Lecture Notes

Monday October 11 and Tuesday October 12, 2004

Upper Air Data: Convection

Objectives

By the end of this class, students should be able to:

describe the differences between shallow and deep convection
use a skew-T sounding profile to quantitatively identify
- the CCL,
- the CT,
- the MCL,
- the operational LI,
- the SI,
qualitatively assess the amount of CAPE and CIN associated with a sounding profile

Convection

Shallow Convection

confined to a limited layer of the atmosphere, usually the boundary layer.
induced by a rapid localized destabilization of an atmospheric layer; the most common and important example is the destabilization of the boundary layer associated with the diurnal solar heating of the earth's surface.
associated with thermal updrafts that generate low-level cumulus cloud
animation below illustrates characteristics of shallow "dry" (no condensation) convection; it shows high-resolution 2-d model run of a typical thermal updraft; the "bubble" of warm air can be interpreted as being formed by heating at the surface of the earth
- thermal is about 1 km in length;
- the rising parcel lasts about 10 minutes before mixing with its surroundings; it breaks into smaller turbulent eddies that mix the higher potential air with its surroundings.
- it acts to mix the air in the lowest 1 km of the atmosphere (i.e. below 900 mb)

Deep Convection

occurs in humid, conditionally unstable atmospheres when lifted air parcels accelerate upward to the tropopause. It appears in satellite photos as bright localized cloud bands owing to high cloud tops.

induced by forcing air parcels to the level of free convection (LCL).

Initialization of deep convection: Ground view

The following animation shows a southward facing view of Mt. Lemmon near Flagstaff, AZ. Note that the south side of the mountain is heated more directly from the sun than the surrounding countryside, making it prone to deep convection.

Click image for animation (animation courtesy Arizona State University)

Beginning of animation shows shallow convective cloud forming as surface parcels reach the LCL.
Convective cloud deepens as shallow convection mixes and destabilizes the boundary layer. The boundary layer deepens gradually.
Finally, surface parcels elevated by shallow convection reach the LFC. At this point, deep convection is initiated as thermals accelerate until they reach the EL in the upper troposphere.

Initialization of deep convection: Satellite view

In this animated sequence over Florida, deep convection produces strong thunderstorms.

The initial deep convective cell is initiallized by a sea-breeze along the eastern coast of Florida.

The Initial storm cells generate strong downdrafts associated with falling rain. The converging outflow from these downdrafts triggers other stroms to the west of the original storm cells. Thus storm formation moves gainst the prevailing large-scale flow.

Predicting convection

Operational convective indices

Because shallow convection mixes the boundry layer before afternoon convection is initiated, it makes sense to graphically estimate mean values of mixing ratio and potential temperature and to use them to determine the LCL, LFC, and EL. These are usually estimated for the bottom 1 km (100 mb) of the atmosphere, although many forecasters use other definitions. We will use the bottom 100 mb of the atmosphere as the mixing layer.

The potential temperature for the mixing layer can be calculated graphically by tracing the dry adiabat that passes closest to all parts of the temperature profile. [Click here for example]
Similarly, a mean mixing ratio can be determined by drawing a line along the mixing ratio line that passest closest to all points on the moisture profile in the lowest 100 mb. [Click here for example]
A new LCL (called the mixing condensation level or MCL) can be calculated where the two mixed profiles meet. [Click here for example]
LFC's, LI, and EL can be determined for the MCL as well. [Click here for example]
Showalter index - is the lifted index for the 850-500 mb layer.

Convective temperature

CCL - Convective Condensation Level - is found by following w_s for mixed boundary layer (line of constant mixing ratio) up to ambient temperature profile. Calculate boundary layer w_s by taking mean of w_s at top and bottom of boundary layer. CCL is the LCL for a parcel reaching the CT. [Click here for example]
CT - Convective temperature - is found by following the dry adiabat down from the CCL. This is the temperature that the surface layer of the atmosphere must reach to initiate cloud formation. If the CCL is also the LFC, then deep convection will also be initiated at the CT. [Click here for example]
Knowing the CT allows the forecaster to observe hourly temperatures when awaiting convective outbreaks. Areas that should be monitered include:
- cloud-free areas - they heat up to the CT faster
- areas along fronts and troughs - low level convergence often provides a lift that triggers convection here

Predicting convection: Energetics

Note that a thin black line is often included in sounding profiles. This shows the temperature of a parcel lifted from the boundary layer. The soundings may use a surface parcel, a mixed layer parcel, a parcel at the CT, or a parcel at the forecast high for the day.
CAPE - Convective available potential energy - is the amount of bouyant energy available to air parcels for conversion to kinetic energy. CAPE over 1500 J/kg usually yields thunderstorms. CAPE over 3000 J/kg usually yields severe thunderstorms.
CAPE is expressed in J/kg (energy per unit mass) and is proportional to the area between the lifted parcel temperature and atmospheric temperature if T(parcel) > T(atmosphere) (red area on maps above). The sounding at left has 511 J/kg of CAPE. This is not enough to induce deep convection. The sounding at right has over 3000 J/kg of CAPE, and a severe storm warning should be issued. Indeed, hurricane Isadore was not far from Florida at the time of the sounding. The LCL and LFC are at the same level. The tropopause and EL also coincide.
CIN - Convective Inhibition - defines the work required to bring parcel to the LFC. It is proportional to the light blue shading between the parcel and ambient temperature profile above. Note that there is no CIN in the souding at right.
w_max - Maximum updraft - The maximum updraft can be calculated by assuming that all CAPE is transformed into kinetic energy (KE).
Convective overshoot - A parcel reaching the equlibrium level will be at its maximum upward speed. It will overshoot the EL by a considerable amount (up to 2.5 km have been observed). The amount on a given day can be calculated from w_max and if we assume an isothermal atmosphere above the EL.
- T = T of stratosphere.
- c_p - heat capacity of dry air - 1004 J/K/kg
- g - gravitation constant (9.8 m/s)

Convective parameters and lifted air parcels: Morning soundings

Morning soundings usually feature a stable boundary layer. They are used to make predictions about afternoon convection in the vicinity. Lifted parameters are calculated using lifted parcels where: a) a mean mixing ratio of the lowest km of the atmosphere; b) the estimated high for the day. This is to provide a more meaningful estimate of CAPE, CIN, LFC, EL,etc. than actual surface conditions.
The following is a morning sounding in Key West, Florida.

This is a very unstable sounding. A large CAPE can be infered from the large area of red shading between parcel and ambient temperature profiles. Note that CAPE greater than 3000 J/kg indicates the probability of severe convection. Indeed, hurricane Isadore was not far from Florida at the time of the sounding. The LCL and LFC are at the same level in his sounding. The tropopause and EL also coincide.
This soundingings illustrate the effect of a humid boundary layer on the potential for convection. If the relative humidity near the surface is high, the LCL will be reached sooner, more latent heat will be released, and the parcel lapse rate will be smaller.

Exercise

Click here for solutions
From the given soundings, calculate the following. Round your answer off to the nearest K:
Miami, FL [pdf], San Juan, PR [pdf]

2) The CCL using the mixing ratio for the lowest 100 mb of the atmosphere.
3) The CT.
4) The LCL using the CT as your surface parcel temperature.
5) The LFC using the CT as your surface parcel temperature.
6) The LI using the CT as your surface parcel temperature.
7) The EL
8) Shade in and indicate areas representing CAPE and CIN.
9) Assume CAPE for San Juan = 2312 J/kg
- a) Calculate w_max
- b) Calculate height of convective overshoot h_overshoot