Unformatted text preview: CONVECTION
CONVECTION MSC 243 Lecture #10
3 November 2011 Websites
Websites Sounding Archive: Make your own SkewT’s!
Useful Sounding Help:
Recent Soundings (a week):
Helpful tutorial on buoyancy and CAPE:
http://meted.ucar.edu/mesoprim/cape/cape.htm Temperature and Moisture
Temperature and Moisture
here: at 700 mb Data from a
each level are:
- Dew Point - Mixing Ratio - 313K Potential
Temperature - 1 C 5 g/kg 11 g/kg Temperature Saturation
Mixing Ratio Clouds are more likely to occur when T and Td are similar Parcel Motion
Parcel Motion As a parcel moves up and down, so long as it is not saturated, its temperature will change at the dry adiabatic lapse rate (9.8 C / km). As a parcel moves up and down, so long as it is not saturated, it will conserve its ‘mixing ratio’, the ratio of water molecules to air molecules. Parcel Motion
Parcel Motion If a parcel becomes saturated, continued cooling will result in condensation into water vapor. The latent heat released by condensation offsets the cooling from expansion, and the parcel will rise at the moist adiabatic lapse rate. This moist adiabatic lapse rate is generally around 56 C / km, not as high as the dry adiabatic lapse rate. Cloud Formation
First, consider an air parcel at the surface. When it is warmed at the surface, it is buoyant: it rises and its temperature falls. If no heat is added or released, the parcel rises “dry adiabatically”. Adiabatic cooling of rising air is the dominant cause of cloud formation…
[On a SkewT diagram, move up along the dry adiabat from the surface] Cloud formation
Cloud formation The rising unsaturated air parcel reaches the lifting condensation level (LCL), where it begins to condense.
At the LCL, the air parcel becomes saturated as its temperature reaches the dew point (of the parcel). This is where the cloud base may begin.
LCL = the height at which a parcel of air would become saturated if lifted dry adiabatically (point at which saturation would Lifting Condensation Level (LCL)
Lifting Condensation Level (LCL) LCL
(~ 860 mb)
• At surface pressure find T Draw a line up parallel to dry adiabat At surface pressure find Td Draw a line up parallel to mixing ratio line The intersection of the 2 lines is the LCL Read pressure and label in mb NOTE: if T and Td vary abruptly near the surface, use an average value of T and Td in the lowest 50
100 mb. Above the LCL: Cloud Above the LCL: Cloud Formation Now that the air parcel is saturated at the LCL, it might be able to rise even higher under the right conditions Condensation is a warming process: latent heat release may assist further rising.
But, this is partially offset by the cooling due to expansion.
As the parcel rises above the LCL, it cools at a slower rate: the moist adiabatic lapse rate. Above the LCL: Cloud Above the LCL: Cloud Formation Under favorable instability conditions, the air may rise to a level where it becomes warmer than its environment.
This is the Level of Free Convection (LFC) The level beyond (above) which the air parcel becomes buoyant. An air parcel above this level can rise upward even in the absence of any forced lifting ON A SKEWT: Start at Lifting Condensation Level (LCL) and follow up the moist adiabat until you reach the temperature line from the sounding Key Concepts: Buoyancy
Key Concepts: Buoyancy
Buoyancy: upward force that acts on a parcel of air due to density difference
Higher potential temperature and water vapor content increase buoyancy.
Precipitation acts to decrease buoyancy.
Main thing to look at: is the temperature of the air parcel warmer or cooler than the environment (i.e. the sounding temperature)? Atmospheric Convection
Atmospheric Convection As an air parcel moves up from the LFC, its temperature is higher than that of the environment, and it is positively buoyant.
Q: What is the air parcel going to do?
A: It will rise further, with no extra help!
The air parcel will continue to cool at the moist adiabatic lapse rate. Atmospheric Convection
Atmospheric Convection The air parcel remains just saturated.
Hence, as it continues to rise, it condenses more water and forms more clouds and precipitation.
Therefore, convection is associated with clouds and rainfall.
The larger the difference in temperature between the parcel and environment, the faster the updraft, the faster the condensation, and the more severe storms! Atmospheric Convection
Atmospheric Convection This condensation continues until the air parcel finally reaches the Equilibrium Level (EL). EL = The height at which the temperature of the buoyant parcel again becomes equal to the environmental temperature. The EL is where the cloud anvil forms. Sometimes, air parcels with upward momentum may push up through the EL, but then they are heavier than their surroundings and sink back. ON A SKEWT: From the point where the LFC was found, follow a moist adiabat up until crossing the temperature line again.
That level is the equilibrium level, at which a parcel of air no longer accelerates upward. Levels: LCL < LFC < EL
Levels: LCL < LFC < EL
LCL Convective Convection Level (CCL)
Convective Convection Level (CCL)
The height to which parcel of air, if heated sufficiently from below, would rise adiabatically until it is just saturated.
(Often the height of the base of cumuliform clouds which are produced by thermal convection solely from surface heating)
On a SkewT map: move up the mixingratio line from the surface dewpoint, until this line intersects the temperature reading. CCL on a sounding
CCL on a sounding Convection temperature: surface temperature that must be reached to start the formation of convection clouds Stability Indices
Stability Indices Several measures have been devised to quantify how likely it is that a parcel will be able to rise if forced, and how high the parcel will rise. Most of these indices are related to the concept of stability and lapse rates. CAPE.
CAPE. CAPE represents the amount of buoyant energy available to accelerate a parcel vertically, or the amount of work a parcel does on the environment. CAPE is the positive area on a sounding between the parcel's assumed ascent along a moist adiabat and the environmental temperature curve from the level of free convection (LFC) to the equilibrium level (EL). EL ∫ CAPE = g [(Tparcel Tenvir) / Tenvir] dz (in J/kg) LFC
CAPE = 0: Stable. CAPE < 1000: Marginally unstable.
CAPE = 1000 to 2500: Moderately unstable. CAPE = 2500 to 3500: Very unstable. CAPE above 3500: Extremely unstable. Area of white shadedregion: CAPE Area of white shaded region: CAPE CIN: “Negative Cape”
CIN: “Negative Cape” To overcome CIN (Convective Inhibition), some external work is needed to lift the parcel to overcome the tendency to sink.
1. Heating. 2. Moistening. 3. Forced Lifting Lifted Index
Lifted Index The LI is a commonly utilized measure of stability which measures the difference between a lifted parcel's temperature at 500 mb and the environmental temperature at 500 mb. LI = T(500 mb envir) T(500 mb parcel)
in degrees C, where T (500 mb envir) represents the 500 mb environmental temperature and T (500 mb parcel) is the rising air parcel's 500 mb temperature. LI > 0: Stable but weak convection possible for LI = 13 if strong lifting is present. LI = 0 to 3: Marginally unstable. LI = 3 to 6: Moderately unstable. LI = 6 to 9: Very unstable. LI below 9: Extremely unstable. Lifted Index (KMPX)
Lifted Index (KMPX)
EQ LEV Convective
quite likely LI LFC K Index
K Index The K index is a measure of thunderstorm potential based on the vertical temperature lapse rate, and the amount and vertical extent of lowlevel moisture in the atmosphere.
K = T(850 mb) + Td(850 mb) T(500 mb) DD(700 mb)
in degrees C, where T represents temperature, Td represents dewpoint temperature, and DD represents dewpoint depression (the difference between the dewpoint temperature and the actual temperature) at the indicated level. From SPC: Values greater than 3040 tend to be associated with deep convection, while thunderstorms are much less common with lesser values. Lapse Rates and Stability
Lapse Rates and Stability
Moist adiabatic lapse rate < dry adiabatic lapse rate CONDITI
Y UNSTABLE ABSOLUTEL
Y STABLE Absolute Stability
If the environmental lapse rate < moist adiabatic lapse rate, an air parcel cools more quickly than the environment as it is lifted upward. The air parcel is denser than the environment and we have absolute stability. The air parcel will sink back to its original height. Absolutely Stable
Absolutely Stable Absolute Instability
If environmental lapse rate > dry adiabatic lapse rate, the environment is absolutely unstable. Conditional Instability
If the moist adiabatic lapse rate < environmental lapse rate < dry adiabatic lapse rate, we have conditional instability.
An air parcel at the environmental temperature is (a) unstable to upward displacements if it is saturated, but (b) stable to upward displacements if it is unsaturated.
This is the most common situation. Conditionally Unstable
Conditionally Unstable Equivalent Potential Temperature: Thetae
Equivalent Potential Temperature: Thetae The temperature that a parcel of air would have if all its moisture were condensed out by a pseudo
adiabatic process (i.e. with latent heat of condensation used to heat the parcel), and the parcel then brought dryadiabatically back to 1000 mb. Directly related to the amount of heat and moisture present in an air parcel
Useful in diagnosing atmospheric instability If Thetae increases with altitude, the atmosphere is stable
If Thetae decreases with altitude, convection may be possible Thetae on a sounding
Thetae on a sounding Example of Thetae
Local maximum of 850 mb Thetae indicates a local area of instability. Easterly flow is advecting Thetae into NE Colorado. More on http://www.spc.noaa.gov Summary of SkewTs
Summary of SkewTs Why do we need SkewT LogP diagrams? Can assess stability of atmosphere Can see weather elements at every layer Determine clouds, capping inversions Determine character of severe weather Data is assimilated into weather forecast models (GFS etc) What are the disadvantages? Only available twice per day (00 and 12 UTC) Weather can change dramatically between soundings Wind blows balloon downstream, not instantaneous measurements. ...
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- Fall '08
- Convection, LCL, parcel, LFC