The Mixing Ratio

[Patrick Marsh]

Thanks for pointing out that methods differ on how PW is calculated-I will have too look into that.

The key is knowing what levels are being used in the calculations. Some calculations go from SFC to computed EL, some go to fixed levels, such as 300mb, etc. On any given sounding if you compared the PW from SPC, RAP, University of Wyoming, AWIPS, you are bound to find at least one of them differ slightly. This is almost entirely based on what the cut-off level is for the computation.

 

[Greg Blumberg]

As for the mid level moisture you indicated Mike, I would not always consider it bad, but you'd have to consider how close the layer is to saturation when it experiences the lift that helps create the lapse rates we need. If the layer becomes saturated in that scenario, it won't cool nearly as strongly as an un-saturated layer would because of the effects from latent heating. It's only when the mid level moisture be considered "bad" is when that lifting causes it to reach saturation.

 

[Jeff Duda]

As for the mid level moisture you indicated Mike, I would not always consider it bad, but you'd have to consider how close the layer is to saturation when it experiences the lift that helps create the lapse rates we need. If the layer becomes saturated in that scenario, it won't cool nearly as strongly as an un-saturated layer would because of the effects from latent heating. It's only when the mid level moisture be considered "bad" is when that lifting causes it to reach saturation.

Additionally, a fairly moist (i.e., relatively higher values of RH) in the mid layers limits the amount of evaporational cooling that would occur with precip loaded downdrafts. Thus downdrafts would be less cool and outflows would have less tendency to escape the storm and cause it to go "outflow-dominant".

 

[Jeff Snyder]

Additionally, a fairly moist (i.e., relatively higher values of RH) in the mid layers limits the amount of evaporational cooling that would occur with precip loaded downdrafts. Thus downdrafts would be less cool and outflows would have less tendency to escape the storm and cause it to go "outflow-dominant".

That is (errr, was) conventional wisdom. However, a recent paper by James and Markowski looked at the effects of dry air aloft on deep convection, and their results are contrary to what most of us had thought about when it comes to mid-level moisture and evaporational cooling. In their supercell case, the drier mid-level air initially caused greater evaporation of hydrometeors, but this reduced precipitation/water loading [precip loading imparts negative buoyancy that can reduce updraft intensity / increase downdraft intensity]. The reduced water loading makes up for (+/-) evaporational cooling in their simulations.

See: James and Markowski, 2010: A numerical investigation of the effects of dry air aloft on deep donvection

I'll take the following snippet from the abstract:
"A three-dimensional cloud model was used to investigate the sensitivity of deep convective storms to dry air above the cloud base. In simulations of both quasi-linear convective systems and supercells, dry air aloft was found to reduce the intensity of the convection, as measured by updraft mass flux and total condensation and rainfall. In high-CAPE line-type simulations, the downdraft mass flux and cold pool strength were enhanced at the rear of the trailing stratiform region in a drier environment. However, the downdraft and cold pool strengths were unchanged in the convective region, and were also unchanged or reduced in simulations of supercells and of line-type systems at lower CAPE. This result contrasts with previous interpretations of the role of dry air aloft in the development of severe low-level outflow winds....

When dry air was present, the decline in hydrometeor mass exerted a negative tendency on the diabatic cooling rates and acted to offset the favorable effects of dry air for cooling by evaporation. Thus, with the exception of the rearward portions of the high-CAPE line-type simulations, dry air was unable to strengthen the downdrafts and cold pool. A review of the literature demonstrates that observational evidence does not unambiguously support the concept that dry air aloft favors downdraft and outflow strength. It is also shown that the use of warm rain microphysics in previous modeling studies may have reinforced the tendency to overemphasize the role of dry air aloft."

(emphasis added by me)

 

[Mike Kovalchick]

As for the mid level moisture you indicated Mike, I would not always consider it bad, but you'd have to consider how close the layer is to saturation when it experiences the lift that helps create the lapse rates we need. If the layer becomes saturated in that scenario, it won't cool nearly as strongly as an un-saturated layer would because of the effects from latent heating. It's only when the mid level moisture be considered "bad" is when that lifting causes it to reach saturation.

Unfortunately, I skipped meterology school and studied much more boring subjects. (Engineering.) As a result, I am self taught in meterology and skipped all the boring stuff this time around!

Living in Michigan, 2 things kill our tornadic potential almost every time. Uni-directional winds and moisture in the mid-levels. (Or rather the lack of an EML or decent capping of any sort which leads to wide spread storm coverage, massive seeding of storms etc. It doesn't help that SE winds here come from downsloping/mountains of West Virginia rather than the GOM. Mid-level lapse rates here are almost always low.)

As you know, the best tornadic storms are generally associated with the loaded gun sounding or some variation of it. http://www.erh.noaa.gov/gsp/localdat/cases/2008/IdesOfMarchSupercells/LoadedGun.gif

Nice recent real example of this occurred during the Bowdle wedge. Take a look at the 0Z Aberdeen sounding..700 mb dewpoints were nice and low! (I shouldn't call it a loaded gun..the gun had already fired in this case!)
http://w1.spc.woc.noaa.gov/exper/archive/events/100522/index.html
(Too bad it doesn't show the PW or mixing ratio. Note also the the 12Z soundings which I assume was well North of the warm front was very moist in the mid-levels.)

I'm sure the mixing ratio was high as the dewpoints were high and the low-level moisture layer was quite thick.

No doubt there are plenty of situations with mid-level moisture is present but I always mentally add it to my list of things that I'm not happy about in a particular chasing setup.

 

[Jeff Duda]

That is (errr, was) conventional wisdom. However, a recent paper by James and Markowski looked at the effects of dry air aloft on deep convection, and their results are contrary to what most of us had thought about when it comes to mid-level moisture and evaporational cooling. In their supercell case, the drier mid-level air initially caused greater evaporation of hydrometeors, but this reduced precipitation/water loading [precip loading imparts negative buoyancy that can reduce updraft intensity / increase downdraft intensity]. The reduced water loading makes up for (+/-) evaporational cooling in their simulations.

See: James and Markowski, 2010: A numerical investigation of the effects of dry air aloft on deep donvection

I'll take the following snippet from the abstract:
"A three-dimensional cloud model was used to investigate the sensitivity of deep convective storms to dry air above the cloud base. In simulations of both quasi-linear convective systems and supercells, dry air aloft was found to reduce the intensity of the convection, as measured by updraft mass flux and total condensation and rainfall. In high-CAPE line-type simulations, the downdraft mass flux and cold pool strength were enhanced at the rear of the trailing stratiform region in a drier environment. However, the downdraft and cold pool strengths were unchanged in the convective region, and were also unchanged or reduced in simulations of supercells and of line-type systems at lower CAPE. This result contrasts with previous interpretations of the role of dry air aloft in the development of severe low-level outflow winds....

When dry air was present, the decline in hydrometeor mass exerted a negative tendency on the diabatic cooling rates and acted to offset the favorable effects of dry air for cooling by evaporation. Thus, with the exception of the rearward portions of the high-CAPE line-type simulations, dry air was unable to strengthen the downdrafts and cold pool. A review of the literature demonstrates that observational evidence does not unambiguously support the concept that dry air aloft favors downdraft and outflow strength. It is also shown that the use of warm rain microphysics in previous modeling studies may have reinforced the tendency to overemphasize the role of dry air aloft."

(emphasis added by me)

What about moist air aloft? That's what I was trying to get at.