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A Method to Determine Precipitation Types

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A Method to Determine Precipitation Types OCTOBER 2000 BOURGOUIN 583 A Method to Determine Precipitation Types PIERRE BOURGOUIN Canadian Meteorological Centre, Dorval, Quebec, Canada (Manuscript received 23 September 1999, in ®nal form 31 May 2000) ABSTRACT A method to diagnose surface precipitation types is suggested. Most cases of freezing rain and ice pellets occur with a layer warmer than 08C extending above a surface-based layer of air colder than 08C. The procedure uses predictors proportional to the product of the mean temperature of a layer and its depth. These predictors can be seen as areas on aerological diagrams. A positive area is associated with a layer warmer than 08C, conversely a negative area is associated with a layer colder than 08C. The same predictor is used to discriminate snow from rain. A statistical analysis was applied using the North American aerological stations network to determine a set of criteria for discriminating freezing rain, ice pellets, snow, and rain. Once the criteria are known, the precipitation type can be easily diagnosed using temperature pro®les from upper-air observations or from numerical weather prediction models. The method has been in operational use at the Canadian Meteoro- logical Centre since 1995. 1. Introduction ature variation of only 18C is suf®cient to induce a tran- sition between different phases, for example between Forecasts of winter precipitation types have long been freezing rain and rain or between snow and rain. This a concern for weather forecasters (Brooks 1920) because implies that an accurate vertical temperature pro®le they may have important consequences for human ac- forecast is needed if one hopes to correctly determine tivities. Inconveniences associated with freezing rain, snow, or ice pellets can range from disturbing local precipitation types. Although temperature is most im- ground transportation or aircraft operations to paralyz- portant, ideally any diagnostic for precipitation type will ing a large region. For example, the great ice storm of take account of all related atmospheric parameters. January 1998 dramatically affected eastern Canada from One way to include all the physics associated with 4 to 10 January 1998. The accumulation of freezing rain precipitation type is to develop a sophisticated explicit and ice pellets exceeded 100 mm in many areas of south- microphysics model. However, these models are expen- western Quebec and eastern Ontario during that period. sive in computer time, limiting their operational appli- As a result, thousands of trees were downed, hydro wires cations. Furthermore, a knowledge of initial hydrome- and pylons were destroyed, and transportation was teor phases or distributions, cloud condensation, and ice greatly affected. At least 25 people died in the storm nuclei distributions is needed but not routinely ob- and nearly one million people were left without heat or served. Some optimized microphysics models will like- electricity for more than a week (Reagan 1998). This ly become available in the future, but in the meantime was rather an extreme case, but it illustrates the need other alternatives must be used to routinely forecast pre- of issuing accurate forecasts of precipitation types. cipitation types. The most common approach is to derive The observed precipitation type depends on speci®c statistical relationships between some predictors and atmospheric conditions that include thermal and mois- different precipitation types. Relationships are simple ture distributions, vertical motion, cloud, and ice nuclei to derive using an adequate database of easily available distributions. However, it has long been recognized that parameters, generally related to temperature and mois- the vertical temperature pro®le is of prime importance ture. Once, developed, this approach requires very little (see, e.g., Wagner 1957; Koolwine 1975; Bocchieri computer time to produce forecasts. Other techniques, 1980; Czys et al. 1996). In some situations, a temper- such as that of Czyz et al. (1996), are physically based and remain to be evaluated. This paper describes the development and testing of a new method for diagnosing precipitation type, called Corresponding author address: Pierre Bourgouin, Canadian Me- the area method. A general description of the problem teorological Centre, 2121 Trans Canada Highway, North Service Road, Dorval PQ H9P 1J3, Canada. of precipitation type diagnosis is given in section 2, E-mail: [email protected] along with a discussion of some existing objective di- q 2000 American Meteorological Society 584 WEATHER AND FORECASTING VOLUME 15 FIG. 1. Schematic diagram showing typical vertical temperature pro®les leading to (a) freezing rain or ice pellets, (b) ice pellets or rain, (c) snow or rain, and (d) snow. Positive (PA) and negative (NA) areas are indicated (see text for details). agnosis techniques. The area method is presented in remaining trapped in valleys. However, if the layer section 3. Section 4 includes results of tests of the new warmer than 08C aloft in Fig. 1a is too small, precipi- method and results of comparison tests with some of tation will be snow, but not ice pellets or freezing rain. the techniques described in section 2. This paper con- If there is a warming suf®cient to bring surface tem- cludes with a discussion. peratures above 08C and little temperature change aloft, a vertical pro®le as given by Fig. 1a will become as given in Fig. 1b. Just above the surface-based warm 2. Diagnosis of precipitation type layer, precipitation may be in the form of freezing rain As stated previously, it is the vertical temperature or ice pellets (if the above freezing layer aloft is large pro®le that is the main determinant of precipitation type. enough) as discussed above. If freezing rain is present, If the pro®le crosses the 08C isotherm one or more times, water drops may remain supercooled, especially large then rain, freezing rain, ice pellets, snow, or mixtures and rapidly falling ones, as they reach the surface. of these hydrometeors can occur. In this section, typical Freezing rain is reported by observers when drops temperature pro®les associated with different precipi- freeze on impact with an ice accretion indicator, the tation types are described. ground, or other objects at or near the earth's surface Freezing rain or ice pellets are possible whenever according to reporting standards. Freezing rain may be precipitation falls through a layer warmer than 08C and reported with surface temperatures above 08C. However, through a subfreezing layer underneath (Fig. 1a). This it is a rare occurrence, because ice accretion indicators occurs when warm air advection, generally associated or other objects rapidly adjust their temperature to the with extratropical cyclones, is stronger aloft than near ambient temperature. For that reason, we decided to the surface. Conditions favorable to freezing rain or ice exclude freezing rain from possible precipitation type pellets will persist longer if low-level cold air advection associated with a sounding similar to Fig. 1b and will is present, which is frequently associated with topo- not be considered further. If freezing rain is present just graphically induced cold air drainage, or with cold air above the surface-based warm layer, we expect rain to OCTOBER 2000 BOURGOUIN 585 be reported at the surface. If ice pellets rather than freez- 700 hPa) generally indicates a layer with temperatures ing rain are present, they may melt and be reported as warmer than 08C. On the other hand, a thickness lower rain at the surface or they may survive the fall and be than 131 dam in the lower layer (1000±850 hPa) sug- reported as ice pellets, possibly mixed with rain. Again, gests temperatures under 08C at or near the surface. For if the above freezing layer aloft is too small, snow will example, conditions leading to freezing rain are a 850± be present above the surface-based warm layer. In that 700-hPa thickness larger than 154 dam combined with case, rain or snow may be reported depending on con- a 1000±850-hPa thickness between 129 and 131 dam. ditions. As a result, a pro®le as in Fig. 1b may produce The same upper-layer thickness combined with a lower- rain, ice pellets, mixed rain and ice pellets, or snow. layer thickness less than 129 dam would yield ice pel- While an above freezing layer aloft is required for lets, while a value above 131 dam would result in rain. freezing rain and ice pellets situations, diagnosis of pre- An advantage of this technique is that a synoptic map cipitation type also requires discrimination between rain of the location of precipitation types can be produced. and snow. This may be dif®cult when temperatures in It should be noted that the criteria associated with this the low levels are slightly above 08C (Fig. 1c). With method are not ®xed in operational forecasting. They soundings as in Fig. 1c, snow, rain, or mixed snow and are modi®ed by forecasters in each situation based on rain are possible. If there is no above freezing layer subjective evaluations of different parameters such as (Fig. 1d), snow is generally expected, but it is also pos- the intensity of vertical motion or the cold air advection sible to get freezing drizzle (Bluestein 1993; Huffman or typical synoptic patterns. Also, the technique was and Norman 1988). This phenomenon is due to the fact developed for eastern Canada and is not directly appli- that droplets of drizzle are very small and hence more cable to other geographical areas, especially over moun- likely to remain in liquid form than larger droplets at tainous areas. the same below freezing temperatures. This situation will not be considered further. Many operational techniques have been developed to c. Ramer (1993) method associate speci®c atmospheric conditions to different precipitation types. When the method proposed in this The technique proposed by Ramer uses temperature paper was developed, the Derouin (1973) method was (T), relative humidity (RH), and wet-bulb temperature used operationally at the Canadian Meteorological Cen- (TW) on different pressure levels to diagnose precipi- tre (CMC) and the Cantin and Bachand (1993) method tation types.
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