A Multisensor Approach to Detecting Drizzle on ASOS*

820 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 20 A Multisensor Approach to Detecting Drizzle on ASOS* CHARLES G. WADE National Center for Atmospheric Research, Boulder, Colorado (Manuscript received 15 October 2002, in ®nal form 3 January 2003) ABSTRACT National Weather Service Automated Surface Observing System (ASOS) stations do not currently report drizzle because the precipitation identi®cation sensor, called the light-emitting diode weather identi®er (LEDWI), is thought not to have the capability to be able to detect particles smaller than about 1 mm in diameter. An analysis of the LEDWI 1-min channel data has revealed, however, that the signal levels in these data are suf®ciently strong when drizzle occurs; thus, they can be used to detect drizzle and distinguish it from light rain or snow. In particular, it is shown that there is important information in the LEDWI particle channel that has not been previously used for precipitation identi®cation. A drizzle detection algorithm is developed, based on these data, and is presented in the paper. Since noise in the LEDWI channels can sometimes obscure the drizzle signal, a technique is proposed that uses data from other ASOS sensors to identify nondrizzle periods and eliminate them from consideration in the drizzle algorithm. These sensors include the ASOS ceilometer, temperature, and dewpoint sensors, and the visibility sensor. Data from freezing rain and freezing drizzle events are used to illustrate how the algorithm can differentiate between these precipitation types. A comparison is made between the results obtained using the algorithm presented here and those obtained from the Ramsay freezing drizzle algorithm, and precipitation type recorded by the ASOS observer. The paper shows that by using data from the LEDWI particle channel, in combination with data from other ASOS sensors, the ability exists to detect drizzle with the current suite of ASOS instrumentation. 1. Introduction Surface Observing System (ASOS), developed jointly by the National Weather Service (NWS), the Federal Drizzle is a form of liquid precipitation that is char- Aviation Administration (FAA), and the Department of acterized by ``very small, numerous and uniformly dis- Defense (DOD). Precipitation detection and identi®ca- persed water drops. .that fall to the ground. .and (are) frequently accompanied by low visibility and fog'' tion on ASOS is accomplished using an optical sensor (Huschke 1959, p. 179). Drizzle has been recognized called the light-emitting diode weather identi®er (LED- as a distinct precipitation type since 1806 when Sir Fran- WI) manufactured by Optical Scienti®c, Inc. [formerly cis Beaufort ®rst developed a system of weather nota- Scienti®c Technology, Inc. (ScTI)]. LEDWI provides tions designed to facilitate the recording of weather ob- observations of rain and snow in accordance with the servations (Heidorn 1998). While the human observer speci®cation for the ASOS precipitation identi®er (PI; can detect and identify drizzle rather easily, it is much NOAA 1991), which states that the PI sensor must 1) more dif®cult for automated precipitation identi®cation be able to detect precipitation 99% of the time when sensors to do the same. With the trend toward replacing precipitation rates are greater than or equal to 0.25 mm the human observer with automated weather observing h21 (0.01 in. h21), and 2) once detected, be able to stations, the number of drizzle observations in the Unit- identify precipitation type 97% of the time when the ed States has decreased signi®cantly during the past precipitation is solid (snow) and 90% of the time when decade. the precipitation is liquid. False alarm rates (reporting One of the systems of automated weather stations that precipitation when none is occurring) are also required provides continuous observations of weather conditions to be less than 0.2%. Extensive testing of LEDWI during at nearly 900 sites in the United States is the Automated the early 1990s has shown that it generally meets or exceeds the ASOS speci®cation (Burgas and Laster * The National Center for Atmospheric Research is sponsored by 1995). the National Science Foundation. Drizzle is not currently reported on ASOS except at those stations where an observer may augment the ob- servation (approximately 15% of the nearly 900 ASOS Corresponding author address: Charles G. Wade, NCAR/RAP, 3450 Mitchell Lane, Boulder, CO 80301. stations provide observations that are augmented). E-mail: [email protected] There are two reasons for this. First, drizzle generally q 2003 American Meteorological Society Unauthenticated | Downloaded 09/30/21 10:53 PM UTC JUNE 2003 WADE 821 occurs at rates that are below the detection threshold of format and describes how precipitation type and inten- the PI speci®cation (i.e., ,0.25 mm h21). Thus, there sity is determined using the current LEDWI algorithm. is no requirement for LEDWI to detect drizzle. Second, Section 4 discusses how the LEDWI channel data can there is a perception that LEDWI is incapable of de- be used to help identify drizzle and shows illustrations tecting drizzle-sized particles (i.e., particles smaller than of data from a freezing rain and freezing drizzle event 0.5 mm in diameter). This arises from a statement in an that occurred in Peoria, Illinois, on 18 February 2000. ScTI document (Scienti®c Technology, Inc. 1995) that Section 5 compares the results of the drizzle detection states that LEDWI was ``designed to be sensitive to drop analysis presented here with output from the Ramsay sizes .1 mm in diameter'' (Burgas 1998). In other freezing drizzle algorithm, while section 6 shows ad- words, LEDWI was not designed to detect drizzle. ditional comparisons between LEDWI and the ASOS In recent years there has been increased interest in icing sensor. Section 7 discusses the problem of non- being able to detect drizzle on ASOS, particularly when precipitation noise in the LEDWI data and shows how temperatures are near or below freezing. Ramsay (1999) data from ASOS sensors can be used to eliminate this has developed a technique that infers the existence of problem. Section 8 discusses the problem of establishing freezing drizzle when the ASOS icing sensor reports ice drizzle intensity. Section 9 provides a summary of the accumulation on the probe and LEDWI reports no pre- drizzle detection algorithm. cipitation. This method assumes that LEDWI correctly identi®es rain when it occurs (we will show later that 2. A description of the LEDWI sensor some LEDWI ``rain'' is actually drizzle) and, of course, it does not detect drizzle at temperatures above freezing. LEDWI operates on the principle that a partially co- The NWS has begun a search for an ``enhanced'' PI herent infrared or visible light beam, when passed sensor that is capable of detecting and identifying driz- through an irregular medium, will have its frequency zle at all temperatures, as well as identifying other forms altered by the irregularities in the medium (Starr and of precipitation such as ice pellets, snow pellets, and Wang 1989). The phenomenon is called scintillation, hail. The enhanced PI sensor would replace the LEDWI examples of which are the twinkling of stars at night on ASOS. or the shimmering effect seen over a heated surface on An analysis of LEDWI channel data, recorded each a summer day. When precipitation falls through an in- minute and retained on each ASOS station for a period frared beam it also introduces frequencies in the beam of 12 h, shows that there is an elevated signal level in that are a function of the size and fall speed of the the channel data that occurs when drizzle is being re- particles. LEDWI measures the frequency composition ported by the station's observer. By comparing the val- of the beam after it has passed through precipitation, ues in the various channels there is evidence that drizzle performs a Fourier transformation on the received sig- produces a unique and characteristic pattern in the chan- nal, and deduces the type and intensity of the precipi- nel data that allows it to be distinguished from rain or tation based on the power levels generated in two fre- snow. quency bands: a low-frequency band from 75 to 250 Hz The purpose of this paper is to describe the LEDWI that is more sensitive to slower-falling precipitation, and channel data and show the relationships that exist be- a high-frequency band from 1000 to 4000 Hz that is tween the various channels when different types of pre- more sensitive to faster falling precipitation. cipitation are occurring. Based on these relationships, a Figure 1 shows a schematic diagram of the LEDWI function is developed that allows drizzle to be detected sensor. The transmitter and receiver heads are mounted and distinguished from light rain or light snow. Drizzle approximately 3 m above the ground and are oriented intensity is established using the LEDWI data in much so that the receiving head faces north. Figure 2 is a the same way that rain intensity is determined. schematic diagram illustrating how LEDWI works. The A complicating factor in detecting drizzle using LED- transmitter, located on the right side of the ®gure, directs WI data is that background ``noise'' exists in the LEDWI a pulsed infrared beam toward the receiver, located 0.8 channels that makes it dif®cult to distinguish drizzle m away. The infrared beam is 50 mm in diameter and from the noise. The noise is due primarily to index of is pulsed at 50 000 Hz. The receiving lens contains a refraction gradients that occur near the earth's surface horizontal slit aperture that has a vertical dimension of as a result of heat exchange processes. Thus, the noise 1 mm. The purpose of the slit is to make LEDWI more generated by these gradients is particularly high during sensitive to the vertical motion of the particles rather sunny, nonprecipitating weather and is minimal during than their horizontal motion.

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