Some Observations of the Clearing of Cumulus Clouds Downwind from Snow-Covered Areas

JULY 1999 NOTES AND CORRESPONDENCE 1687 Some Observations of the Clearing of Cumulus Clouds Downwind from Snow-Covered Areas M. SEGAL, C.ANDERSON, AND R. W. ARRITT Department of Agronomy, Iowa State University of Science and Technology, Ames, Iowa R. M. RABIN Cooperative Institute for Meteorological Satellite Studies, NOAA/National Severe Storms Laboratory, Norman, Oklahoma, and Space Science and Engineering Center, University of Wisconsin Madison, Madison, Wisconsin D. W. MARTIN Cooperative Institute for Meteorological Satellite Studies, Space Science and Engineering Center, University of Wisconsin Madison, Madison, Wisconsin 18 February 1998 and 27 June 1998 ABSTRACT Satellite images that illustrate the clearing of cumulus clouds downwind from snow-covered areas are presented. The cloud clearing resembles that occasionally observed with lakes during warm advection, supporting the suggestion that the thermal forcing associated with a uniform snow-covered area is comparable to that of a coldwater lake of similar size. Analysis of snow cover patterns in the central United States suggests that the climatological probability for situations conducive to the cloud clearing is at most once per month. 1. Introduction In recent years there has been increasing interest in studying landscape effects on mesoscale atmospheric processes. One of the most frequently investigated situations is that of heterogeneous landscapes, which in the daytime produce horizontal gradients in sensible heat fluxes and, consequently, horizontal gradients in boundary layer thermal characteristics. Numerical model simulations have suggested that some of these gradients induce thermal circulations that might reach seabreeze intensity, but observational support for such intense circulations is, in general, weak or absent (e.g., Segal and Arritt 1992; Zhong and Doran 1995; Hubbe et al. 1997). As suggested and illustrated quantitatively in these studies, the discrepancy between model results and observations for situations related to vegetation breezes may be due in part to overly idealized simulations: the models typically assume more extended, uniform, and strongly contrasting land use than commonly exists in nature. It appears that further observational information on the intensity of the various types Corresponding author address: Moti Segal, Department of Agronomy, Iowa State University of Science and Technology, Ames, IA 50011-1010. of landscape-induced thermal circulations is needed in order to quantify these circulations. In the absence of sufficient observational data, which typically are obtained in a special field project, any indirect support from routine observations should be beneficial. One well-defined type of landscape variation is occasionally associated with snow cover. Snow cover is often extensive and sharply bounded, a situation that should be conducive to the generation of thermally driven circulations. Numerical model simulations have implied that circulations ( snow breezes ) as strong as sea breezes may be associated with these situations during appropriate weather conditions (Segal et al. 1991a). According to this study for an ideal snow bare-soil contrast and the solar conditions of March, in the absence of background flow the snow breeze may reach speeds of6ms 1 and extend 30 km into the snow-free area. Some observations support the existence of snow breezes (Segal et al. 1991b and references therein); however, the observations are too sparse for supporting general conclusions about their intensity. Indirect evaluation of the generation of thermally induced flow generated by large snow-covered areas was pursued by Segal and Kubesh (1996). They evaluated the lake breezes of large lakes during late winter early spring when the water surface temperatures were near 0 C (as would be about the snow surface temperature in fair weather condi- 1999 American Meteorological Society

1688 MONTHLY WEATHER REVIEW VOLUME 127 tions). Because the areal extent of large lakes may be similar to that of snow-covered areas, they inferred the potential intensity of snow breezes from their results. The present note provides additional indirect evidence that snow swaths induce similar thermodynamic and dynamic effects as lakes. Clearing of cumulus clouds is commonly observed downwind from lakes under warm advection conditions (e.g., Purdom 1990; Rabin et al. 1990). Occasionally, cumulus clearing is observed downwind from the Great Lakes during the spring, when the water surface temperature is a few degrees Celsius above freezing and, thus, almost resembles the temperature of a snow surface in warm daytime conditions. Segal et al. (1997) attributed cloud clearing downwind of a lake to thermodynamic and dynamic processes forced by the lake in the presence of warm advection, in particular (i) suppression of the convective boundary layer (CBL) as warm air is advected over a cooler surface and (ii) subsidence induced by the coupled background flow and lake breezes that can produce drying in the lower atmosphere with a corresponding cloud clearing downwind from the lakes as the air moves onshore. In this note we present observational illustrations of cumulus cloud clearing downwind from snow-covered areas in patterns analogous to those observed downwind of lakes. Additionally, we estimate the potential real-world frequency of snow cover situations that are conducive to downwind cumulus cloud clearing or snow breezes. 2. Illustrative cases of cloud clearing downwind from snow cover a. 10 April 1989 FIG. 1. Case of 10 April 1989. (a) GOES visible image of snow cover (from northeast Colorado to northwest Kansas) and the associated cloud cover at 1331 CST. Clearing surrounding the snowcovered area and surface wind direction are indicated by the white arrows and the dark streamline, respectively. The encircled area in Oklahoma indicates a separate, rapidly melting snow swath. (b) The 1231 CST GOES brightness temperature (K) across the main snow swath. A band of uniform snow cover about 100 km wide and several hundred kilometers long covered an area extending from northwest Kansas toward northeast Colorado on 10 April 1989 at 1331 CST (Fig. 1a). Based on two reporting stations the snow depth over Kansas was about 10 cm at 0600 CST. Around noon local time the Geostationary Operational Environmental Satellite (GOES) imagery for a pertinent section of the snow cover (Fig. 1b) indicated that the brightness temperature (blackbody IR temperature) over the snow cover was near 0 C, whereas in the surrounding snow-free areas the brightness temperatures were as much as 20 C warmer. The area surrounding the southern edge of the snow had a shelter air temperature around 6 C and over the snow the shelter temperature was about 0 C [based on National Weather Service (NWS) surface reports]. A weak high pressure system resided in the lower atmosphere over the snow-covered area. The surface flow at 1300 CST was about 5 m s 1 and southerly along the western half of the snow cover, turning to northwesterly along its eastern portion (as depicted by the dark streamline in Fig. 1a). Overall these characteristics, when compared with those observed in Segal et al. (1991b), suggest appropriate thermal conditions for the development of a snow breeze. The orientation of the background flow also suggests warm advection over the snow swath. Over Kansas the GOES visible imagery for 1331 CST indicates clearing in the shallow cumulus cloud field downwind from and perpendicular to the snow cover (Fig. 1a). This pattern persisted for several hours. Note that the cloud clearing pattern resembles cumulus cloud clearing downwind from lakes. Adopting the Iribarne and Godson (1981, p 141) formulation for estimation of the lifting condensation level (LCL) (using the shelter temperature and dewpoint temperature), and using the few available observations in the region, we have estimated that the clouds bases were at 1000 1500 m. The mechanisms suggested by Segal et al. (1997) to

JULY 1999 NOTES AND CORRESPONDENCE 1689 support these cloud patterns would be suppression of the CBL over the snow-covered area due to the warm advection, likely accompanied by subsidence associated with the snow breeze. Finally, the encircled area in the panhandle of Oklahoma was snow covered in the morning but almost entirely melted by 1331 CST (some residual snow is evident). It appears that the northern edge of this area confined the southward development of the observed cumulus clouds. A cloud line that is likely to be associated with some convergence marks this location. One might suggest that it is a result of thermal flow between the melting snow area and its snow-free surroundings. b. 14 April 1997 A spring storm generated an extensive snow-covered area in southeast Wisconsin that shrank considerably by the morning on 14 April 1997 (Fig. 2a). During the daytime, surface southwesterly flow of about 5 m s 1 associated with a surface high pressure system dominated the Lake Michigan area. The morning and afternoon radiosondes at Green Bay, Wisconsin (indicated by G in Fig. 2a), suggest that the characteristic wind speed in the overlying 1000-m layer was about 10 m s 1. Brightness surface temperature is not presented in this case since a large portion of the pertinent domain was cloudy. However, maximum shelter temperatures in the area (obtained from the Office of the Wisconsin State Climatologist) indicated that the temperature over the snow-free area reached as high as about 13 C near the western shore of Lake Michigan. Over the snowcovered area several stations indicated an air temperature of 5 C. We suggest that this pattern is a result of somewhat nonuniform snow cover where the snow-free fraction is warmer than 0 C (as supported by Fig. 2b). The warm advection may have accelerated the melting of the snow, which is evident in a comparison of Figs. 2a and 2b. Toward noon, shallow cumulus clouds developed west of Lake Michigan (Fig. 2b). Their suppression downwind of both the snow-covered area and the eastern shore of Lake Michigan was evident in a time series of satellite images from late morning to early afternoon (the wind direction is revealed by the orientation of the cloud rows). The most noticeable clearing downwind from the snow cover occurred at 1345 CST (Fig. 2b). Estimation of the LCL based on shelter temperature and dewpoint observations suggests that cloud bases were at 1000 1500 m. Cloud clearing at the lateral sides of the snow cover and perpendicular to the prevailing wind direction might imply subsidence associated with a snow breeze. Worth noting is that cumulus clearing is also evident northward from Lake Michigan (indicated by a dashed arrow). A light southeasterly lake breeze was observed in this location but was confined to the lake shore. Similar cloud clearing was also observed in FIG. 2. GOES visible imagery of the snow swath of 14 April 1997 in southeastern Wisconsin at (a) 0845 CST and (b) 1345 CST. Clearing surrounding the snow-covered area and Lake Michigan are indicated by solid arrows. the same location the next day (not shown), which might imply land use or topographic effects. 3. Estimating the potential frequency of snow effect on cloud clearing and snow breezes in the central United States In order to evaluate the significance of situations such as those illustrated in section 2 in weather forecasting or climatic analysis, an estimate of their frequency is needed. A bulk estimation can be obtained through the simple analysis approach outlined as follows. Snow

1690 MONTHLY WEATHER REVIEW VOLUME 127 breezes and cumulus cloud clearing downwind from snow-covered areas are most likely in relatively warm weather (i.e., shelter temperature over the snow-free surface 0 C). For cooler air temperature situations the solar irradiance is likely to be relatively low, or the daytime downward soil heat flux increases whereas sensible heat flux decreases over snow-free areas. Therefore, the daytime thermal difference in the lower atmosphere between the snow-free surface and the snowcovered surface area will be small, and, correspondingly, any induced snow breeze should be weak. Additionally, in these conditions it is unlikely that the CBL will be thermodynamically supportive of cumulus cloud development. The most likely period to observe snow breezes and downwind cloud clearing is therefore in fall and spring. By similar reasoning such effects may also be more common when snow cover is present in more southern latitudes, although the occurrence of snow cover is less frequent there. We have estimated the potential frequency of situations conducive for snow breezes and downwind cloud clearing in the central United States, where these situations should be most prevalent (Fig. 3). We used 30 years (1961 90) of shelter temperature and snow depth observations available on the SAMSON CD-ROM produced by the National Climatic Data Center in Asheville, North Carolina. Seven sites representing different locations in the central United States over relatively flat terrain were considered. Four different constraints were used: (i) days in which any snow depth is reported and the air temperature 1.7 C (29 F); (ii) as (i) but with air temperature 0 C; (iii) and (iv) constrained by air temperature as in (i) and (ii), respectively, but with snow depth 5 cm (under this constraint the snow-covered area is more likely to be uniform, i.e., with fewer snowfree patches). We have computed the monthly average frequency of occurrence for each class. The frequencies provided in Fig. 3 suggest the potential number of days per month having conditions that are conducive to snow cover effect on thermal circulation and cloud clearing. At some distance from the meteorological stations there will exist a boundary between the snow-covered and snow-free areas. The location of this boundary changes during snow-melt periods. For the southern stations in our sample, snow accumulates more often from localized heavy snow falls and melts more quickly compared with the northern stations. This suggests that for these southern stations the boundary of the snow-covered area might be closer to the station (probably on the order of a meso- scale length). Therefore, the actual frequency of cloud clearing or snow breeze occurrence at any of these stations, but especially the northern stations, is likely to be lower than the potential frequency shown in Fig. 3. As might be expected the number of snow cover cases drops with increase of shelter temperature, because higher temperatures suggest a tendency toward shallower snow depth and faster melting. The frequencies for FIG. 3. Monthly frequency of days reporting snow cover at seven NWS stations in the central United States (DDC: Dodge City, KS; SPG: Springfield, MO; MSN: Madison, WI; BIS: Bismarck, ND; GTF: Great Falls, MT; IND: Indianapolis, IN; OMA: Omaha, NE). (a) Days in which any snow depth is reported and the air temperature 1.7 C (29 F); (b) as in (a) but with air temperature 0 C; (c) and (d) constrained by air temperature as in (a) and (b), respectively, but with snow depth 5 cm. both temperature thresholds peak in the winter when snow ablation is low; however, in this period the warm ambient atmospheric conditions conducive for snow breeze cloud clearing are infrequent. In fall and in

JULY 1999 NOTES AND CORRESPONDENCE 1691 TABLE 1. Frequency of shallow cumulus clouds types 1 and 2 (Cu 1 and Cu 2 ) and daytime completely clear sky in the central United States (in the greater area represented by the sites selected in Fig. 3). Based on Warren et al. (1986). Fall (Sep Nov) Winter (Dec Feb) Spring (Mar May) Frequency of Cu 1 and Cu 2 (%) 4 8 4 7 15 Frequency of daytime completely clear sky (%) 10 20 10 20 7 15 spring (when conducive conditions are likely to prevail) their frequency is less than 3 days month 1 in northern latitudes and significantly less in southern latitudes. The evaluations presented above for the central United States imply that, climatologically, the occasions that might be conducive to snow breezes or cumulus clearing downwind from snow cover are likely to occur at most 4 8 days month 1 (depending on month and site locations). The frequency should be lower, however, considering the need for favorable meteorological conditions (i.e., appropriate thermodynamic stratification in the lower atmosphere when cloud clearing is considered; clear sky and weak synoptic wind when identified snow breeze is considered). Table 1 summarizes probabilities for shallow cumulus clouds (Cu 1 and Cu 2 based on day and night observations) and for completely clear skies during the daytime, extracted from Warren et al. (1986). Their analysis is based on the reporting synoptic stations during the period 1971 81, and it is presented on a relatively coarse grid of 5 5. Using this information permits refinement in the climatological estimation of the likely frequency of cumulus cloud clearing downwind from snow cover and snow breeze. For simplicity it can be assumed that the probabilities for the event of snow cover and for the event of observed shallow cumulus clouds are statistically independent. Both events should be reported in the event of cumulus cloud clearing downwind from snow cover. Therefore, it is estimated, using Table 1 for the spring frequency of observed cumulus ( 15%), that the frequency of downwind cumulus clearing in spring is lower by an order of magnitude than the snow cover frequencies shown in Fig. 3. For winter and fall (for which the frequency of observed cumulus is 4% and 8%, respectively), it is even lower. A similar evaluation is done for the frequency of a situation ideal for the development of a snow breeze, which is assumed to occur only in the event of completely clear daytime sky. Under these constraints, following Table 1, the snow-breeze frequency would be 5 to 10 times lower than the frequency indicated in Fig. 3. Snow breezes may develop even when some cloudiness exists, and under clear-sky conditions when the background flow is strong. However, estimation for the frequency of these situations would be affected by various uncertainties. 4. Discussion In this note, clearing of cumulus clouds downwind from snow-covered areas has been illustrated for two cases during early spring. The background flow indicated that warm air was moving across the snow cover in both cases. Cumulus cloud clearing patterns were analogous to those associated with lakes when the lake surface water temperature is lower than that of the advected air. These observations suggest that snow-covered areas and cold lakes are similar in terms of their thermodynamic and dynamic impacts on the daytime convective boundary layer. During spring, the frequency of snow cover in the central United States decreases and the daily rate of melting increases. Shallow convection is more likely to occur in this period, especially outside snow-covered areas. We examined such situations for April 1997 (in addition to the 14 April case), when the north-central United States was affected by extensive snow cover. From a subjective evaluation of GOES satellite images, we could not identify downwind cloud clearing associated with snow swaths nor formation of clouds associated with the edges of snow-covered areas that might imply convergence induced by snow breezes. This is in agreement with the frequency analysis presented in the note. We performed a statistical analysis of snow depth observations in several representative geographical locations in the central United States in which snow cover may generate a snow breeze or downwind cloud clearing in order to determine the maximum frequency of these events. The analysis suggested that even in spring these situations are likely to occur climatologically at most 1 day month 1 for cloud clearing and 2 days month 1 for a snow breeze to occur under ideal synoptic conditions. Refinement of the presented evaluations can be obtained by performing climatological analysis of snow cover patterns based on satellite imagery. Finally, it is worth pointing out that for the vegetation breeze a semiseasonal persistence of the surface thermal contrast is anticipated. Therefore, its effect on cumulus clouds would be easier to identify compared with those in the snow cover case. Triggering of convection or cumulus clouds under supportive atmospheric conditions by flow convergence associated with vegetation breezes was proposed long ago. Adopting an analogous approach to the present study should be beneficial for evaluating the vegetation breeze. Focusing on relevant locations for vegetation breezes (e.g., Cutrim et al. 1995) and analyzing satellite images for formation of convective cloud lines during days with supportive environmental conditions would provide an indirect evaluation of the frequency and intensity of this type of circulation. Acknowledgments. Support for this study was provided by NSF Grants ATM9319455 and ATM9616728.

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