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Observations and Models of Boundary-Layer Processes Over Complex Terrain. What is the planetary boundary layer (PBL)? What are the effects of irregular terrain on the basic PBL structure? How do we observe the PBL over complex terrain? What do models tell us?

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Observations and models of boundary layer processes over complex terrain l.jpg
Observations and Models of Boundary-Layer Processes Over Complex Terrain

  • What is the planetary boundary layer (PBL)?

  • What are the effects of irregular terrain on the basic PBL structure?

  • How do we observe the PBL over complex terrain?

  • What do models tell us?

  • What is our current understanding of the PBL and what are the outstanding problems to be addressed?


Observing the pbl over complex terrain l.jpg

Most of our observational and modeling results are relevant to the homogeneous PBL because of its relative simplicity. However, most of the real world land surfaces are far from homogeneous. It is necessary to utilize techniques and instruments that can deal with this extra dimension.

Observing the PBL over complex terrain


Techniques for probing over inhomogeneous terrain l.jpg
Techniques for Probing Over Inhomogeneous Terrain relevant to the homogeneous PBL because of its relative simplicity. However, most of the real world land surfaces are far from homogeneous. It is necessary to utilize techniques and instruments that can deal with this extra dimension.

  • Remote sensing – lidars and radars – can scan both

  • horizontally and vertically, but have sensitivity and

  • resolution limitations

  • Aircraft – fast and mobile, but cannot do long-term

  • observations

  • Arrays of instruments – limited by number of sensors

  • and deployment difficulties


Slide7 l.jpg

DYCOMS-II Platform and Payload relevant to the homogeneous PBL because of its relative simplicity. However, most of the real world land surfaces are far from homogeneous. It is necessary to utilize techniques and instruments that can deal with this extra dimension.


Slide8 l.jpg

free relevant to the homogeneous PBL because of its relative simplicity. However, most of the real world land surfaces are far from homogeneous. It is necessary to utilize techniques and instruments that can deal with this extra dimension.→

troposphere

mixed →

layer

surface →

layer


Slide10 l.jpg

Normalized mixed-layer relevant to the homogeneous PBL because of its relative simplicity. However, most of the real world land surfaces are far from homogeneous. It is necessary to utilize techniques and instruments that can deal with this extra dimension.

spectra for the 3 velocity

components. The two curves

define the envelopes of

spectra that fall within the z/zi

range indicated. The dashed

blue lines indicate contri-

butions to the u and v spectra

due to mesoscale variability,

both from synoptic systems

and from surface hetero-

geneity.


Slide11 l.jpg

Map of gully substudy for CASES99 in central Kansas, USA. H refers to 2-d sonic

anemometers (at 1 m height) and TH to thermistors (at 0.5 m height).


Slide12 l.jpg

Shallow Drainage Flows – Mahrt, Vickers, Nakamura, Soler, Sun,

Burns, & Lenschow – BLM, 101, 2001.

Schematic cross-section of prevailing southerly synoptic flow, northerly surface flow down

The gully, and easterly flow likely drainage flow from Flint Hills. Numbers identify the

Sonic anemometers on the E-W transect. E is to the right and N into the paper.


Slide13 l.jpg

9-day composited diurnal variation of the 1-m height wind speed (from 2-d sonic

anemometers averaged for 5 minutes For gully stations H1 – H5

(Mahrt et al., BLM, 2001).


Slide14 l.jpg

Composited diurnal variation of 5-min averaged temperature for the 9.7 m and 0.3 m

Levels on the gully tower, and for thermistors on the gully bottom (TH4) and at the

Top of the slope (TH9) versus local time (Mahrt et al., BLM, 2001).


Slide15 l.jpg

Vertical profiles of temperature during the early-evening very stable period with gully

Flow (2000 LT) and after mixing (2300 LT) on 26 – 27 October.


Slide16 l.jpg

Temperature time-height cross- very stable period with gully

Section and 1-hr averaged wind

Vectors for 26-27 October 1999

showing northerly gully flow in

the gully bottom before 2130 LT

and southerly ambient flow at all

stations between 2130 and 0130.

reference vector on the left

represents 0.25 m/s.


Slide17 l.jpg

Surface energy balance for 26-27 October. H is sensible heat flux; Rn net radiation, G soil

heat flux and Res the imbalance = Rn - H - G


Slide18 l.jpg

Vector composite (resultant flux; Rn net radiation, G soil

winds) for the 9 most stable

nights for the 5 sonic

anemometer sites.


Slide19 l.jpg

Radiation Richardson number provides flux; Rn net radiation, G soil

predictor for local slope (gully) flows:


Slide21 l.jpg

Two RHI lidar scans from HRDL in flux; Rn net radiation, G soil

CASES-99 illustrating the time de-

pendence of waves. (a) at 05:30:49

UTC; (b) at 05:34:24 UTC. Note that

the vertical scale is 7.5 times the hori-

Zontal scale. Lidar Resolution 30 m.

(From Blumen et al., Dynamics of

Atmospheres and Oceans, 2001, Fig. 4,

p. 197).


Slide22 l.jpg

Banta, et al., 1990: Doppler lidar observations of the 9 Jan 1989 severe downslope windstorm in Boulder Colorado. 5th AMS Conf. On Mt. Meteor., 23 – 29 June 1990, Boulder, CO, USA.2222


Slide25 l.jpg

430 m 1989 severe downslope windstorm in Boulder Colorado. 5

3060 m

3040 m

3060 m

3040 m


What s going on with the co l.jpg
What’s going on with the CO 1989 severe downslope windstorm in Boulder Colorado. 5?

  • Stably-stratified flow going down gully at night (up to ~0630 LT)

  • At 0630 LT flow starts to reverse and go upslope

  • During transition (0630 to 0830 LT) highest CO occurs as CO is advected in from nearby forested area and very complicated structure occurs

  • After upslope is well-established (after 1000 LT) horizontal gradients disappear – CBL is well-established