Highlights of Refractivity Observations by Radar (and Some More) during IHOP_2002 Frédéric Fabry and ShinJu Park McGill University Montréal, Canada

A Few Definitions IHOP_2002 IHOP_2002: International H 2 O Project. Its goal: Determine how much does knowing 4-D H 2 O distribution help in QPF. Refractivity (N) Refractivity (N): A quantity related to the index of refraction (n) of air: N = 10 6 (n−1) = 77.6 P/T e/T 2 N = 10 6 (n−1) = 77.6 P/T e/T 2. Surface N can be measured by radar used to infer e and T d Surface N can be measured by radar up to a range of about 50 km using ground targets. Since over that range, P and T are (fairly) uniform, N can be used to infer e and T d.

IHOP: Instrumentation Deployment

IHOP: S-Pol

Early fears that ground targets would be in short supply in the Panhandle were unfounded, thanks to unburied power/phone lines, farms, elevators… Liberal Hooker Beaver Perryton Booker Beaver R. valley KS OK TX IHOP: Ground Targets at S-Pol Homestead

Real-Time/Default Display Storm Outflow 60 Rapid moistening Wet Dry Diurnal cycle (mostly)

Example: Surface Moistening, 25 May No wind (!) Maximizes local effects Sunny, cool, calm morning; will warm quickly Anomalous propagation echoes Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain

Example: Surface Moistening, 25 May Previous rain (g m -2 s -1 ) 120 W 360 W Flux computed thanks to a lot of crude assumptions (too high contrast expected)

Example: Surface Moistening, 25 May

Previous rain Note how “fragile” the moisture patch was under light winds

Contrasting Example: 29 May 72 hrs later, on a sunny day prior to 29 May No wind again. Maximizes local effects Some weak differentialmoistening

Contrasting Example: 29 May

East winds building up

Contrasting Example: 29 May East winds building up

Contrasting Example: 29 May East winds building up

Contrasting Example: 29 May East winds building up Moistureboundary

Contrasting Example: 29 May Moistureboundary

Moistureboundary

Moistureboundary

Winds shift to SE behind 2 nd boundary Moistureboundaries

Contrasting Example: 29 May Winds shift to SE behind 2 nd boundary Moistureboundaries

Contrasting Example: 29 May Winds shift to SE behind 2 nd boundary Moistureboundaries

Contrasting Example: 29 May Winds shift to SE behind 2 nd boundary Moistureboundaries

Contrasting Example: 29 May

Windsshifting to South Boundary still lurking

Contrasting Example: 29 May Windsshifting to South Boundary still lurking

Contrasting Example: 29 May Windsshifting to South Boundary still lurking

Contrasting Example: 29 May Windsshifting to South Boundary still lurking

Contrasting Example: 29 May

And now to the SW, and then to the West

Contrasting Example: 29 May And now to the SW, and then to the West

Contrasting Example: 29 May And now to the SW, and then to the West

Contrasting Example: 29 May And now to the SW, and then to the West

Contrasting Example: 29 MayBoundary makes a comeback

Contrasting Example: 29 MayBoundary makes a comeback

Contrasting Example: 29 MayBoundary makes a comeback

Contrasting Example: 29 MayBoundary makes a comeback

Contrasting Example: 29 May

Advection of drier air on the dry side

Contrasting Example: 29 May Advection of drier air on the dry side

Contrasting Example: 29 May King Air overflights start Winds are not very strong but not light. Advectedboundariesintersects flight path

Contrasting Example: 29 May King Air overflights in progress Winds are not very strong but not light. Advectedboundariesintersects flight path

Contrasting Example: 29 May Winds are not very strong but not light. King Air overflights in progressAdvectedboundariesintersects flight path

Contrasting Example: 29 May  A strength of N maps: Provide context to ABL work

Example: Boundary Evolution

Example: Convection Initiation Confused boundary (more in Z than in N) that will sharpen with time. Rolls-like bands Cold front approaching

Example: Convection Initiation

Dry line Cold front approaching

Example: Convection Initiation Dry line Cold front approaching Moisture

Example: Convection Initiation Dry line Cold front approaching Moisture New cells

Example: Convection Initiation Dry line Collision Moisture New cells

Example: Convection Initiation Dry line Collision Moisture New cells New cell

Example: Convection Initiation Rain-induced moist patch New cells New cell

Example: Convection Initiation Rain-induced moist patch New cells

Example: Convection Initiation

Anothercollision

Anothercollision

Anothercollision

Anothercell

Anothercell

Anothercell

ShinJu Park (Ph.D. student) is investigating this event. Advices welcome. More on this event coming…

Phenomena Observed during IHOP Boundary layer processes: Convective rolls; Uneven moistening of BL by surface fluxes. A variety of moisture discontinuities: Fronts; Drylines and other convergence lines; Gust fronts and outflow boundaries; More diffuse (10-20 km wide) gradients. (Primarily) nocturnal wave phenomena: Nocturnal bores; Other waves by themselves or embedded in fronts. Now we need to digest at least some of it…

Data Processing: IHOP “Climatology” Broad WNW-ESE refractivity and daily N gradient observed. Mimics (reflects?) climatological moisture gradient in the area.

Small-Scale Structure of N Thanks to the distributed measurements and the near-continuous coverage in time, one can perform some statistics that would be very hard to do with other sensors. Ex.: Systematic study of the spatial variability of N. How fast does N change with distance?

Small-Scale Structure of N Lesser variability in along-wind direction than across: More mixing in along-wind direction via surface friction? More advection-driven E-W gradients (across wind).

Small-Scale Structure of N Greatest variability in the afternoon until after sunset; Smallest just around sunrise (some wind dependence).  Implications on representativeness of in-situ data.

Small-Scale Structure of Humidity The amount of afternoon-time small-scale variability changes significantly from day to day. Causes? Possibilities: Surface-flux driven; BL top driven (dry air entrainment); Large-scale driven. More?

Small-Scale Structure of Humidity First (and only) hypothesis tested: BL top driven. If true, small-scale variability should be well correlated with the amount and dryness of inversion air entrained. Sounding now Sounding later Inversion air now included in BL will dry BL

MAD measured by surface data; Expected drying computed from Homestead soundings and measured surface warming. Small-Scale Structure of Humidity On days with a well-developed BL, correlation between BL drying by entrainment and small-scale variability is high.  Possible to predict? If yes, it could be useful to evaluate the expected errors in BL humidity from sondes.

Distance (km) Spatial Structure of T, q, w How important is the observed variability in N or humidity for CI?  Need to contrast with effect on CIN of heat, updrafts. Step 1: Measure the variability of all these parameters in the (upper) boundary layer (King Air data used).

Spatial Structure of CIN At large scales: Temperature is the biggest player. At small scales: Updrafts are the most important. Moisture variability generally plays a lesser role (!) Step 2: Convert the spatial variability of all these parameters into CIN variability.

A More Detailed Look at a CI Event (1) Cold front (2) Dry line (3) Moist air (4) Extensions of waves on the dry line in the moist air? (5) New cells forming on these extensions (6) Cold front-dry line collision in the making

A More Detailed Look at a CI Event (5) New cells forming on these extensions (6) Cold front-dry line collision in the making (7) Cell resulting from that collision