Lowlevel jets Various types and causes

1
Low-level jets Various types and causes

• pre-frontal (warm conveyor belt)
• nocturnal
• barrier
• West coast (Baja jet)
• East of Andes
• East of Front Range (Appalachians)
• SE coast of S. Africa, Australia
• Great Plains

2
Prefrontal low-level jet
3
Nocturnal Low-level jet

• Nighttime radiative cooling
• Where is the NBL cooling (per unit
  area)
• Development of a nocturnal BL (NBL)
• not usually steady-state (steady-state is
  possible, eg Antarctic winter night)
• Depth and strength a function of
• sfc cooling rate
• Pre-existing temperature profiles
• Pre-existing wind profile
• Low-level jet
• Wind maximum just above top of NBL (300-500 m
  AGL)
• Speed 0-30 stronger than geostrophic wind
• Decoupled from sfc friction layer
• (plus some inertial response)
• peaks in strength midnight-dawn

Z(m)
day
night
Mean LLJ max winds for 16 July days (height
AGL) (Bonner 1968)
4
Great Plains LLJ southerly vs northerly

• Example of northerly (barrier) jet
• http//www.cira.colostate.edu/ramm/picoday/020320
  /mar8_vis.html
• http//www.cira.colostate.edu/ramm/picoday/020320
  /mar8_ir.html
• http//www.cira.colostate.edu/ramm/picoday/020320
  /mar8_sfc.html

5
Great Plains southerly LLJ
Hodograph, Norman OK 12 Z 5/3/97

• strong diurnal variation nocturnal decoupled
• present up to 50 of time in spring/early summer
• strongest/most common in central OK/KS
• peak winds 15-30 m/s
• Potential from significant moisture/thermal
  advection
• Potential for severe storms (elevated
  convection)
• Potential for aircraft hazards (shear-induced
  turbulence)

LLJ frequency (divide by 1462 days or about 4
years to obtain actual frequency) source Bonner
1968
6
Great Plains southerly vs northerly LLJ
S-LLJ only (weak synoptic forcing)
N-LLJ and S-LLJ (strong synoptic forcing)
southerly
northerly
climatology for 702 cases of southerly LLJ and
658 cases of northerly LLJ over the ARM-CART site
(Whiteman et al 1997)
7
Central Great Plains southerly low-level jet
causes

• Synoptic-scale (colorado lows)
• Nocturnal jet (decoupling from NBL)
• Thermal wind
• Inertial oscillation
• winds in convective BL (CBL) subgeostrophic
• assume friction now disappears (nighttime
  decoupling)
• result is inertial oscillation
• inertial period is 2p/f (midlatitude about 17
  hours)
• wind becomes super-geostrophic (Fu,Fv) is
  departure from geostrophy, more so if daytime
  retardation is larger

Mean LLJ winds for 702 cases of southerly LLJ
over ARM-CART (Whiteman et al 1997)
Rawinsonde wind profiles on 31 July 1994
(Whiteman et al 1997)
8
Now the PGF also varies diurnally in the Great
Plains
night

• The PGF varies diurnally (think of it as buoyancy
  forcing with a varying horizontal
  terrain-following component)
• This forces a diurnally-varying acceleration
  (terrain-following)
• And this creates a geostrophic wind
  (terrain-following, southerly at night and
  northerly at day)
• Simpler explanation
• At day the west is warmer than the east (at say
  900-850 mb), ie there is a heat low to the west
• This creates a northerly geostrophic wind at day
• at night this heat low weakens, sometimes
  disappears
• Weaker southerly geostrophic wind at night

day
9
LLJ off the California coast (Baja jet west
coast jet)
Flight track
west
east
Source Parish 2000
What is the thermal wind shear, say at b, at 500
m?
10
By flying at constant pressure, the height of an
isobaric surface can be determined by the radar
altimeter.
coast
meridional
VT calculated geostrophic wind profile obtained
by adding thermal wind components to lowest-level
geostrophic wind Vg
Conclusions The coastal jet is close to
geostrophic balance Wind shear due to the
sloping MBL depth The large-scale structure of
sloping MBL top and its attendant low-level jet
is consistent with the geostrophic adjustment of
thermally direct circulation forced by the
horizontal temperature contrast between land and
ocean lasting more than just an afternoon (sea
breeze)
11
Barrier jet east of central Andes
Santa Cruz Bolivia
Mean 12 Z wind profile over Santa Cruz in January
12
references

• Great Plains
• Walters CK, Winkler JA, 2001 Airflow
  configurations of warm season southerly low-level
  wind maxima in the Great Plains. Part I spatial
  and temporal characteristics and relationship to
  convection. WEATHER FORECAST 16 (5) 513-530.
• Whiteman CD, Bian XD, Zhong SY, 1997 Low-level
  jet climatology from enhanced rawinsonde
  observations at a site in the southern Great
  Plains. J APPL METEOROL 36 (10) 1363-1376.
• Helfand HM, Schubert SD, 1995 Climatology of the
  simulated great-plains low-level jet and its
  contribution to the continental moisture budget
  of the United States. J CLIMATE 8 (4) 784-806.
• Mark J, Arritt RW, Labas K, 1995 A climatology
  of the warm-season great-plains low-level jet
  using wind profiler observations. WEATHER
  FORECAST 10 (3) 576-591.
• Lanicci JM, Warner TT, 1991 A synoptic
  climatology of the elevated mixed-layer inversion
  over the southern Great Plains in spring. 3.
  relationship to severe-storms climatology.
  WEATHER FORECAST 6 (2) 214-226.
• West coast
• Parish TR, 2000 Forcing of the summertime
  low-level jet along the California coast. J APPL
  METEOROL 39 (12) 2421-2433.
• Douglas MW, Valdez-Manzanilla A, Cueto RG, 1998
  Diurnal variation and horizontal extent of the
  low-level jet over the northern Gulf of
  California. MON WEATHER REV 126 (7) 2017-2025.
• Appalachians
• Sjostedt DW, Sigmon JT, Colucci SJ, 1990 The
  Carolina nocturnal low-level jet - synoptic
  climatology and a case-study. WEATHER FORECAST 5
  (3) 404-410.
• South America, China, South Africa
• Seluchi ME, Marengo JA, 2000 Tropical-midlatitude
  exchange of air masses during summer and winter
  in South America Climatic aspects and examples
  of intense events. INT J CLIMATOL 20 (10)
  1167-1190.
• Ding YH, 1992 Summer monsoon rainfalls in China.
  J METEOROL SOC JPN 70 (1B) 373-396.
• Ross KE, Piketh SJ, Swap RJ, et al., 2001
  Controls governing airflow over the South African
  lowveld. S AFR J SCI 97 (1-2) 29-40.