WESTERN REGION TECHNICAL ATTACHMENT
NO. 97-40
DECEMBER 30, 1997

USING VARIED ENHANCEMENT CURVES TO BETTER EVALUATE FOG AND STRATUS WITH RAMSDIS

Dan St. Jean - NWS Boise ID

Many examples and uses of the fog-reflectivity product imagery derived from the GOES-8 and GOES-9 imagers have been recently documented. (See NWS Western Region TA-Lites 96-1, 96-4, 96-8 and 96-15, and TA 95-25.) Detection of fog and stratus using the RAMSDIS fog-reflectivity product has proven to be an invaluable tool for forecast operations, not only in detecting fog and stratus, but also in distinguishing cloud phase, forest fires and snow cover vs. stratus when used in conjunction with other imager channels. Furthermore, the ability to toggle RAMSDIS's enhancement curves on a given image loop can be significant in making optimum qualitative use of the fog-reflectivity product.

During the nighttime hours, the fog product is displayed on the RAMSDIS fog/reflectivity loop. The default enhancement is a grey, or SVGAVIS (hereafter referred to as VIS), enhancement curve. Fig. 1 shows two areas of fog and stratus at 0630 UTC, 11 December 1997, in eastern Oregon and southern Idaho; the first extending from near La Grande, OR (LGD) southward to Rome, OR (P88), and the second from south of Mountain Home AFB, ID (MUO) eastward to near Pocatello, ID (PIH). (Corresponding 0600 UTC surface observations can be found in Table 1.) The VIS enhancement makes it difficult not only to determine the depth of the fog/stratus layer but also to better recognize where development or dissipation of fog/stratus may be occurring. Fig. 2 displays the same image with the RAMSDIS fog, or SVGAFOG (hereafter referred to as FOG) enhancement curve. The image with the FOG enhancement more clearly shows the following: (1) The thickest fog/stratus is near Twin Falls, ID (TWF); (2) A region of thinner stratus is present across much of northern Nevada (Surface observations from Winnemucca, NV (WMC) and Elko, NV (EKO) listed in Table 2 confirm this); (3) There is a break in the fog between Burley ID (BYI) and PIH, which was not as apparent with the VIS enhancement.

Conversely, during daylight hours the reflectivity product is displayed on the RAMSDIS fog/reflectivity loop. Fig. 3 displays the FOG enhancement at 1900 UTC the same day. Certainly fog and stratus are still discernible through much of the Snake River Valley from PIH into eastern Oregon, confirmed by surface observations in Table 3. It is unclear, however, if the imagery is suggesting the presence of fog farther south between P88 and WMC. The same image with VIS enhancement (Fig. 4) indicates a marked difference between this region and the stratus to the north, making a clearer distinction between fog/stratus and cloud-free regions during daylight hours. Furthermore, examination of the 1900 UTC visible image (Fig. 5) verifies the absence of clouds or snow cover in the area between P88 and WMC, with snow cover evident over southwestern Idaho and south central Oregon.

This example implies that the fog/reflectivity product can be evaluated with greater certainty if forecasters are able to toggle between the VIS and FOG enhancement curves when displaying the RAMSDIS loop. By pressing Alt-F9 on the RAMSDIS keyboard, the VIS enhancement curve is displayed on the current loop. Here's how your RAMSDIS focal point can implement a hotkey which will change the enhancement curve of the current loop to the FOG enhancement:

(1) At the RAMSDIS keyboard, press Ctrl-Esc to return to the OS2 desktop.

(2) Double click on the "OS2 Window" icon.

(3) Go to the data directory by typing:

CD MCIDAS\DATA

(4) Open the file which contains the Alt-F* hotkey commands (KEYAF.CMD):

E KEYAF.CMD

(5) Insert the following line at the top of the document:

TE KEYAF4 "EU REST SVGAFOG LOOP=YES

(This will set up the Alt-F4 key for the FOG enhancement curve)

(6) Save the file, exit the OS2 window and return to RAMSDIS by double-clicking the MCIDAS icon on the desktop.

(7) At the applications terminal (monochrome screen), type the following to activate the Alt-F4 macro:

BATCH "KEYAF.CMD

RAMSDIS is now enabled to toggle between the VIS (Alt-F9 key) and FOG (Alt-F4 key) enhancement curves.