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Sailing Colorado Lakes

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					Sailing Colorado Lakes
….Or….how to integrate frustration into
 your recreational time,

……………..and learn to enjoy it.
Part I I
       by John McGinley
         CLSC
Fact
◊ Denver….the largest city in the US
  farthest away from a major water body
  (Great Lake or Ocean).
Despite this we have a
large sailing community.

As a Colorado sailor you
have seen some unique
things…..
….two downwind boats on opposing
courses
                      Slide 0




……….massed boats knocked down in every compass
direction….and then float semi-submerged in a dead calm
….every leg of a race a beat
….major sailing voyages three
miles long and usually
circular….
…..a tornado from your boat
     ….races like this!
                                5 min

                 1 hour
RC    Finish

               Start                    3 min
                       30 Sec
…..never had to deal with this
on your lake




     Unless you’ve sailed on McConaughy!
…..an anchor used as a tactical
tool, racing or cruising, where
there is no current or tide




                    yikes
Colorado Winds and Sailing
Weather – an attempt to explain
this craziness
•   Colorado wind regimes
•   Lake breezes
•   Daily wind evolution - sailing strategies
•   Thunderstorms and lightning
•   Waves on lakes
•   Summary
The summer weather pattern. Winds in the
Plains are dominated by the Bermuda High;
mountains are dominated by monsoon and
local circulations: Denver sits between.




              Monsoon
In Eastern Colorado we get four
main wind regimes

  Westerly downslope - dangerous gusts
  Post cold frontal upslope – cloudy with showers
  Mountain upslope breeze - light and nutty winds
  Bermuda high southerlies - Denver Cyclone and
  thunderstorms
Some days are dominated by downslope or
mountain wave flow
                         Dryline




                                   Reliable
                                   Southerly
                                   Winds
Downslope Day
                         Lenticular clouds
Strong Winds



 Cap cloud
 or
 Foehn wall                                  Rotor



               Dangerous winds
Lenticular Clouds
Lenticular Clouds
Weak Summer cold fronts.
Cool upslope with clouds.
Banding is possible near the Front Range.
Showers and thunderstorms likely.
Cloudy Upslope Day
         Stratocumulus



                              steady winds




                gusty winds        steady winds
Cloud Bands



      Stratocumulus            Strong winds above




                Best sailing
Looking down on cloud bands




  Best Winds
Stratus and Stratocumulus
Stratus and Stratocumulus




                  Edge of cloud


                   steady         gusty



    Bands
  Mountain Upslope
  Dryline (red-dash) is key feature separating
  Bermuda High flow from mountain-disturbed flow - near the
  Front Range we get a light upslope mountain breeze and
  unpredictable winds. Winds are dominated by local effects.


SW
Monsoon
                                                      Reliable
                                                      Southerly
                                                      Winds
Mountain Breeze
            Cold
            Warm




  Thermal




                   Good breeze   20-30 miles
  Sunny Day – thermals form



Cumulus cloud




                  also Invisible
The Infamous Thermal




                          Cat’s paws
                            3-4 kts


         0.5 to 2 miles
Looking down on a cumulus/
thermal . Moves along with
steering flow.
           3-4 kts + translation speed

   Best Winds
Cumulus cloud and winds
- find the edge of the
cloud/thermal
Size relative to our
favorite lakes …..
         These things
  Cherry
  Creek
           can dominate   Carter
           lake winds!

 Chatfield
Cumulus clouds
Cumulus clouds – visible tracer
of thermals
Dryline sometimes is right on the Front
Range (late summer) - forms what
meteorologists call the “DENVER
CYCLONE”
Cumulus cloud    Stormy Day




 Towering
 cumulus cloud          Hail
      Rain
                                           Cumulonimbus


                                       Shelf Cloud


                      Outflow Gust front
Formation of downbursts




         Evaporative
         Cooling
Schematic of Downburst
Downburst example
Downburst example
Downburst example
LAKE BREEZES



    Do they happen here?
 Why Breezes Form –
 Land versus Water Heating




Warming   T

              L                         H

                  density difference
                  pressure difference
                     LARGE LAKE
For breeze, lake must maintain a
temperature deficit to create pressure gradient
Easy to do on large lake


               Thermal – appx ½ to 2 mi in diameter
               Governs how much mixing can take place



Warm                      Cool
Low pressure              High pressure                 Warm


                            Lake > 3 - 4 miles wide
“Gradient” wind impact on
breezes: Breeze Index
                   wind = P/(2rRS) = 589 x P/S
                                               sin (lat)
 TL= 95               Wind Speed = 22.9 kts
          Tw=65
                         Breeze Index = Wind Speed2
                                   T
                  T is the air temperature difference from lake
                      to land (F)
                   Over lake air temp = watertemp +10F
                  If BI is greater than 14, there will
                  be no breeze…. Here BI = 26, no breeze
Lake Breeze on Medium Sized
Lake




           Light winds
  Lake Breeze on Medium Sized
  Lake – with light background
  flow

Background
wind



              Light winds
Lake Breeze on Large Lake –
Sometimes forms mid lake
convergence zone ---------


            Light winds




            Light winds
  Lake Breeze on Large Lake –
  with light background wind

Background
wind
              Light winds




              Light winds
Elevated Lake Breezes in
Dammed Canyons




             Dead zones
Limits to Breezes

• Size of Lake – need to maintain a deep
  layer temperature difference
• Background Gradient Wind – if it is too
  strong breeze will be erased
Size and configuration limits:
Why Colorado lakes rarely have
breezes
• Tough to get a breeze with a lake smaller
  than 2 miles wide
• Tough to get a breeze where lake has
  steep shores (canyon, surrounded by
  mountains, etc)
                       SMALL LAKE

     Thermal contrast is mixed away by the thermal
     eddies
               Thermal – appx ½-2 mi in diameter
               effectively eliminates water-land    Top of
               temperature difference – no breeze   mixing layer

Warm
Low pressure




                             Lake – 1-2 mile wide
When breezes don’t form

• Lakes that are too small ( < 2 mi)
• Lakes dominated by gradient flow


• Lake winds are controlled by land effects
 on winds as if the lakes aren’t there
On a Colorado lake, what you
            see
     On see…..
What you a Colorado lake,
  what you see
 What the atmosphere sees
                  So see…..
             What you you are subject to all the
Hey, is
that
combine on
                     you see
              What occur over eddies, etc
               small thermals,
                that           land
starboard
or port?
Features that influence winds
on Lakes
• Channeling
• Land heating over time
Channeled flow in high-terrain
bounded lakes – air is lazy
and often goes around rather
than over




                   Near shore flow lines
                   mimic height
                   lines on a topo map
  Concept of Air Flow
   Air flows on surfaces of constant potential temperature
   These surfaces are called “isentropic surfaces”
Winds on each layer
are pushed along by
force differences
on the surface




                                                       Air flow is mostly
                                                       smooth and
                                                       steady on these
                                                       surfaces - good
                                                       for sailing


                                    As these surfaces get
                                    farther apart in the vertical,
                                    winds get more turbulent and unsteady
Evolution of winds thru a day
-active over lakes with or without a
breeze

• Morning – Cool layer – light winds
• Mid Morning – Laminar flow – steady wind
• Late morning - Steady wind with beginning thermals
    – gusts and calms
•   Noon – Cellular Clouds – wind areas and calms
•   Afternoon – Rolls – oscillating flows
•   Evening – Cellular to Laminar – steady winds return
•   Night – Laminar – sometimes great winds
•   Early morning – cool buffer over lake from drainage
    flow
Early Morning- Decoupled Flow


     Governed by background pressure field

     Flow is steady just aloft

     Winds are nearly calm on lake




   Cool layer               Lake
Late Morning- First Thermals
    Laminar flow moves up

    Cool layer erodes; steady dying winds at middle of lake

    Gusty winds develop at shore line




                          Lake
Morning Steady Winds and
Thermals
   Thermals begin to drift across lake pushed by
   background winds; steady winds
   punctuated by lulls and brief gusts; shifts are
   disorganized
Noon- Organized Cells
    Hexagonal cells form on lake

    Periods of good air and calms

    Shifts get more predictable




                         Lake
Late Morning/Noon: Hexagonal
Cells              Winds may shift 60 deg
                         At 2-4 min intervals




         Up-lgt
         winds
Afternoon- Deep Rolls
    Requires wind shear aloft (changing wind direction)
    Common with fronts; might not see if no front
    Banded rolls often visible with clouds
    Forms pulsing, oscillating, ribbon winds on lake




                         Lake
    Afternoon – Deep Rolls
  Free air wind
  Surface wind
                         1-2 nmi




                              As these features
                              drift they cause
                              wind surges and
                              oscillations at
Best Winds
                              5-10 minute
                              intervals
Early Evening- Last Thermals


    Steady flow settles back on lake; sometimes best sailing
    of day

    Waning thermals




                         Lake
Night- Laminar Flow


    Governed by background pressure field

    Wind flow is steady

    Cool layer starts to develop from cold
    air drainage




                          Lake
Early Morning- Decoupled flow


     Governed by background pressure field

     Flow is steady just aloft; cut off from lake

     Winds are nearly calm on lake




   Cool layer               Lake
Waves on Lakes
Wind
                       Lake Fetch




• Fetch to attain a fully developed sea (FDS) - Ffds
   Ffds (nm) = 3.17 x Wind 4/3 (kts)
• On a lake waves happen sooner and grow faster
Wave Height on Lake = .020 x Wind 2 x (Fetch/Ffds)1/2
                        = .011 x Wind 4/3 x Fetch (nm)
• Wind duration for fully developed waves
   Time (hrs) = H (ft)
Wave Case Study: Lake
McConaughy
• 60 kt WNW wind
                               Ffds=744nm; 72 ft wave

              20nm




     Wave Height = .011 x Wind 4/3 x Fetch 1/2

                  = .011 x 60 4/3 x 201/2
                                            Carter: 4.5 ft
                                            Chatfield: 3 ft
                   = 11.5 ft                Cherry Creek: 2.5ft
Thunderstorm Hazards
Thunderstorms, shelf clouds
and outflows
Thunderstorm wind flow
Lightning with Thunderstorms
Charge in a Thunderstorm
A mast in a thunderstorm – a
target or not?
  Cloud base
   - - - - - - - - - - + - + - + - + - + - -
                          + + + + +
                          + + + +
      + +                   ++ + +
      + +                     + +
      + +                     + +
      + +                     + +
      + +                     + +


   + + + + + + + + + + + + + + + + + + +
    Electrical field lines
A mast on a lake – a target or
not?
  Cloud base
   - - - - - - - - - - + - + - + - + - + - -
                          + + + + +
                          + + + +
      + +                   ++ + +
      + +                     + +
      + +                     + +
      + +                     + +
      + +                     + +


   + + + + + + + + + + + + + + + + + + +
    Electrical field lines
               Summary
Colorado is in a complex wind regime with 4
basic patterns

Colorado lakes are minimally impacted by
lake breezes.

Wind evolution over lakes are dominated by
processes that act over a land surface going
through a heating cycle: steady winds-
imbedded thermals-hexagonal cells –
longitudinal rolls- steady winds – morning
calms

Colorado summer hazards: thunderstorm
outflows/downbursts and lightning
Some good metro Denver/
Colorado web sites
Both these sites use analysis and forecast techniques developed at
    our NOAA laboratory: the Forecast Systems Lab…now known
    as the Earth System Research Lab
1. Rocky Mountain Center page: www.fs.fed.us/rmc Sponsored by
   USFS - best product for sailors is “High Res Data” …click on
   Forecasts and you’ll get a 24 hr forecasts of winds every 600m
   over the front range. There are also point forecasts for a
   number of CO locations.
2. LAPS web page: laps.fsl.noaa.gov Sponsored by NOAA. Has
   forecasts out to 24 hrs for a variety of weather including surface
   winds and temperature - click on “Forecasts for Colorado” then
   “Rocky Mtn 10km”; click on “SfcTempWind” at desired hour or
   loop - “Precip” might be something else you’d want to see.

				
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posted:10/2/2011
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