Ceilometers – Chances of Rain

In Slim’s post yesterday he spoke of a ceilometer being used to measure cloud height and the amount of cloudiness and how they differentiate between cloudy, partly cloudy, etc skies.

A ceilometer is a device for measuring the height of cloud bases and overall cloud thickness. One important use of the ceilometer is to determine cloud ceilings at airports. The device works day or night by shining an intense beam of light (often produced by an infrared or ultraviolet transmitter or a laser), modulated at an audio frequency, at overhead clouds. A photocell detects reflections of this light from the base of the clouds in the receiver of the ceilometer. There are two basic types of ceilometers: the scanning receiver and the rotating transmitter.

The scanning-receiver ceilometer has its separate light transmitter fixed to direct its beam vertically. The receiver is stationed a known distance away. The parabolic collector of the receiver continuously scans up and down the vertical beam, searching for the point where the light intersects a cloud base. When a reflection is detected, the ceilometer measures the vertical angle to the spot; a simple trigonometric calculation then yields the height of the cloud ceiling. Many modern scanning-receiver ceilometers use a laser pulse to identify the height of a cloud’s base and top and various points in between to create a vertical profile of the cloud.

The rotating-transmitter ceilometer has its separate receiver fixed to direct reflections only from directly overhead while the transmitter sweeps the sky. When the modulated beam intersects a cloud base directly over the receiver, light is reflected downward and detected. (Britannica).

The cost of a ceilometer is $30000 to $100000 so it is not something most couldn’t afford to put in their own backyard.  I think you will find most of them around airports.

 

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We will see showers and isolated thunderstorms developing today mainly south of I-96 which could produce a good soaking in some locations, while many others receive lighter amounts or stay dry. More sunshine is expected farther north.

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Forecast Discussion

-- Showers/t-storms today, mainly south --

A weak/diffuse surface low in IL this morning will continue
northeastward today and undergo modest deepening, reaching Lake Erie
tonight. Lift accompanying this feature and its associated shortwave
aloft will aid the development of showers/t-storms today, with
greatest coverage (and highest PoPs) across southern Lower Michigan.
RAP forecast profiles at JXN indicate rich moisture (PW near 1.75
inches), tall/skinny CAPE profiles (MLCAPE of 800-1200 J/kg), and a
deep (14 kft) warm-cloud layer. These environmental factors support
efficient warm-rain processes, and suggest that heavy rain rates are
possible. Additionally, cell motion will likely be quite slow this
afternoon/evening, with a mean cloud-layer wind under 10 kts.

The latest HREF 24-h LPMM precip through 12z Mon suggests that
isolated rainfall amounts around 1.5 inches are possible in the
southern CWA, with a relatively higher risk of heavy rainfall just
SE of the area. The bottom line is that showers/storms today will be
monitored for heavy rainfall, and that urban areas will be most
prone to minor flooding if any slow-moving showers/t-storms persist
locally.

-- Warm/dry weather expected Mon and Tue --

Subsidence/drying behind the departing wave should provide
clearing on Mon as a low-level ridge builds into the region.

For Mon night, some guidance (i.e., the 00z NAM-nest, along with EPS
and GEFS probs) suggests that an MCS may approach the region from
the NW, but will likely weaken prior to reaching the forecast area.
This signal will be reassessed in later updates, but no PoPs are
introduced on Mon night with this package.

-- Strong system Tue night and Wed --

Global ensemble guidance remains consistent in the timing/placement
of a potent shortwave impulse that will crest the western CONUS
ridge Mon and Mon night, then approach the region on Tue. The
associated PV anomaly is evident in water-vapor imagery near
Vancouver Island at present.

The main surface cyclone associated with this impulse will likely
be located near ND / Manitoba at 12z Tue, with MI situated well
within a broad warm sector by Tue evening. Aloft, an approaching
midlevel jet streak (60 kts at 500 mb) will contribute to
increasing deep-layer shear over Lower MI on Tue night, with 0-6 km
bulk shear reaching 40-50 kts by 12z Wed.

The primary question remains the extent of instability (surface
based or elevated) that will be present over the forecast area prior
to cold frontal passage--likely occuring between Wed morning and
early afternoon. If sufficient instability can become established
near/west of the region on Tue night, then severe thunderstorms will
be possible on Tue night. It is also plausible that the main risk
for severe storms materializes on Wed, and primarily in the eastern
CWA. Overall, many mesoscale aspects of this period remain uncertain
as of this update, including the possible evolution of antecedent
convection, but operational CAMs will provide some guidance
starting on Mon.