.
Name the advantages of using gauge based QPE. (Hint 4)
Surface-based precipitation estimates are often considered “ground truth” since they directly measure precipitation at the surface.
Manual gauges may allow for more accurate liquid equivalent measurements with frozen precipitation. Snow and hail are melted and measured manually. The observer may collect a “core” measurement of snow from the ground to give a more representative sample.
Weighting sensors, such as snow pillows used at SNOTEL sites in the western US, generally provide better estimates of snow water equivalent than automated gauges.
Manual, or recording, gauges require physical interaction to obtain a precipitation estimate, which has advantages and disadvantages. Biggest advantage is that errors can be identified and fixed more quickly. Insect or bird nests, leakage, overflow, blockage, vandalism.
Name and describe the types of rain gauges.
· Tipping bucket: Measures precipitation as weight of rain causes a pendulum to tip. Measurement resolution dependent on the weight required to tip the pendulum.
· Weighing: Measures precipitation based on the change in weight of water in an enclosed volume. Best used to measure precipitation accumulation.
· Recording or manual: Rainfall is held in an enclosed volume, which has a graduated display to manually describe the amount of rainfall (or liquid equivalent) that fell since the last time the gauge was checked.
Name the limitations of gage based QPE. (10 with 2 concerning precipitation rate and 4 concerning phase.)
Rain gauges are direct ground-based measurements, but cannot resolve the spatial detail of precipitation patterns.
Nearby obstructions can alter precipitation reaching the surface.
Rain gages are limited to point measurements that are valid for the area of the gauge.
Rain which falls at an angle due to wind reduces the effective size of the rain gage opening which may lead to an underestimation of rainfall. Frozen precipitation, especially snow, is more severely impacted by wind than liquid precipitation. The magnitude of the under-catch will vary with snowflake characteristics. Denser crystals will have fewer gauge catch errors than low density crystals.
The low temporal resolution (usually daily estimates) with varying recording times makes manual reports less suitable for software programs that need fast access to high resolution gauge estimates.
Precipitation rate:
· If the precipitation rate is too fast, then splash and overflow effects can cause substantial under-catch.
· If the precipitation rate is too slow, then evaporation and surface tension can reduce estimates.
Precipitation phase:
· Freezing rain or frozen precipitation can clog apertures.
· Tipping mechanism may become inoperable due to frozen water.
· To obtain a liquid equivalent measurement of frozen precipitation with a tipping bucket gauge, the gauge must be heated to turn the ice to liquid. A heated gage can lead to enhanced sublimation/evaporation of precipitation which reduces estimates.
· If the snowfall rate is too high, the melting will not occur fast enough to prevent gauge overflow.
What is good about precipitation climatology?
Precipitation climatology guidance can be used to help fill in the gaps where estimates of observed precipitation are poor. Also, it can be very useful in regions where precipitation distribution, and the ability to observe it, is greatly affected by terrain features.
What climate tool is often used to create precipitation climatologies and what is this tool based on?
The Parameter-elevation Regressions on an Independent Slopes Model (PRISM) provides a commonly-used precipitation climatology tool.
PRISM precipitation climatologies are based on:
· Historic record of measured precipitation at point locations.
· Geographic input, especially terrain information.
· Prevailing wind direction (in some cases).
· Stream flow discharge measurements.
