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) superadiabatic 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 occurrence of nonsurface SLRs within RAOB data.

1. Introduction original vertical movement. The superadiabatic condition 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 9.76ЊC/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 instabilities 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

᭧ 1996 American Meteorological Society

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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 temperatures are erroneously cold and, often, falsely indicate 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 85Њ to 105ЊW longitude. The lat- cloud cover for the late afternoon sounding shown in itude ranges from 31Њ to 45ЊN. 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- 09ЊC). These neighboring sites all indicate tempera- cast Systems Laboratory [National Oceanic and At- tures of roughly 021Њ and 038ЊC 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 15ЊC. 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 16.7ЊC/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 22.6ЊC/ of binning soundings according to the number of re- km, resulting in a 6.6-K decrease in potential tem-

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FIG. 1. RAOB soundings for (a) Dodge City, KS (DDC), exhibiting superadiabatic lapse rates at the surface and between 500 and 400 mb; (b) Nashville, TN (BNA), exhibiting superadiabatic lapse rates within the following layers: 900±886 mb, 750±747 mb, and 663±627 mb; (c) Denver, CO (DEN), displaying a deep boundary layer from the surface to 550 mb with some sublayers exhibiting superadiabatic lapse rates; (d) Salem, IL (SLO), exhibiting superadiabatic lapse rates within the following layers: 654±646 mb, 606±594 mb, 557±550 mb, 445± 441 mb, and 379±370 mb; and (e) Green Bay, WI (GRB), exhibiting superadiabatic lapse rate between 866 and 850 mb. perature. As inferred from the dewpoint depression, air interface due to adiabatic lifting and the inherent the radiosonde exits cloud into dry air. The wet evaporative cooling at cloud top. thermistor, now cooler than its environment due to A very deep mixed layer is evident for the early evaporation, indicates an SLR. Upon drying, the spring sounding at Denver, as shown in Fig. 1c. The thermistor warms to the proper atmospheric temper- surface-based SLR exhibits a 0.8-K decrease in poten- ature. Hodge (1956) suggests that cloud-top SLRs tial temperature through its 7-mb depth. From 817 to may exist partly due to actual physical processes 550 mb, the lapse rate is essentially dry adiabatic. This rather than solely from measurement error. The ra- boundary layer comprises 10 separate levels of reported tionale offered includes cooling at the saturated±dry data. Many of these sublayers indicate slightly super-

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The ®ve examples shown in Fig. 1 represent different types of atmospheric situations. SLR reports do not appear to be con®ned to a speci®c RAOB site, season, time of day, or vertical location. For this reason, a more detailed analysis of SLR occurrence was performed.

4. Climatology of superadiabatic lapse rates in RAOB data Table 1 shows that approximately 60% of examined RAOB soundings reported at least one nonsurface SLR. To quantify the superadiabatic incidence rate, each layer of each sounding was considered separately. The lowest 50-mb layer was exempted to exclude SLRs created by diabatic surface heating. As shown in Fig. 1c, surface-based SLRs can extend upward beyond 50 mb, altering the statistics slightly. Due to the ¯uctuating nature of signi®cant pressure levels, atmospheric layers vary in depth from 1 mb to over 50 mb. Adjacent superadiabatic layers were not combined for these statistics. In this case, two thinner SLRs were reported rather FIG.1.(Continued) than a solitary thicker one. This sample of RAOB data contains a grand total of 2 653 890 atmospheric layers that are at least 50 mb removed from the surface. Of adiabatic conditions, probably due to a small temper- these, 86 446 exhibit superadiabatic lapse rates, yield- ature or round-off error of the otherwise dry adiabatic ing an incidence rate of 3.26%. It is possible that le- lapse rate. Occurrence of these deep boundary layers is gitimate atmospheric processes are responsible for not unusual for dry elevated surface locations such as some of the SLRs within these data. However, the in- Denver. The ``elevated heat source'' effect and reduced cidence rate of nonsurface SLRs shown here is higher moisture allow the surface conditions to mix out easily. than expected. A late summer sounding from Salem, Illinois In an attempt to discover correlations for these SLR (SLO), is shown in Fig. 1d. Five individual layers ex- reports, the data were binned according to time of day, hibit superadiabatic lapse rates. The most extreme SLR month, year, and location. Plots of superadiabatic in- is located between 606 and 594 mb. The potential tem- cidence rates (for individual atmospheric layers) as a perature decreases 4.4 K while ascending through this function of month, year, and geographic position are 12-mb layer. The equivalent lapse rate is 33.7ЊC/km. presented in Fig. 2. The majority of the data were taken The most probable cause of this troublesome sounding at the standard RAOB release times of 0000 and 1200 is the release of the radiosonde into a convective cell. UTC. The incidence rates for 1200 and 0000 UTC are A rising surface parcel traces out the dry adiabat until 2.9% and 3.6%, respectively, indicating a small diurnal saturation. Disregarding the erroneous middle tropo- signal. For the MAD region, 0000 UTC corresponds to sphere, the parcel then ascends approximately along the local times in the late afternoon and early evening. In- moist adiabat until the tropopause is reached. Radar cidence rates of SLRs appear to be correlated with imagery and surface observations (not shown) indicate warmer surface temperatures. Their occurrence is re- numerous thunderstorms in the vicinity at the time of duced somewhat during the early morning when sur- radiosonde release. The volatile winds of the convec- face temperatures are at a mean daily minimum. tive environment are capable of causing erratic balloon The annual time series in Fig. 2a demonstrates a ascents and descents that can result in erroneous pres- strong seasonal signal for nonsurface superadiabatic sure measurements. Additionally, sporadic precipita- occurrence. The incidence rate for SLRs is correlated tion can lead to wetting and drying of the thermistor. with summer and associated atmospheric conditions The RAOB pro®le presented in Fig. 1e is taken at (e.g., steeper lapse rates, warmer surface temperatures, Green Bay, Wisconsin (GRB), during winter. A su- convection, etc.). At 4.1%, the superadiabatic inci- peradiabatic layer is indicated between 866 and 850 dence rate for July is nearly double the 2.1% exhibited mb. Saturated conditions surround the superadiabatic in January. layer and are indicative of cloud and, possibly, precip- The incidence rates for SLRs as a function of year itation. This case exhibits a lapse rate of 19.3ЊC/km for the 5 years of data considered are shown in Fig. and a potential temperature reduction of 1.5 K during 2b. The small drop in superadiabatic occurrences for ascent. No speculation is offered regarding the cause 1986 and the large drop for 1987 coincided with the of this SLR. introduction of the Automated Radio Theodolite

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FIG. 2. Incidence rate of superadiabatic layers (as a percentage of the total number of atmospheric layers) based on (a) month, (b) year, and (c) geographic location.

(ART) system and the associated automation of data Differences in superadiabatic occurrence between processing by the National Weather Service (NWS). RAOB stations are listed in Table 2. The stations are The ART system incorporates a minicomputer to in- listed in order, from highest to lowest, of their re- gest the ¯ight data, perform data reduction compu- spective superadiabatic incidence rates. There is a tations, and produce the transmitted report for ulti- signi®cant difference between stations. The rate of mate distribution to forecasters and data collection SLR occurrence for Midland, Texas (MAF), at centers (Schwartz 1990). Although the number of 4.39%, is nearly double the rate of 2.22% character- reported nonsurface SLRs drops substantially after izing the Rapid City, South Dakota (RAP), site. Fig- the implementation of ART by NWS, the additional ure 2c more clearly shows the geographic depen- ®ltering causes only a slight decrease in the aggre- dence of SLR incidence rate. The higher superadi- gate amount of upper-air data. Even though some as- abatic incidence rates are generally correlated with pects of ART proved troublesome (Schwartz 1990; the location of a RAOB station, being more prevalent Schwartz and Doswell 1991), the automated pro- with southern stations. This result corresponds well cessing system appeared to ®lter out some of the with the seasonal signal shown in Fig. 2a. Both re- SLRs. sults suggest a higher incidence rate of reported

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TABLE 2. Incidence rate of superadiabatic layers for speci®c TABLE 4. Potential temperature (u) decrease RAOB sites (listed in order of decreasing incidence rate). of superadiabatic layers.

Superadiabatic Potential temperature North West incidence decrease bin (K) Percent of SLRs in bin (%) RAOB station (location) latitude longitude rate 00.2 £ du õ 0 65.5 MAF (Midland, TX) 31Њ56؅ 102Њ12؅ 4.39% 00.4 £ du õ00.2 17.2 SLO (Salem, IL) 38Њ39؅ 88Њ58؅ 3.75% 00.6 £ du õ00.4 7.1 UMN (Monett, MO) 36Њ53؅ 93Њ54؅ 3.67% 00.8 £ du õ00.6 3.3 SEP (Stephenville, TX) 32Њ13؅ 98Њ11؅ 3.62% 01.0 £ du õ00.8 1.9 GGG (Longview, TX) 32Њ21؅ 94Њ39؅ 3.54% 01.2 £ du õ01.0 1.2 JAN (Jackson, MS) 32Њ19؅ 90Њ04؅ 3.45% 01.4 £ du õ01.2 0.9 AMA (Amarillo, TX) 35Њ14؅ 101Њ42؅ 3.45% 01.6 £ du õ01.4 0.7 HON (Huron, SD) 44Њ23؅ 98Њ13؅ 3.41% 01.8 £ du õ01.6 0.5 DDC (Dodge City, KS) 37Њ46؅ 99Њ35؅ 3.38% 02.0 £ du õ01.8 0.3 1M1 (N. Little Rock, AR) 34Њ50؅ 92Њ16؅ 3.36% du õ02.0 1.4 DEN (Denver, CO) 39Њ46؅ 104Њ53؅ 3.26% CKL (Centerville, AL) 32Њ54؅ 87Њ15؅ 3.23% OKC (Oklahoma City, OK) 35Њ24؅ 97Њ36؅ 3.10% BNA (Nashville, TN) 36Њ15؅ 86Њ34؅ 3.08% TOP (Topeka, KS) 39Њ04؅ 95Њ37؅ 3.06% The magnitudes of reported SLRs are listed in Table PIA (Peoria, IL) 40Њ40؅ 89Њ41؅ 2.92% 4 according to potential temperature decrease during GRB (Green Bay, WI) 44Њ29؅ 88Њ08؅ 2.74% -LBF (North Platte, NE) 41Њ08؅ 100Њ41؅ 2.65% ascent through the layer. Similar to the lapse rate re OMA (N. Omaha, NE) 41Њ22؅ 96Њ01؅ 2.59% sults, the vast majority of reported SLRs are relatively -RAP (Rapid City, SD) 44Њ03؅ 103Њ04؅ 2.22% small. Over 65% of the cases exhibit a potential tem perature decrease of 0.2 K or less. Potential temperature decrease appears to be a more discriminating criterion than lapse rate for characterizing superadiabatic SLRs with warmer surfaces, steeper lapse rates, and magnitude. Extreme lapse rates often result from a convective activity. small drop in potential temperature occurring within a thin layer. Potential temperature decrease indicates the 5. Magnitude of reported superadiabatic lapse rates integrated effect of lapse rate and thickness of the SLR. Only 1.4% of reported SLRs exhibit a potential tem- To quantify the magnitude, each reported SLR was perature drop of more than 2 K. binned according to lapse rate. Initially, the superadi- Another important criterion is the thickness of the abatic layers were segregated from all nonsuperadi- reported superadiabatic layers. Adjacent SLRs (adjoin- abatic layers. The number of SLRs within a speci®c bin ing SLRs that were previously considered separate) is reported as a percentage of the total number of SLRs. were combined for these statistics, yielding a more ac- Results of the lapse rate binning are shown in Table 3. curate assessment of the thickness of superadiabatic Over 60% of the reported SLRs exhibit lapse rates less layers. Results of SLR thickness binning are given in than 11ЊC/km. The majority of these SLRs are quite Table 5. While most superadiabatic reports are in thin subtle. The number of SLRs with larger lapse rates is layers, some of them occur through a more substantial not trivial, however. Almost 5% of all superadiabatic atmospheric depth. As SLR thickness increases, the layers exhibit lapse rates greater than 20ЊC/km. Figures chances that the cause is a solitary erroneous RAOB 1b and 1d are examples of RAOB data exhibiting these temperature decrease. Over 7% of the SLRs are more large lapse rates. than 50 mb in depth. An SLR through such a consid- erable thickness cannot be dismissed as a temporary aberration. The pro®le shown in Fig. 1a demonstrates TABLE 3. Lapse rates (G) of superadiabatic layers. one of these thick superadiabatic layers. Lapse rate bin (ЊC/km) Percent of SLRs in bin (%)

9.76 õ G £ 11 61.7 11 õ G £ 12 15.5 TABLE 5. Pressure thickness of superadiabatic layers. 12 õ G £ 13 6.2 13 õ G £ 14 3.4 Pressure thickness bin (mb) Percent of SLRs in bin (%) 14 õ G £ 15 2.6 15 õ G £ 16 2.1 0 õ SLR thickness £ 10 46.7 16 õ G £ 17 1.3 10 õ SLR thickness £ 20 26.3 17 õ G £ 18 0.8 20 õ SLR thickness £ 30 11.6 18 õ G £ 19 0.8 30 õ SLR thickness £ 40 5.7 19 õ G £ 20 0.6 40 õ SLR thickness £ 50 2.3 20 õ G 4.9 50 õ SLR thickness 7.4

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TABLE 6. Potential temperature (u) decrease versus year for TABLE 8. Potential temperature (u) decrease versus pressure superadiabatic layers (raw count of number of occurrences). thickness for superadiabatic layers (raw count of number of occurrences). 01 £ du õ 0 02 £ du õ01 du õ02 Year (slight) (moderate) (strong) Pressure thickness 01 £ du õ 0 02 £ du õ01 du õ02 bin (mb) (slight) (moderate) (strong) 1983 18 391 566 188 1984 18 643 595 208 0 £ thickness õ 10 30 138 1511 681 1985 18 998 690 258 10 £ thickness õ 20 16 889 904 434 1986 15 407 844 331 20 £ thickness õ 30 7642 284 143 1987 10 725 410 192 30 £ thickness õ 40 3718 145 52 40 £ thickness õ 50 1543 52 23 50 £ thickness 4914 150 31

Several two-dimensional bins of reported superadiabatic layers were constructed to correlate the severity of the SLR with other criteria. Three categories of su- reports exhibiting various levels of severity even after peradiabatic magnitude were de®ned using potential the implementation of ART. temperature decrease reported during ascent through The superadiabatic layers were then binned accord- the atmospheric layer: slight (01 £ du õ 0), moderate ing to both magnitude and vertical location (i.e., pres- (02 £ du õ01), and strong (du õ02). The de®- sure level). The raw count for each of the two-dimen- nitions and categories of superadiabatic magnitude sional bins is given in Table 7. The slightly superadi- were based on our examination of the data but were abatic cases (01 £ du õ 0) peak in the 900±700-mb somewhat arbitrary. Other parameters used to de®ne bin. For moderate and strong cases, peak occurrence is the bins included year, vertical location, thickness, and in the 700±500-mb bin. There is a shift in peak occur- estimated cloud status. Although previous statistics are rence from the lower to the middle troposphere as su- listed as a percentage of the total number of SLRs, the peradiabatic severity increases. Regardless, the broad two-dimensional bins indicate a raw count of the num- distribution indicates that superadiabatic layers of any ber of cases. Otherwise, the large number of slightly magnitude can be reported throughout the atmospheric superadiabatic cases overwhelm the number of mod- column. Even the near-tropopause region is not im- erate and strong cases if reported as a percentage. mune to reports of superadiabatic layers ranging in de- A breakdown of superadiabatic magnitude for each gree from slight to strong. of the 5 years of RAOB data is shown in Table 6. Each Potential temperature decrease and superadiabatic year is dominated by slightly superadiabatic cases. The layer thickness are unique parameters that can be used small reduction in reported SLRs for 1986 is for subtle independently to characterize SLR magnitude. To- cases only. Moderate and strongly superadiabatic oc- gether they represent a multiplicative or integrated currences actually increase. Results for 1987 indicate value quantifying the superadiabatic severity through fewer reported SLRs in all categories, from slight to an atmospheric depth. Two-dimensional binning of re- strong. With respect to the worst cases of reported ported SLRs appears in Table 8 using these variables. SLRs, it appears that there might have been some prob- Similar to Table 5, adjacent SLRs were joined, result- lems associated with data ingest, reduction, or quality ing in more accurate thickness statistics (the total num- control during the introduction of ART hardware and ber of SLRs necessarily decreased, although the same software systems by NWS in 1986. The system re- data were used for these bins). Very few SLRs are ported substantially fewer SLRs by 1987. Nonetheless, within bins that simultaneously exhibit extreme super- there remains a consequential number of superadiabatic adiabatic magnitude on both scales. An attempt was made to correlate the magnitude of reported SLRs with cloud status, in effect, to quantify

TABLE 7. Potential temperature (u) decrease versus vertical location for superadiabatic layers (raw count of number of occurrences). TABLE 9. Potential temperature (u) decrease versus estimated cloud Pressure level 01 £ du õ 0 02 £ du õ01 du õ02 conditions for superadiabatic layers (raw count of number of bin (mb) (slight) (moderate) (strong) occurrences).

100 £ pressure 01 £ du õ 0 02 £ du õ01 du õ02 õ 300 4770 261 140 Cloud condition (slight) (moderate) (strong) 300 £ pressure õ 500 20 009 669 280 No cloud 54 317 1108 344 500 £ pressure Entering cloud 6433 73 11 õ 700 25 804 1283 522 Within cloud 11 222 901 322 700 £ pressure Exiting cloud 3485 756 360 õ 900 28 719 814 224 Unknown 6707 267 140

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FIG. 3. RAOB soundings (a) for Nashville, TN (BNA), exhibiting superadiabatic lapse rate between 667 and 655 mb, and (b) for Rapid City, SD (RAP), exhibiting superadiabatic lapse rate between 590 and 567 mb. the wet-bulb effect. Unfortunately, there were no cloud wholly within the exiting cloud category. While com- observations stored in the RAOB data. Although cloud prising only 5.3% of all reported SLRs, the exiting data could be obtained from archives of local surface cloud category forms the largest contingent of strongly observations and satellite cloud analyses, they were superadiabatic cases. Within cloud cases are well rep- neither convenient nor readily available for the mag- resented at all levels of severity. Clouds cannot be nitude of cases considered here. For our purposes, it blamed for all SLRs however. The no-cloud SLRs was suf®cient to infer cloud status from available hu- dominate the overall number of occurrences, ranging midity data. Birkenheuer (1993) uses an inverted sim- from slight to strong. The unknown cloud status (due ple cloud parameterization scheme from Zivkovic and to missing dewpoint data) cases also contribute signif- Louis (1991) to revise humidity ®elds based on ana- icantly at various levels of superadiabatic severity. lyzed cloud amount. This algorithm varies relative humidity in cloudy regions between 60% and 100%, de- 6. A quality control technique for superadiabatic pending on cloud fraction. Only a crude estimate using lapse rates dewpoint depression was needed. For our purposes, a 3ЊC dewpoint depression was utilized as the cloud±no- Reports of nonsurface SLRs within the RAOB data cloud threshold. Cloud existence was assumed when- used to initialize MAD were prevalent. Removal of all ever the dewpoint depression is 3ЊC or below. Other- pro®les containing nonsurface SLRs was impractical wise, clear conditions were assumed. This criterion since they composed about 60% of all input soundings. provided a simple, yet reasonable, cloud estimate. By Additionally, some data, indicating milder cases of monitoring past and present dewpoint depressions nonsurface SLRs, may have accurately re¯ected the en- (during radiosonde ascent), statistics were recorded re- vironmental conditions due to real atmospheric pro- garding the relation of a reported SLR to clouds. The cesses occurring there. Although some correlations are ®ve categories of cloud status included no cloud, en- shown, the SLRs exist for different atmospheric re- tering cloud, within cloud, exiting cloud, and ``un- gimes and different temporal, spatial, and vertical lo- known.'' The unknown category was reserved for su- cations. The majority of reported SLRs are quite subtle peradiabatic cases with missing dewpoint data (usually in nature. Some of the soundings contain severely su- due to cold temperatures in the upper troposphere). peradiabatic layers, probably erroneous. Most of the The results of two-dimensional binning of reported examples shown in Fig. 1 are within the latter category. SLRs according to magnitude and cloud status are pre- However, the pro®le shown in Fig. 1c does not appear sented in Table 9. Almost all occurrences reported for inaccurate. Including this sounding in a meteorological the entering cloud case are benign in nature, with few or climatological analysis would not cause alarm. Other in the moderate or strongly superadiabatic classi®ca- pro®les given in Fig. 1 do present some problems, how- tions. The wet-bulb effect is presumably contained ever. Both temperature and moisture data of these pro-

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®les are suspect for various reasons. A check for non- zarre behavior. The temperature and dewpoint pro®les surface SLRs would ¯ag these soundings. in Fig. 3a suddenly jump to lower values and then re- For performing RAOB quality control, lapse rate turn. There exists an anomalous spike toward higher was not chosen as the criterion due to excessive sen- temperatures and dewpoints in Fig. 3b. In each case, sitivity in thin layers. The decrease of potential tem- there is no explanation for the erratic behavior of the perature while ascending through an atmospheric layer temperature and moisture pro®les. was used as the superadiabatic ®ltering parameter since Erroneous temperature and humidity pro®les can it more effectively quanti®ed the magnitude of the adversely affect meteorological parameters derived SLR. For quality control of input RAOB data to MAD, from them. If the aberrations shown in Fig. 3 occur the threshold for allowable potential temperature de- near 500 mb, resulting lifted index computations crease ascending through any nonsurface layer was set would be nonsensical. Precipitable water and iso- at 1 K. Any layer with a bottom level more than 50 mb baric height change by a nontrivial amount. For ex- removed from the surface was subject to the test. If the ample, the precipitable water calculated from the decrease in potential temperature, from bottom level to sounding in Fig. 3a is 4.6 cm. If the pro®le anomaly top level, was more than 1 K, the layer was ¯agged as is corrected by reasonably assuming a moist adi- failing superadiabatic quality control. Using this abatic lapse rate and constant dewpoint depression, threshold allowed 95.0% of all reported superadiabatic the precipitable water becomes 4.9 cm. This correc- layers to pass quality control. Since some pro®les con- tion increases the 500-mb height by approximately tained multiple superadiabatic failures, the quality con- 22 m. For Fig. 3b, the pro®le is best corrected by trol acceptance rate for soundings themselves was simply omitting the troublesome points between 610 higher than for individual atmospheric layers. Speci®- and 590 mb. The resulting value of precipitable water cally, 95.7% of RAOB soundings passed the quality drops from 4.5 cm for the original pro®le to 4.2 cm control check for nonsurface SLRs. Of all the example for the corrected one. The 500-mb height is reduced pro®les, only the one shown in Fig. 1c passes this cri- by 24 m. These errors affect current analyses and terion. This threshold is conservative enough to ®lter may adversely affect numerical weather prediction problematic pro®les while allowing the vast majority model forecasts. Using a check for nonsurface SLRs, of subtle SLRs to pass. these pro®les can be ¯agged for either correction or deletion. 7. Conclusions The meteorological and climatological communities should be aware of the multitude of reported This paper documents an abundance of nonsurface SLRs within RAOB data. For our purposes, a quality SLRs in upper-air data. Excluding the 50-mb atmo- control criterion was created for nonsurface SLR re- spheric layer at the surface, 60% of all input RAOB ports. The 50-mb atmospheric layer at the surface soundings reported a superadiabatic layer. One omis- was exempted due to SLR occurrence there via the sion of previous quality control efforts applied by the diabatic heating of solar insolation. Above this sur- National Climatic Data Center and Forecast Systems face layer, the potential temperature was monitored Laboratory (NOAA) was a ¯ag for these nonsurface at every level reported (by the National Climatic SLRs. Data Center) during ascent. If the potential temper- The incidence rate of SLRs is directly correlated ature decrease through any layer was more than 1 K, with warmer surfaces, steeper lapse rates, and convec- the pro®le was ¯agged for failing superadiabatic tive activity. Speci®cally, the number of reported non- quality control and was discarded. Our RAOB sound- surface SLRs increases during summer versus winter, ings exhibited a 4.3% rejection rate. Other users of during late afternoon versus early morning, and for upper-air data may want to develop their own criteria southern locations versus northern ones. or generate algorithms to correct erroneous super- The majority of SLRs are subtle, almost unnotice- adiabatic layers. All RAOB data users should realize able. Some of these can result from a small inaccuracy that their pro®les may contain an excessive number within a dry adiabatic lapse rate caused by instrument of nonsurface SLRs. Some of these may indicate error or by the reported parameter precision within the gross errors in the data. data archive. Nonsurface SLRs may actually exist due to atmospheric phenomena such as gravity waves or Acknowledgments. This work has been made possible the physical environment of cloud tops. Actual causes through a graduate fellowship from NOAA/ERL/Fore- remain unknown. cast Systems Laboratory. Additional funding during Other superadiabatic reports are quite extreme and preparation of this work has been provided by NASA obviously erroneous, capable of corrupting meteoro- Order Number W-18,077. Appreciation is due to the re- logical parameters, analyses, and (possibly) forecasts viewers of this manuscript for many helpful suggestions. derived from them. The RAOB data illustrated in Figs. The authors would like to thank Thomas Schlatter and 3a and 3b are problematic. In both examples, a nearly John McGinley of the NOAA/ERL/Forecast Systems saturated segment of the pro®le is interrupted by bi- Laboratory for their valuable insight. We have also ben-

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Unauthenticated | Downloaded 09/29/21 02:44 AM UTC SEPTEMBER 1996 SLONAKER ET AL. 359 e®ted from discussions with Chuck Wade (NCAR, Re- Schwartz, B. E., 1990: Regarding the automation of rawinsonde ob- search Application Program) and Steve Albers (NOAA/ servations. Wea. Forecasting, 5, 167±171. , and C. A. Doswell III, 1991: North American rawinsonde ob- ERL/Forecast Systems Laboratory). servations: Problems, concerns, and a call to action. Bull. Amer. Meteor. Soc., 72, 1885±1896. REFERENCES , and M. Govett, 1992: A hydrostatically consistent North Amer- Battan, L. J., 1984: Fundamentals of Meteorology. 2d ed. Prentice- ican radiosonde data base at the Forecast Systems Laboratory, Hall, 304 pp. 1946±present. NOAA Tech. Memo. ERL FSL-4, 81 pp. [Avail- Birkenheuer, D., 1993: An update of the LAPS speci®c humidity able from the National Technical Information Service, 5285 Port analysis. Preprints, Eighth Symp. on Meteorological Observa- Royal Road, Spring®eld, VA 22061.] tions and Instrumentation, Special Sessions on Water Vapor and Slonaker, R. L., 1994: Data base of meteorological conditions and Ultraviolet Measurements, Anaheim, CA, Amer. Meteor. Soc., corresponding satellite radiances for the central United States. J157±J164. Ph.D. thesis, University of Colorado, Boulder, CO, 241 pp. Hodge, M. W., 1956: Superadiabatic lapse rates of temperature in Weinstock, J., 1986: Finite amplitude gravity waves: Harmonics, ad- radiosonde observations. Mon. Wea. Rev., 84, 103±106. vective steepening and saturation. J. Atmos. Sci., 43, 688±704. Klostermeyer, J., and R. Ruster, 1981: Further study of a jet stream- , 1987: The turbulence ®eld generated by a linear gravity wave. generated Kelvin±Helmholtz instability. J. Geophys. Res., 86, J. Atmos. Sci., 44, 410±420. 6631±6637. Zivkovic, M., and J. F. Louis, 1991: A cloud parameterization based on Saucier, W. J., 1955: Principles of Meteorological Analysis. Dover, cluster analysis. Preprints, Ninth Conf. on Numerical Weather Pre- 438 pp. diction, Denver, CO, Amer. Meteor. Soc., 127±130.

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