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Occurrence of Nonsurface Superadiabatic Lapse Rates Within RAOB Data

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Occurrence of Nonsurface Superadiabatic Lapse Rates Within RAOB Data 350 WEATHER AND FORECASTING VOLUME 11 Occurrence of Nonsurface Superadiabatic Lapse Rates within RAOB Data RICHARD L. SLONAKER Colorado Center for Astrodynamics Research, University of Colorado, Boulder, Colorado BARRY E. SCHWARTZ NOAA/ERL/Forecast Systems Laboratory, Boulder, Colorado WILLIAM J. EMERY Colorado Center for Astrodynamics Research, University of Colorado, Boulder, Colorado (Manuscript received 13 January 1995, in ®nal form 16 January 1996) ABSTRACT As part of creating an atmospheric database for research purposes, 73 497 radiosonde observation (RAOB) soundings from 1983 through 1987 were checked for nonsurface (at least 50 mb above the surface) superadi- abatic lapse rates (SLRs). About 60% of the input pro®les contain a nonsurface SLR, most of which are subtle. Some of the superadiabatic reports are extreme, indicating probable RAOB error. These erroneous upper-air data are capable of corrupting derived meteorological parameters and analyses. A check for nonsurface SLRs allows these suspect data to be ¯agged for deletion or correction. The occurrence of superadiabatic reports is somewhat correlated with season and geographic location. However, all meteorological conditions are prone to these reports of nonsurface SLRs. A quality control criterion is developed to check for nonsurface SLRs using potential temperature, which is not overly sensitive in thin layers (as opposed to lapse rate). During RAOB ascent, any nonsurface report of a potential temperature decrease of more than1Kis¯agged for superadiabatic quality control failure. This threshold rejects the worst 4.3% of input upper-air pro®les, allowing the vast majority of minor occurrences to pass. The meteorological and climatological communities should be aware of the occur- rence of nonsurface SLRs within RAOB data. 1. Introduction original vertical movement. The superadiabatic condi- tion does not imply spontaneous movement of air par- The dry adiabatic lapse rate is given by Saucier cels in the vertical (as would exceedance of the auto- (1955) as 9.767C/km. Lapse rate values larger than dry convective lapse rate) but rather acceleration in the di- adiabatic are referred to as superadiabatic. Likewise, rection of vertical perturbation. Thus, SLRs should any decrease in potential temperature with increasing exist for relatively short periods of time and should be height indicates superadiabatic conditions. ``Super sampled by the upper-air network infrequently. moist adiabatic lapse rates'' refer to lapse rates beyond SLRs are not uncommon near the surface. They are moist adiabatic. These are not addressed in this paper. relatively temporary events existing in thin layers near All references to superadiabatic herein refer to lapse surfaces much warmer than the overlying air (Saucier rates above dry adiabatic. 1955). Examples include a cold air mass advecting Superadiabatic lapse rates (SLRs) are statically un- over warm water and strong diabatic surface heating stable with respect to vertical displacement. An as- from solar insolation (Battan 1984). cending air parcel undergoes adiabatic cooling, while The existence and character of nonsurface SLRs is a descending parcel adiabatically warms. When the en- much less studied and, as such, is not often reported in vironmental lapse rate is superadiabatic, the density of the literature. Klostermeyer and Ruster (1981) notice the atmospheric parcel relative to its surroundings re- periodic signal power bursts in radar measurements sults in a buoyancy force in the same direction as the during a jet stream passage. Their model computations indicate the power bursts are produced by static insta- bilities due to SLRs induced from Kelvin±Helmholtz instabilities. Weinstock (1986), in a study of ®nite am- Corresponding author address: Dr. Richard L. Slonaker, Naval Research Laboratory, Code 7223, 4555 Overlook Ave., S.W., Wash- plitude gravity waves, shows that an SLR is an indi- ington, DC 20375. cation of wave velocity growth saturation. Weinstock E-mail: [email protected] (1987) determines an a priori prediction of the degree q 1996 American Meteorological Society /3q05 0216 Mp 350 Tuesday Aug 06 01:13 PM AMS: Forecasting (September 96) 0216 Unauthenticated | Downloaded 09/29/21 02:44 AM UTC SEPTEMBER 1996 SLONAKER ET AL. 351 of superadiabaticity for a linear gravity wave at satu- TABLE 1. RAOB soundings binned according to the number of ration. Hodge (1956) suggests that cloud-top SLRs reported nonsurface SLRs (at least 50 mb removed from the surface). may result from adiabatic lifting through the saturated± dry air interface or from the inherent evaporative cool- No. reported superadiabatic Percent of total ing occurring there. layers per sounding No. soundings soundings Some reported SLRs result from a phenomenon pe- culiar to the radiosonde instrument itself known as the 0 29 432 40.04% 1 21 377 29.08% ``wet-bulb effect'' (Hodge 1956). Within a cloud, wa- 2 11 984 16.31% ter droplets impinge on the thermistor as the radiosonde 3 5936 8.08% ascends. Upon exiting the cloud, the wet (icy) therm- 4 2551 3.47% istor cools due to evaporation (sublimation) in the drier 4/ 2217 3.02% air similar to a wet-bulb thermometer. Resulting tem- peratures are erroneously cold and, often, falsely indi- cate an SLR. After the thermistor dries, it warms back ported nonsurface SLRs are shown in Table 1. It was to the environmental temperature. unlikely that real physical processes such as gravity Some SLRs are attributable to bad pressure cells waves and the physical environment of cloud tops were aboard the radiosondes. Although the thermistor data responsible for all of the SLRs found in this sample of may be accurate, if they are assigned to incorrect pres- RAOB data. Rather than simply allowing quality con- sure data, the resulting lapse rates can erroneously in- trol to remove 60% of the input pro®les, the occurrence dicate superadiabatic conditions. of nonsurface SLRs within RAOB data was further in- vestigated. 2. The Midwest Atmospheric Data Base 3. Examples of superadiabatic lapse rates A quality control check was implemented to ¯ag nonsurface SLRs during the development of an atmo- Several examples of reported SLRs are shown in Fig. spheric database for meteorological research. The Mid- 1. Speculations are offered concerning some possible west Atmospheric Data Base (MAD) was created to causes of these SLRs, because the actual causes are provide an accurate estimate of atmospheric conditions unknown. The occurrence of nonsurface SLRs has not for initialization of the satellite sounding retrieval prob- been explored in research nor documented in the lit- lem (Slonaker 1994). The database encompasses a erature. mesoscale region of the central United States, extend- Dewpoint depressions do not suggest substantial ing speci®cally from 857 to 1057W longitude. The lat- cloud cover for the late afternoon sounding shown in itude ranges from 317 to 457N. In general, this area Fig. 1a. Mid-September solar insolation at Dodge City, extends from the Front Range of the Rocky Mountains Kansas (DDC), probably created the surface-based on the west to central Indiana on the east. From north SLR evident there. When strong diabatic surface heat- to south, the range is from South Dakota to central ing exists, this phenomenon is quite common. The SLR Texas. MAD was initialized with 5 years of data from between 500 and 400 mb in Fig. 1a appears to result 1983 through 1987. Radiosonde observation (RAOB) from fallacious pressure data. A comparison with sur- sites for the MAD region remained stable for these rounding RAOB station data at Denver, Colorado years, neither initiating nor terminating their data col- (DEN), North Platte, Nebraska (LBF), Topeka, Kan- lection activities. The 20 RAOB sites within the region sas (TOP), and Amarillo, Texas (AMA) for the same yielded 73 497 input atmospheric pro®les for this time time reveals similar 500-mb temperatures (near line. These upper-air data were obtained from the Fore- 097C). These neighboring sites all indicate tempera- cast Systems Laboratory [National Oceanic and At- tures of roughly 0217 and 0387C for the 400- and 300- mospheric Administration (NOAA)] and originated mb levels, respectively, while the Dodge City data are from the National Climatic Data Center. (Schwartz and erroneously low by about 157C. This type of error sug- Govett 1992). Both organizations applied independent gests a faulty pressure cell aboard the radiosonde. A quality control measures. One omission of these pre- thermistor value from a lower pressure (higher alti- vious efforts was a ¯ag for nonsurface SLRs. tude) was erroneously assigned to 400 mb. The result- The 50-mb layer adjoining the surface was exempted ing lapse rate is 16.77C/km, while the reported poten- from the quality control ®lter since SLRs were known tial temperature drops 14.4 K during ascent through this to realistically exist there. Beyond this near-surface re- 100-mb layer. gion, any layer in which the potential temperature de- An autumn sounding from Nashville, Tennessee creased with increasing height was ¯agged as super- (BNA), indicates three separate superadiabatic lay- adiabatic. Only 40% of all RAOB soundings were ers in Fig. 1b. The wet-bulb effect is probably re- found to be devoid of nonsurface SLRs. Many of the sponsible, at least in part, for the most prominent ¯agged pro®les contained multiple SLRs. The results SLR. From 663 to 627 mb, the lapse rate is 22.67C/ of binning soundings according to the number of re- km, resulting in a
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