ApacheTrout Posted March 9, 2011 Share Posted March 9, 2011 I've a question on measuring & estimating wind speeds. During big storms (such as the recent slam of Vermont) I'll often see whole trees swaying, which would be indicative of 32-38 mph, or twigs and small limbs littered on the snow, which would be indicative of 39 to 54 mph. Yet individual stations and metars from nearby stations will report winds of 10 to 15 mph and gusts to 25 mph. What could explain the disconnect between my observations and the recorded measurements? I don't think I live in a wind tunnel area, so I think that can be ruled out. Any ideas? Link to comment Share on other sites More sharing options...
isohume Posted March 9, 2011 Share Posted March 9, 2011 I've a question on measuring & estimating wind speeds. During big storms (such as the recent slam of Vermont) I'll often see whole trees swaying, which would be indicative of 32-38 mph, or twigs and small limbs littered on the snow, which would be indicative of 39 to 54 mph. Yet individual stations and metars from nearby stations will report winds of 10 to 15 mph and gusts to 25 mph. What could explain the disconnect between my observations and the recorded measurements? I don't think I live in a wind tunnel area, so I think that can be ruled out. Any ideas? METARs report in knots, but that does seem like a disconnect depending on how far away the METAR site is located from you. In any given storm tho...the strongest winds are rarely measured at a METAR site due to the sparse network. Link to comment Share on other sites More sharing options...
Ellinwood Posted March 9, 2011 Share Posted March 9, 2011 I've a question on measuring & estimating wind speeds. During big storms (such as the recent slam of Vermont) I'll often see whole trees swaying, which would be indicative of 32-38 mph, or twigs and small limbs littered on the snow, which would be indicative of 39 to 54 mph. Yet individual stations and metars from nearby stations will report winds of 10 to 15 mph and gusts to 25 mph. What could explain the disconnect between my observations and the recorded measurements? I don't think I live in a wind tunnel area, so I think that can be ruled out. Any ideas? In addition to what Isohume said, I would also double-check elevation differences between you and the METAR stations just to rule it out as a variable (probably a non-issue since it looks like you live in a low-lying area). Also, what direction is the wind blowing when you notice these discrepancies? Link to comment Share on other sites More sharing options...
ApacheTrout Posted March 9, 2011 Share Posted March 9, 2011 In addition to what Isohume said, I would also double-check elevation differences between you and the METAR stations just to rule it out as a variable (probably a non-issue since it looks like you live in a low-lying area). Also, what direction is the wind blowing when you notice these discrepancies? I seem to notice it with north and south winds. Less with east and west winds, as my area is protected by trees on the east and west. regarding elevation, I'll check nearby stations to see if they differ. Or perhaps they're more sheltered. Link to comment Share on other sites More sharing options...
CooL Posted March 10, 2011 Share Posted March 10, 2011 How do you decipher between an east based NAO and a west based NAO? does it have to do with which side of Greenland where the ridging is more pronounced? And why is a west based NAO better for snowstorms in the northeast/mid atlantic? thanks Link to comment Share on other sites More sharing options...
baroclinic_instability Posted March 10, 2011 Share Posted March 10, 2011 I've a question on measuring & estimating wind speeds. During big storms (such as the recent slam of Vermont) I'll often see whole trees swaying, which would be indicative of 32-38 mph, or twigs and small limbs littered on the snow, which would be indicative of 39 to 54 mph. Yet individual stations and metars from nearby stations will report winds of 10 to 15 mph and gusts to 25 mph. What could explain the disconnect between my observations and the recorded measurements? I don't think I live in a wind tunnel area, so I think that can be ruled out. Any ideas? Some strong wind events out there can feature strong terrain induced lee waves. Terrain can act to significantly enhance local winds as stably stratified air ascends mountains then oscillates as gravity acts as the restoring force. Wave ducting is one way strong lee winds can be enhanced--vertically propagating waves and wave breaking/overturning is another. Mountain wave dynamics are extremely complex--but that is a possible and potentially likely reason why you will often see such significant localized differences in winds between different observing sites that may not match with your site. Terrain enhancement of winds is highly localized. For more on mountain waves and wind--here is a recent post I did from a big downslope windstorm out W last month. It produced gusts in excess of 100 MPH. http://www.americanw...torm-12-13-feb/ Neat site talking about a previous big wind event last month. Note the presence of long trapped wave wavetrains owing to wave ducting. http://cimss.ssec.wi...g/archives/7627 Link to comment Share on other sites More sharing options...
ApacheTrout Posted March 11, 2011 Share Posted March 11, 2011 Some strong wind events out there can feature strong terrain induced lee waves. Terrain can act to significantly enhance local winds as stably stratified air ascends mountains then oscillates as gravity acts as the restoring force. Wave ducting is one way strong lee winds can be enhanced--vertically propagating waves and wave breaking/overturning is another. Mountain wave dynamics are extremely complex--but that is a possible and potentially likely reason why you will often see such significant localized differences in winds between different observing sites that may not match with your site. Terrain enhancement of winds is highly localized. For more on mountain waves and wind--here is a recent post I did from a big downslope windstorm out W last month. It produced gusts in excess of 100 MPH. http://www.americanw...torm-12-13-feb/ Neat site talking about a previous big wind event last month. Note the presence of long trapped wave wavetrains owing to wave ducting. http://cimss.ssec.wi...g/archives/7627 Thanks for the information. I'm going to document the times/dates when my observations don't match up well with measured wind speeds. Perhaps there's a pattern there. It also may just come back to me making poor judgments, but then again I use the observational guidelines posted by NWS to come up with my estimates. Link to comment Share on other sites More sharing options...
LithiaWx Posted March 17, 2011 Share Posted March 17, 2011 What is the difference in the arctic and polar jet streams? Link to comment Share on other sites More sharing options...
isohume Posted March 19, 2011 Share Posted March 19, 2011 What is the difference in the arctic and polar jet streams? The Air Force defines the the arctic jet (AJ) in the lower 2-3 km of the troposphere with the degree of baroclincity or thermal gradient present. The AJ separates the polar airmass from the arctic airmass, which is defined generally by temperatures lower than -18C at h85 or a h10/h5 thickness less than 5080 m during the winter. The polar jet (PFJ) is defined through a deeper layer in the troposphere than the AJ...mainly by h3 temps and wind speeds. The PFJ is often analyzed for imbalances between p-grad accelerations and coriolis accelerations at the entrance and exit region of the jet cores (highest wind maxes)...which helps to identify areas of enhanced vertical motion. The AJ is cyclonic and it's normally removed from the PFJ...but it can and does interact with the nrn and srn branches of the PFJ...thus creating stronger frontal zones or triple point cyclones at the sfc. Here's some of the interactions btw the three main jets and the two branches of the PFJ. Link to comment Share on other sites More sharing options...
Analog96 Posted March 19, 2011 Share Posted March 19, 2011 I've a question on measuring & estimating wind speeds. During big storms (such as the recent slam of Vermont) I'll often see whole trees swaying, which would be indicative of 32-38 mph, or twigs and small limbs littered on the snow, which would be indicative of 39 to 54 mph. Yet individual stations and metars from nearby stations will report winds of 10 to 15 mph and gusts to 25 mph. What could explain the disconnect between my observations and the recorded measurements? I don't think I live in a wind tunnel area, so I think that can be ruled out. Any ideas? They probably don't have the anenometer high enough or sheltered from trees. Link to comment Share on other sites More sharing options...
isohume Posted March 19, 2011 Share Posted March 19, 2011 They probably don't have the anenometer high enough or sheltered from trees. The ASOS would be. That's one of the reason's I never trust "backyard" weather obs. Link to comment Share on other sites More sharing options...
Amped Posted March 20, 2011 Share Posted March 20, 2011 The Air Force defines the the arctic jet (AJ) in the lower 2-3 km of the troposphere with the degree of baroclincity or thermal gradient present. The AJ separates the polar airmass from the arctic airmass, which is defined generally by temperatures lower than -18C at h85 or a h10/h5 thickness less than 5080 m during the winter. The polar jet (PFJ) is defined through a deeper layer in the troposphere than the AJ...mainly by h3 temps and wind speeds. The PFJ is often analyzed for imbalances between p-grad accelerations and coriolis accelerations at the entrance and exit region of the jet cores (highest wind maxes)...which helps to identify areas of enhanced vertical motion. The AJ is cyclonic and it's normally removed from the PFJ...but it can and does interact with the nrn and srn branches of the PFJ...thus creating stronger frontal zones or triple point cyclones at the sfc. Here's some of the interactions btw the three main jets and the two branches of the PFJ. Earlier this year when I was sick of hearing tripple phaser completely made up a definition based on H5 heights alone. Can't find the thread but here's what I think it was. >552DM STJ 552-516DM Northern Jet <516DM Actic jet. Link to comment Share on other sites More sharing options...
Analog96 Posted March 20, 2011 Share Posted March 20, 2011 The ASOS would be. That's one of the reason's I never trust "backyard" weather obs. Yeah, of course the ASOS would be. My anenometer is on my roof, away from trees. I usually measure very close to what EWR measures, maybe a touch lower, because the airport is more open. Link to comment Share on other sites More sharing options...
dendrite Posted March 20, 2011 Share Posted March 20, 2011 I've a question on measuring & estimating wind speeds. During big storms (such as the recent slam of Vermont) I'll often see whole trees swaying, which would be indicative of 32-38 mph, or twigs and small limbs littered on the snow, which would be indicative of 39 to 54 mph. Yet individual stations and metars from nearby stations will report winds of 10 to 15 mph and gusts to 25 mph. What could explain the disconnect between my observations and the recorded measurements? I don't think I live in a wind tunnel area, so I think that can be ruled out. Any ideas? People often overestimate winds when using the Beaufort Scale. Link to comment Share on other sites More sharing options...
isohume Posted March 20, 2011 Share Posted March 20, 2011 Yeah, of course the ASOS would be. My anenometer is on my roof, away from trees. I usually measure very close to what EWR measures, maybe a touch lower, because the airport is more open. It's not just winds I don't trust from PWSs. Who knows how these sensors are sited? Channeled, downwash and wake winds along with thermal contamination from large structures, trees etc make these obs useless. I've seen very odd wind and T/Td inconsistencies with these stations. PWS obs are available in AWIPS, but I never look at them. Link to comment Share on other sites More sharing options...
bkviking Posted March 20, 2011 Share Posted March 20, 2011 I know this isn't technically a meterological issue, but can someone explain why the northern hemisphere receives 12 hours of daylight BEFORE the Vernal Equinox, and also why, say in December, the latest morning for sunrise is usually several weeks after the solstice, and why there isn't an equal gain or loss of light moving from/to the solstice. Thus, here on LI we have a peak late sunrise of 7:15 am during second week of January while the afternoon sunsets were increasing much more. Link to comment Share on other sites More sharing options...
NortheastPAWx Posted March 21, 2011 Share Posted March 21, 2011 Two METAR questions, doing this for an assignment: In this METAR: METAR KBTR 141153Z 22009KT 2 1/2 SM +RA BR SCT005 BKN012 OVC025 18/18 A2998 RMK AO2 TWR VIS 3 VIS 1 3/4V3 TSB1105E43 SLP151 What does the "TSB1105E43" mean? (Just figured that out, NVM) And then: METAR KAOO 041550Z 16010KT 1SM -SNSG BR BKN009 OVC016 M00/M02 A3040 RMK VIS 3/4V 1 1/2 PEB01E29SGB50 SLP302 What is the meaning of "VIS 3/4V 1 1/2"? Link to comment Share on other sites More sharing options...
isohume Posted March 22, 2011 Share Posted March 22, 2011 Two METAR questions, doing this for an assignment: In this METAR: METAR KBTR 141153Z 22009KT 2 1/2 SM +RA BR SCT005 BKN012 OVC025 18/18 A2998 RMK AO2 TWR VIS 3 VIS 1 3/4V3 TSB1105E43 SLP151 What does the "TSB1105E43" mean? (Just figured that out, NVM) And then: METAR KAOO 041550Z 16010KT 1SM -SNSG BR BKN009 OVC016 M00/M02 A3040 RMK VIS 3/4V 1 1/2 PEB01E29SGB50 SLP302 What is the meaning of "VIS 3/4V 1 1/2"? The prevailing vsby is 1 statute mile at ob time and it has been varying between 3/4 and 1 1/2 miles. This remark is only included if the prevailing visibility is <3 statute miles and has been variable. Link to comment Share on other sites More sharing options...
hm8 Posted March 25, 2011 Share Posted March 25, 2011 Explain. http://www.todaysbigthing.com/technology/2011/03/22 Link to comment Share on other sites More sharing options...
isohume Posted March 27, 2011 Share Posted March 27, 2011 Explain. http://www.todaysbig...logy/2011/03/22 This is a nice representation of lee side wave clouds. Cool time-lapse. We see them all the time here in the foothills of the Appalachians. The upstream flow is perturbed by higher topography and this sets off a wave downstream that enhances cloud formation on the ascending portion of the wave. In this example the atmos is very moist downstream, hence the clouds are seen in the descending portion of the wave as well. If the moisture levels are lower you will see lenticular or rotor clouds downstream. These clouds remain generally in the same location over time and they're a big danger sign for pilots. Link to comment Share on other sites More sharing options...
baroclinic_instability Posted March 29, 2011 Share Posted March 29, 2011 This is a nice representation of lee side wave clouds. Cool time-lapse. We see them all the time here in the foothills of the Appalachians. The upstream flow is perturbed by higher topography and this sets off a wave downstream that enhances cloud formation on the ascending portion of the wave. In this example the atmos is very moist downstream, hence the clouds are seen in the descending portion of the wave as well. If the moisture levels are lower you will see lenticular or rotor clouds downstream. These clouds remain generally in the same location over time and they're a big danger sign for pilots. Not quite. That is not a trapped lee wave. This wave looks like a soliton--trapped lee waves do not propagate like this. Trapped mtn waves can "move" from their location depending on changes in wind speed/vertical stability, placement of the mountain top inversion, etc, but not like this example. Link to comment Share on other sites More sharing options...
isohume Posted March 29, 2011 Share Posted March 29, 2011 Not quite. That is not a trapped lee wave. This wave looks like a soliton--trapped lee waves do not propagate like this. Trapped mtn waves can "move" from their location depending on changes in wind speed/vertical stability, placement of the mountain top inversion, etc, but not like this example. That's not evident from the waves we've seen here in the lee of the Apps. They most certainly propagate and attenuate as they move downstream. You just normally don't see the attenuated waves downstream due to lack of moisture. We often see lenticular crossing the Upstate and foothills associated with their downstream propagating waves. The instigating (primary) wave nearest the escarpment is the wave that remains quasi-stationary. Here are a couple photos of moist propagating wave clouds seen around here Link to comment Share on other sites More sharing options...
baroclinic_instability Posted March 29, 2011 Share Posted March 29, 2011 Explain. http://www.todaysbig...logy/2011/03/22 This is a nice representation of lee side wave clouds. Cool time-lapse. We see them all the time here in the foothills of the Appalachians. The upstream flow is perturbed by higher topography and this sets off a wave downstream that enhances cloud formation on the ascending portion of the wave. In this example the atmos is very moist downstream, hence the clouds are seen in the descending portion of the wave as well. If the moisture levels are lower you will see lenticular or rotor clouds downstream. These clouds remain generally in the same location over time and they're a big danger sign for pilots. The wave train in question here was an event in Iowa a few years ago associated with deep convection, not trapped mtn waves. http://science.nasa.gov/science-news/science-at-nasa/2008/19mar_grits/ Link to comment Share on other sites More sharing options...
baroclinic_instability Posted March 29, 2011 Share Posted March 29, 2011 That's not evident from the waves we've seen here in the lee of the Apps. They most certainly propagate and attenuate as they move downstream. You just normally don't see the attenuated waves downstream due to lack of moisture. We often see lenticular crossing the Upstate and foothills associated with their downstream propagating waves. The instigating (primary) wave nearest the escarpment is the wave that remains quasi-stationary. Here are a couple photos of moist propagating wave clouds seen around here Explain that part. I have never heard of or seen a "primary" wave that remains stationary with the downstream waves propagating like a soliton. Trapped mtn/terrain waves can indeed develop in large trains well down wind of the block in the flow under stable conditions--and it happens quite often--but they aren't "moving" through the flow. Dale Durran--one of my favorite mountain meteorologists has a number of good writeups regarding mountain wave dynamics. Here is a good one to start with. http://www.atmos.washington.edu/academics/classes/2010Q1/536/2003AP_lee_waves.pdf Link to comment Share on other sites More sharing options...
isohume Posted March 29, 2011 Share Posted March 29, 2011 Explain that part. I have never heard of or seen a "primary" wave that remains stationary with the downstream waves propagating like a soliton. Trapped mtn/terrain waves can indeed develop in large trains well down wind of the block in the flow under stable conditions--and it happens quite often--but they aren't "moving" through the flow. Dale Durran--one of my favorite mountain meteorologists has a number of good writeups regarding mountain wave dynamics. Here is a good one to start with. http://www.atmos.was...P_lee_waves.pdf I was just referring to what we've observed here in the lee. That the first (primary) wave is the one that produces the stationary lenticular and the attenuated waves downstream seem to have an eastward propagation across the cwfa. I doubt "primary" is the technical name for those waves. Link to comment Share on other sites More sharing options...
Nic Posted March 30, 2011 Share Posted March 30, 2011 What is the difference between an upper level low and a surface low? Link to comment Share on other sites More sharing options...
tornadojay Posted March 30, 2011 Share Posted March 30, 2011 What is the difference between an upper level low and a surface low? A surface low is a relative minimum air pressure reading with respect to surrounding environments measured barometerically at the surface.. so any extratropical cyclones, which are your normal storm systems that occur here in the U.S. generally have an central area of lower pressure than it's surrounding environment. Upper lever lows can be found by looking at charts higher up in the atmosphere.. generally at 500 mb is where we look at these features. Here, these charts give you the height at which that particular pressure is at for any particular location. The center of an upper low would be a relative minumum height for that particular pressure level. The lower the height, the deeper the low is. The distance between two pressure levels has a relationship to temperature. We call these thickness values. A typical thickness measured is the distance from 1,000 mb to 500 mb pressure surfaces. The smaller the distance, the lower the temperature is. Link to comment Share on other sites More sharing options...
trapperman Posted March 31, 2011 Share Posted March 31, 2011 Cap. What is the difference between a strong cap in spring and a strong cap in summer? I've never understood this. If someone could go into detail on this I would appreciate it. I see strong capping this weekend in the plains per the GFS and 700mb temps of "only" 6-8 celcius. In the summer that would be an explosive situation. Please forgive my ignorance. Link to comment Share on other sites More sharing options...
isohume Posted April 2, 2011 Share Posted April 2, 2011 Lol...am I the only one that answers these questions? Anyway...if I understand your question correctly the difference btw a "strong" cap in the cool season and warm season depends on the llvl airmass in place. In the spring, the sfc layered max temps are normally lower than in the summer. Therefore, the magnitude of the capping inversion can be much lower in the spring than in the summer to prevent parcels rising adiabatically alone (w/o nearby triggers or ulvl forcing) and reaching their level of free convection (LFC). If you look on a skew-t you'll notice a weaker cap is needed to prevent parcel rise to the LFC in the spring than in the summer. Skew-t showing cool season cap with h85 temp of ~11C and sfc temp of 77 F Warm season cap with h85 temp of ~23C and sfc temp of 83 F Link to comment Share on other sites More sharing options...
Chris L Posted April 2, 2011 Share Posted April 2, 2011 Is it possible to have 60 dbz snow bands in a Nor'easter? I will say yes given the right conditions, but I have never ever seen it before. Link to comment Share on other sites More sharing options...
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