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WVI - Some additional notes

FUNDAMENTALS

 **  Spacecraft sensors integrate radiant heat energy through the column of the atmosphere within the field of view (FOV), NOT from a fixed level.
 **  WV radiances are detected using Channel 10, which is tuned to 6.7 microns, and allows a resolution of around 5 km (Meteosat & GOES) at the sub-satellite point (SSP)
 **  Radiation "seen" is biased towards nearest WV layer in the FOV, or the underlying (relatively warm) surface, if no cloud or very dry air is present.
 **  Maximum response: 80% of radiation from 620 to 240 mbar, with notional maxima of response for "standard atmosphere" at around 400 mbar. (However, this 'fixed' level is mis-leading: it is best to remember that the radiation is integrated through a layer having diffuse upper & lower bounds.)
 WHAT THE SHADES INDICATE
 BLACK  : "warm" (low altitude / near-surface radiation source; small amounts of water vapour (in the sub-satellite column) in the FOV)
 DARK GREY  : "cool" (slightly colder than low altitude - typical of AC / thin, low AS clouds, therefore typical of low or mid - troposphere around or just below the level of non-divergence (LND)
 LIGHT GREY  : "cold" ( typical of thicker AS levels, or tops of NS, or thinner (but low) CI/CS OR areas of high humidity [ but no clouds evident ]-- around and just above the LND, and at the theoretical maxima of radiation that the sensor is responsive to.
 NEAR WHITE  : "very cold" ( typical of thick / high CI/CS)
 BRIGHT WHITE  : typical of CB clusters which poke out above the general moist / cloudy levels. (Useful for detection of MCS)
 ** Importance of WV imagery lie not just as an instantaneous image, but with loops over time.
 Blacker-with-time: (implies) >  warming / lowering WV content.
   Either (or combination of):
   (a): Descending air or
   (b): Advection of drier air or
   (c): Clouds being replaced by non-cloudy air
 Whiter-with-time: (implies) >  cooling / increasing WV content.
   Either (or combination of):
   (a): Ascending air or
   (b): Advection of moister air or
   (c): Non-cloudy zones turning cloudy

( : but slight changes between moist / non-cloudy and moist / cloudy zones cannot be inferred from WVI, especially at CI levels.)

 **  WVI patterns take up the character of the flow they are in. In the FOV, a WV signature, especially in developmental situations, will almost certainly not be at one level. It is therefore a "tracer" of horizontal and vertical atmospheric motions. Even at jet-stream altitudes, where we tend to assign a 'fixed' level to the jet core, the jet often wanders up and down through several hundreds of metres, and where marked cross-contour flow is involved, then at the entrances and exits to such jets, changes in altitude of at least 1500 m are not unusual.
 **  There are often abrupt changes between "black" and "light-grey" areas: i.e. as between dry / descending and moist / ascending zones. [ US & Canadian Met. sources point to these regions as most likely ones for Clear Air Turbulence (CAT) - indeed they quote a successful detection rate of some 80 % for CAT in these areas. The sharper the boundary, the more likely is CAT to be found, though no inference (as yet detected) can be made as to the severity of the CAT. ]
 **  Very useful in detecting vorticity patterns - which, with attendant moisture indicators, can give useful clues to development.

 FOUR MAIN CATEGORIES WITHIN WHICH WVI CAN PROVE USEFUL
 (A)    VORTICITY DRIVEN DEVELOPMENT (VDD)
   **  Beginning of dry slot ("dry intrusion") cyclonically curved into the vortex centre -- associated centre is "closing / cutting off"
   **  Broadening / blacker dry slot -- indicates development is still in place -- maximum cyclogenesis about to begin. (Intrusion of high tropospheric / lower stratospheric air)
   **  If dry slot is ill-defined / not-warming (darkening) with time -- development of parent system possibly arrested early.
   **  System is weakening when dry / descending air totally encircles the vortex.
[ Sometimes, the dry slot encircles the vortex several times.]
   **  Well defined "hook" on dry imagery: defines classic PVA areas -- indicating strong development.
   **  Short-wave troughs (SWT): active troughs, generating "+SHRA/+RA/TS" rather than just an enhancement of the general shower regime - often marked by a noticeable dry-line (or abrupt change from 'moist' (white) to 'dry' (dark grey/black); found either along or just to the rear of the lower tropospheric trough axis (i.e. ~700hPa).

 


AN EXAMPLE OF A DEVELOPMENT WHERE THE DRY SLOT NEATLY FITS WITH IDEAS OF CYCLONIC DEVELOPMENTS AS OUTLINED ABOVE

 standard analysis at 06Z  This is the conventional surface analysis with the familiar fronts, lows etc. After this point, the depression deepened smartly (but NOT rapidly), to a value of 988 hPa (or mbar) in the Oslo area of Norway some 36 hrs later. 'A' indicates the area influenced by the warm-air conveyor ahead of the driving upper trough - showing how warm, humid air is thrown well ahead of the developing centre to produce the 'white' area seen on the WV image: 'B' indicates the cold low-level air cutting in behind the development.
 image of developing dry slot here  A: 'Dry' slot tucking in behind developing depression: darkening of the dry intrusion indicated that development (falling surface pressure) was to be expected.
 B: High water vapour content detected in the 'conveyor' streams associated with the baroclinic development - leading to thick cloud & rain/snow etc.
 C: The Polar Front Jet lay just to the north of the black / white discontinuity along this clear-cut edge ... however note comments below, as the PFJ can only be placed like this in certain circumstances.
 

 (B)    UPPER-LEVEL BROADSCALE FLOW PATTERNS (JETSTREAMS)
   **  Beware placing jetstreams along WVI boundaries (see introductory note). Water vapour patterns trace the level within which the WV maxima lies, which varies with both space [ all three dimensions ] and time -- e.g. the conveyors associated with extra-tropical depressions.
   **  Some general rules:
> a strong / non-buckling jet can be located by the sharp, well-defined poleward cloud edge of an associated cirrus shield (but see comments above re: placement of jet).
> the stronger the jet -- the better the definition (the greater the ageostrophic forces).
> the location of the dark / descending zone relative to the jet depends on the curvature of the flow:
>> CYCLONIC: darkest zone equatorwards of jet core
>> ANTICYCLONIC: dark zone is poleward of jet core.
(NB: however, I have not found this to be a very true statement, and some reservations are held about this.)
> jet streaks can be traced at the head of a dark zone which is known (from independent analysis) to be within the jet region.
 (C)    POTENTIAL ("CONVECTIVE") INSTABILITY
   **  For release of Convective instability -- decrease of ThetaW with height -- need dry, mid-level air over-running moist low level air. Use (a): WVI loops to trace mid-level moisture and (b): NWP ThetaW fields to trace low-level moist plumes.
   **  Preferred location for maxima of such development is along the leading edge of a dark zone.
 (D)    MONITORING NWP HUMIDITY/DERIVED FIELDS
   **  Use loop to infer movement of edges of moist zones -- i.e. does movement agree with appropriate NWP frames at the same time -- if not, adjust NWP derived output as required.

 Some other notes that might be useful .....
 **  Use change of "whiteness" to ascertain ascent / development or descent / decay.
 **  Broad troughs often have a sharp WV (dry/moist) boundary immediately to the rear of the axis - i.e. along the line that marks the abrupt reversal of the sign of vertical motion / change of sign of relative vorticity.
 **  Sub-tropical jets (STJ): usually show a marked edge which enables the translation / shape change to be monitored, thus confirming, or amending NWP ideas.
 **  NWP relative humidity fields can be cross-checked with WVI with high degree of correlation. Any deviations due to the model 'atmosphere' deviating from the "real" atmosphere can be seen and allowed for.
 **  Sharp WVI discontinuity along jets caused by sensing of lower stratospheric air (low WV content) and dry / descending polar maritime air.