When you've completed this page, you should be able to define conditional instability, as well as assess the stability of a layer (or trends in stability) from its lapse rate (or trends in lapse rate).
In the previous section, you learned that CAPE helps forecasters to assess the potential for strong updrafts, but doesn't directly tell us about atmospheric stability. If you want to assess the stability of a specific layer of the atmosphere, the key is lapse rates. Therefore, forecasters use lapse rates in concert with CAPE to assess stability and the potential for strong updrafts.
In your previous studies, you learned that the lapse rate is the change in temperature with altitude in any given layer of air. As a general rule, the greater the decrease in temperature with height, the greater the likelihood for convective overturning and the development of thunderstorm updrafts. So, how do we assess how "large" or "small" environmental lapse rates are in a given situation? Start by keeping in mind some key "benchmark" lapse rates that will help you as you assess the stability of specific atmospheric layers:
- the dry-adiabatic lapse rate: 9.8 degrees Celsius per kilometer (you can use about 10 degrees Celsius per kilometer as a proxy)
- the moist-adiabatic lapse rate: roughly 6 degrees Celsius per kilometer, but recall that this lapse rate is not constant -- 6 degrees Celsius per kilometer simply serves as a ballpark reference for the lower troposphere
In light of the introduction to CAPE in the previous section, it should come as no surprise that the environmental lapse rate (which you can assess via 12Z or 00Z temperature soundings or by model forecasts) plays a key role in the calculation of CAPE. So when lapse rates are steep, (large decreases in temperature with altitude), CAPE tends to be high. CAPE, however, can also be relatively high when lapse rates are rather modest but the lower troposphere is moist. Indeed, the presence of low-level moisture tends to lower the LFC and increase CAPE accordingly.
Let's take a look at CAPE and lapse rates on a sounding to see what we can tell about the potential for strong updrafts and the stability of individual layers. For starters, check out the skew-T from Miami, Florida, at 00Z on July 9, 2010 (below)
On this skew-T, the vertical profiles of temperature and dew point are the red and green soundings, respectively. The blue curve represents the path of an air parcel lifted from the surface. At the time, there was very little CIN and a CAPE value of 1,649 Joules per kilogram. Note that the temperature profile is dry adiabatic in the boundary layer (from the surface to about 930 mb), which represents a steep lapse rate. But, above that, the temperature profile is actually rather stable in most layers. One exception is the layer between roughly 930 mb and 800 mb, where the temperature profile is conditionally unstable, meaning that the environmental lapse rate is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate.
If you nudge a test parcel originating in this layer upward, its stability depends on whether or not the parcel is saturated (that's the "condition" of the instability). If the parcel is initially saturated and nudged upward from its initial position, it will accelerate away from its initial position because the parcel cools at the moist adiabatic lapse rate (keeping it warmer than the environment). However, if the parcel is unsaturated, it will quickly become cooler than its surroundings (and negatively buoyant) if nudged upward, because it cools at the dry adiabatic lapse rate. An unsaturated parcel would sink back to its initial position.
With the exception of another notable conditionally unstable layer from roughly 600 mb to 520 mb, most other layers above 800 mb are rather stable (small lapse rates). Parcels originating in those layers will sink back to their original positions if nudged upward. So, where does all the CAPE (and potential for strong updrafts) come from? High surface temperatures (and steep lapse rates in the boundary layer), and high dew points in the boundary layer. If you look at the vertical profile of dew points in the boundary layer over Miami at this time, they varied from the upper 60s to the lower 70s degrees Fahrenheit (roughly 20 to 23 degrees Celsius). Yes, the boundary layer was rather moist. As a result, the LFC lay at a relatively low altitude, paving the way for a "tall, skinny" area of CAPE. The fact that lapse rates were relatively small above 800 mb made for relatively small differences a rising parcel's temperature and its surroundings, and a "skinny" positive area. In case you're curious, there were no thunderstorms around Miami at this time, despite the presence of CAPE and the very small amount of CIN present. For more discussion about why, check out the Explore Further section below.
CAPE resulting from steep lapse rates, creates stronger vertical accelerations and updraft velocities, and tends to catch forecasters' attention. Therefore, forecasters find it convenient to have options that allow them to take "shortcuts" and narrow their focus to areas where relatively high CAPE is primarily a result of steep lapse rates (where deep, moist convection tends to be more active, assuming, of course, that there's also ample moisture). As it turns out, so can you!
Consider the lapse rate products available on the SPC Mesoanalysis Page (in the "Thermodynamics" menu). Although the depths of thunderstorms (cloud base to cloud top) vary, forecasters typically look at low-level lapse rates (between the surface and three kilometers), and/or mid-level lapse rates (700-500 mb, or roughly 3-6 kilometers), depending on the local environment in which storms are expected to develop. Below is an example of an analysis of lapse rates, expressed in degrees Celsius per kilometer, in the 700-mb to 500-mb layer at 20Z on July 3, 2010.
In the image above, note the tongue of very steep mid-level lapse rates extending northeastward across western Nebraska and southwest South Dakota (areas with lapse rates greater than 8 degrees Celsius per kilometer are shaded). These lapse rates were technically conditionally unstable since they were greater than the moist adiabatic lapse rate, but weren't quite as great as the dry adiabatic lapse rate. Still, in the real atmosphere, environmental lapse rates rarely exceed the dry adiabatic lapse rate, so you can consider lapse rates approaching 8 and 9 degrees Celsius per kilometer, as "steep" (generally favorable for deep, moist convection).
Low-level lapse rates (between the ground and three kilometers) were pretty steep over western Nebraska and southwest South Dakota, too (20Z low-level lapse rates). In turn, CAPE was also relatively high (20Z analysis of CAPE and CIN), and SBCIN (surface-based CIN) was vanishing in response to surface heating (note surface temperatures well into the 80s on the 20Z analysis of surface temperatures).
In this case, thunderstorms did erupt thanks to convergence associated with an approaching cold front (21Z surface analysis) and some rather weak upslope flow (topographic map). Both of these lifting mechanisms helped parcels overcome the remaining CIN to reach the LFC, setting the stage for thunderstorms (23Z radar reflectivity).
The moral of the story is that you should routinely look at lapse rates (both low- and mid-level) in situations where deep, moist convection might develop. And, as you'll see in the coming sections, analyzing the synoptic-scale pattern helps weather forecasters understand lapse-rate tendency (change in lapse rate over time), which helps forecasters anticipate potential changes to CAPE and CIN.
After identifying regions with relatively high CAPE and steep lapse rates and assessing whether synoptic-scale lift (or mesoscale lift) can get air parcels to the LFC, forecasters turn to the issue of vertical wind shear, which helps me to determine the mode of deep, moist convection. Let's investigate.
Recall that in the example from Miami, Florida above, there was a tall, skinny area of CAPE, with very little CIN. Yet, no thunderstorms formed. To confirm, check out the meteogram from Miami (below) from 02Z on July 8, 2010 through 03Z on July 9.
If you revisit the Miami skew-T from earlier on the page, note that there was a relatively large portion of the troposphere that was not even close to saturation (temperature and dew-point soundings were pretty far apart, indicating low relative humidity). If you're speculating that so much dry air (dew points as low as minus 50 degrees Celsius near 500 mb) would have a negative impact on growing cumulus clouds, you're definitely on the right track.
You might be thinking, what would stop a moist parcel, after reaching the LFC, from staying positively buoyant through a deep layer? Technically, nothing. But, it's not realistic. According to parcel theory (what you learned in your previous studies, and is pervasive throughout meteorology) we don't allow air parcels to interact with their environment when we move them up and down on skew-T diagrams. Parcels in the real atmosphere, however, DO interact with their environments, which means parcel theory has some limitations. We'll discuss some adjustments to classic parcel theory later in the course.
For now, to understand why dry air in the middle troposphere can inhibit growing cumulus clouds, think of the updraft in a growing cumulus cloud as a plume of rising air that does interact with its environment. In the environment depicted on the Miami skew-T, dry air in the middle troposphere mixed into the tops of any growing cumulus clouds (a process called "entrainment"). The entrainment of unsaturated air into the tops of growing cumulus clouds typically weakens updrafts because it promotes evaporation and cooling (evaporational cooling reduces the positive buoyancy associated with the updraft). Thinking about it another way, the evaporation of cloud drops tends to offset the primary source for thunderstorm strength: the release of latent heat of condensation (since it's the release of latent heat that slows the cooling of rising air parcels, keeping them warmer than their surroundings).