Determination of Cloud Bottom Height from Rawinsonde Data. Lt Martin Densham RN 29 August 05

Transcription

1 Determination of Cloud Bottom Height from Rawinsonde Data Lt Martin Densham RN 29 August 05

2 LIST OF CONTENTS TABLE OF FIGURES/TABLES 3 I. INTRODUCTION 4 II. DATA AND METHODS Rawinsondes Met Tower Data.9 3. MODIS Imagery Ceilometer.12 III. RESULTS AND ANALYSIS Calculated LCL Heights Ceilometer Heights Met Tower Observations Entrainment Zone Analysis 16 IV. CONCLUSIONS AND RECOMMENDATIONS.18 REFERENCES

3 LIST OF FIGURES Figure 1. MODIS image from 19 July 2005 showing extent of boundary 4 layer stratus along central Californian coast. Figure 2. Boundary layer cloud formation process. 5 From: Figure 3. R/V Point Sur rawinsonde launch positions from July Figure 4. R/V Point Sur rawinsonde launch positions from July Figure 5. Rawinsonde data retrieved on the 18 and 23 July The 8 dotted lines represent measurements taken on the descending leg. Figure 6. MODIS imagery from July showing the changing geographical 12 extent of the stratus layer along the central Californian coast. Figure 7. Calculated LCL heights for marine layer stratus for July. 13 Figure 8. Watsonville ceilometer heights for marine layer stratus for July. 14 Figure 9. Pressure, wind direction and speed for 19, 21, 22 July from the 15 Met Tower. Figure 10. Air and surface temperature and relative humidity for 19, 21, 22 July 15 from Met Tower. Figure 11. Entrainment Zone (EZ) between the Mixed Layer (ML) and the 16 Free Air (FA). Figure 12. Hypothesis that decoupled mixing in the EZ gives rise to cloud 17 evaporation and results in an increase in cloud base height. Figure 13. Rawinsonde profiles for 19, 21, 22 July respectively showing the 18 lowering of subsidence inversion and increased mixing to the surface. LIST OF TABLES Table 1. Met Tower data for rawinsonde launches with calculated LCL height. 10

4 I. INTRODUCTION The marine boundary layer is of great significance to both civilian and military naval and aviation organizations. The operational impact of the stratocumulus cloud that can form as part of this layer cannot be understated. Electromagnetic, electro-optical and ocular visibility are three tactically significant factors effected by the increased intensity of the marine boundary layer. In addition, the presence of extensive cloud cover has a detrimental effect on the ability to employ and exploit remote sensors and other surveillance measures that would ordinarily enhance operational effectiveness. Within the boundary layer conditions may be favorable for cloud to lower to the surface and the resultant fog impacts dramatically on ship and aircraft operability. The extent of the stratus formed within the marine layer within Monterey Bay is shown in the Moderate Resolution Spectrometer (MODIS) image below. Figure 1. MODIS image from 19 July 2005 showing extent of boundary layer stratus along central Californian coast.

5 Marine layer stratus is a well documented feature found along the Californian coast. The June gloom, as it is know, can last from June until September and shows a great degree of diurnal as well as seasonal variability. The formation of the low cloud is a function of the seasonal southward migration of the eastern Pacific high pressure system in the summer months. The effect of this is two-fold, firstly, strong subsidence occurs aloft that caps the marine layer and creates a strong temperature inversion that traps cool moist air at the surface. Secondly, the circulation of the Pacific high establishes a moderate along shore surface wind that forces surface water off-shore and creates upwelling of cooler water along the coastal zone. The relatively cool surface water helps to condense the moist air above it which is decoupled from the warm air aloft due to the inversion. Entrainment during the day does result in evaporation and lifting of the cloud and helps dissipate the cloud towards the coast. The physical mechanisms behind boundary layer cloud formation are shown below. Figure 2. Boundary layer cloud formation process. From: Manual/SatManu/CMs/index.htm

6 Diurnal variations in short wave radiation increase and reduce this effect, giving rise to daily fluctuations in the extent and thickness of the cloud layer. This variability is characterized by an increase in cloud depth and a lowering of cloud base at night, conversely a reduction in cloud depth and increase in base height occurs during the day. The increased short wave radiation during the day serves to stabilize the cloud base, reducing the transport of moisture into the cloud layer while increasing warming of the cloud top, the net effect of which is to reduce the depth of the cloud layer and raise cloud base. At night long wave radiation cools the cloud top and reduced surface air temperatures act in consort to thicken and lower the cloud layer. In addition to the variability of diurnal heating are the effects of mesoscale circulations, specifically at the land/sea interface. The orographic and local circulations within Monterey Bay give rise to localized modifications of the cloud layer. Thermal variability between land and sea set up sea breeze effects which alter turbulent mixing within the layer that reduce or enhance the stratus layer. In this study the area of analysis is within the Bay itself but the data collections vary geographically within the area and must be taken in to account when analyzing cloud base heights. This project utilizes rawinsonde data obtained from the R/V Pt Sur during a six day period from July at a variety of locations within Monterey Bay. The data for this period was collected under a variety of marine layer conditions and encompassed a period of strong stratus formation followed by the onset of a period of clear conditions when the stratus layer was held in abatement. The aim of the paper is to determine how accurately calculated LCL heights correlate to observed cloud base heights and to what extent meteorological observations can be used to predict the height and extent of cloud. The geographical spread of rawinsonde launches are given in Figures 3 and 4 below:

7 Figure 3. R/V Point Sur rawinsonde launch positions from July Figure 4. R/V Point Sur rawinsonde launch positions from July 2005.

8 II. DATA AND METHODS 1. Rawinsondes. The RS80-15L rawinsonde data is analysed from the 6 days of the cruise. The data consists of 23 ascents and descents taken at a variety of locations extending from Point Sur to the central Monterey Bay and from inshore to 30 miles off-shore. The rawinsondes were configured with a standard 100 gram balloon that was attached to a syringe to enable the balloon to deflate slowly and enable readings to be taken on the decent, these figures appear as dotted lines in Figure 5. Figure 5. Rawinsonde data retrieved on the 18 and 23 July The dotted lines represent measurements taken on the descending leg.

9 On average the launches were carried out every 3 hours with some deviations for the ship s operating routine. The Loran-C variant was used to enable tracking of the radio transmitter. The rawinsonde measured air temperature, humidity and pressure. An onboard software package calculated wind speed and direction in addition to these parameters. The rawinsonde data files were ingested into MATLAB and processed using a plotting code that extrapolated measurements for height, temperature and dew point. Plots were made of height (in meters) vs. temperature (in degrees Celsius) for each ascent/descent. The data from the lower part of the rawinsondes showed uncharacteristically higher temperature and lower humidity values than just after release. The launch position of the balloons was just aft of one the R/V Point Sur s funnel vents which was warmer than the surrounding environment. For this reason the surface data from the Met Tower located on the bridge roof was used to calculate the Lifting Condensation Level (LCL) at each launch point and matched to the appropriate pressure height on the ascents. 2. Met Tower Data. The R/V Point Sur Met Tower measures a range of variables including wind speed and direction, temperature, relative humidity, pressure and sea surface temperature. The Clausius-Clapeyron equation was rearranged to form Equation 1 in order to calculate dew point: T d 1 R v e = ln To L eo 1 (1) where T d is dew point, T o is absolute zero, R v is a constant (461JK -1 kg -1 ), L is the latent heat of vaporization, e is saturation vapor pressure and e o is a constant (0.611kPa). Once the dewpoint was derived the LCL height was determined using Equation 2: z = a ( T T ) (2) LCL d

10 where a is the constant 0.125km/ºC, T is the air temperature and T d is dew point. The Met Tower data and calculated LCL height for each rawinsonde is given in Table 1 below: Rawinsonde Number Time of Launch Location Launch of Air Temp (ºC) Dew Point (ºC) Surface Pressure (mb) Relative Humidity (%) LCL Height (mb) Z N July W Z N July W Z N July W Z N July Z July Z N July W Z N July Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z N July W Z 23 July N W Table 1. Met Tower data for rawinsonde launches with calculated LCL height.

11 3. MODIS Imagery The geographical extent of the stratus layer for the period is shown in Figure 6 below. The images were captured between local time from July. The first period of rawinsonde data corresponds to the extensive stratus layer in the first two images. On the fourth day a clearance of the maritime cloud begins, on the fifth day there is no cloud and on the last day there is some thin less extensive cloud. This calculated LCL heights are compared against these conditions for accuracy in determining the extent of cloud. 18 July 19 July 20 July 21 July 22 July 23 July Figure 6. MODIS imagery from July showing the changing geographical extent of the stratus layer along the central Californian coast.

12 4. Ceilometer. The Automated Surface Observing System (ASOS) at Watsonville Airport includes a ceilometer cloud height sensor that uses laser radiation to detect cloud levels. The ceilometer works by transmitting a pulse of laser light into the atmosphere and sensing the light return as it is reflected back toward the ceilometer by objects in its path. By timing the interval between the transmission and reception, the height of particles (such as water droplets or ice crystals in clouds) above the ceilometer is calculated using Equation 3: ct h = (3) 2 where h is the height of cloud, c is the speed of light and t in time from transmission to reception. This data is used in comparison to the rawinsonde data to correlate cloud base height between the maritime and land-based environments.

13 III. RESULTS AND ANALYSIS 1. Calculated LCL Heights. The calculated LCL heights from the rawinsonde data from the R/V Point Sur are presented for the period July in Figure 7 below. The data is split into two sections representing the two periods the R/V Point Sur was at sea. The first 59 hours are characterized by conditions in which an extensive stratus layer was present. The second period, from hours, is characterized by conditions in which the stratus layer was greatly reduced or absent. During the first period the average cloud height is significantly lower and there is a marked degree a diurnal variability. The cloud base reaches its lowest extent after midnight on both days and reaches a maximum height between 1300 and 1400 local time. During the second period the cloud base lifts, reaching the top of the boundary layer at 1200 on 22 July. There is still a degree of diurnal variability and the lowest cloud base occurs at 2100 on 21 July and at 0500 on 23 July. Calculated LCL Heights Height (m) Time (hours) Figure 7. Calculated LCL heights for marine layer stratus for July.

14 2. Ceilometer Heights. The data collected from the Watsonville Municiple Airport ASOS ceilometer gives cloud base height measured at hourly intervals. The sensor is at an altitude of 49.7m and is located 3.8 miles from the shoreline of Monterey Bay. Although the sensor is land based it is used as a guide to the conditions found in the Bay. It must be noted that due to local climatic effects over land the heights are used to compare the trend of cloud base heights, not actual height. Figure 8 shows the Watsonville cloud height observations and calculated cloud base values from within Monterey Bay. There is good correlation during the first period of the cruise and less during the second. The reason may be that cooling over land in the clear conditions from July caused radiation fog overnight whereas in the Bay it remained clear. Watsonville Ceilometer vs Calculated Cloud Heights Time (hours) Height (m) Ceilometer Calculated LCL July Figure 8. Watsonville ceilometer heights for marine layer stratus for July.

15 3. Meteorological Observations. A variety of data from the Met Tower is analysed in comparison with the calculated cloud base height to determine to what extent these variables can be used to predict cloud height. Figures 9 and 10 shows time series data for three days during the cruise during differing cloud conditions. The 19 July information represents a period with extensive stratus on 21 July there was a clearing of the stratus and on the 22 July there were cloud free conditions. Figure 9. Pressure, wind direction and speed for 19, 21, 22 July from the Met Tower. Figure 10. Air and surface temperature and relative humidity for 19, 21, 22 July from Met Tower.

16 The pressure trend ranges from an average of 1009mb on 19 July, to mb on 21 July and 1011mb on 22 July. The wind speed on 19 July peaks at 12m/s and averages 6m/s, on the 21 July it reduces to an average of 2m/s and peaks at 9m/s and on the 22 July the average increases from 2m/s to 10m/s, peaking at 11m/s. The wind direction averages 330º on 19 July, 360º on 21 July and 350º on 22 July. The air temperature averages 14ºC on 19 and 21 July and 15ºC on 22 July. The relative humidity averages 97% on 19 July, 92% on 21 July and 86% on 22 July. The correlation between pressure, wind direction and relative humidity are the strongest for the varying marine boundary cloud conditions whereas wind speed and air temperature are less significant. These surface variables give a good indication of the cloud extent and depth on a general scale. 4. Entrainment Zone Analysis. In addition to the surface variables the rawinsonde profile is analysed for entrainment effects. Solar radiation and entrainment play a significant role in the extent and depth of marine boundary cloud layer as shown in Figure 11 below: Figure 11. Entrainment Zone (EZ) between the Mixed Layer (ML) and the Free Air (FA).

17 The increased mixing in the EZ results in increased evaporation at the cloud top and increases the cloud base height. Figure 12 below shows the theory behind this comparing virtual potential temperature (θ V ) and mixing ratio (q t ) (Horner, 2005): Cloud layer θ v q T θ v q T θ v q T Figure 12. Hypothesis that decoupled mixing in the EZ gives rise to cloud evaporation and results in an increase in cloud base height. From Horner (2005). The profiles for the three days show whether entrainment is occurring at the top of the boundary layer. In Figure 11 the profiles show the varying degrees of entrainment and mixing. On the 19 July the profile shows the inversion starting at 420m. On 21 July greater mixing occurs throughout the profile and the inversion weakens and lowers to 350m. On 22 July the inversion lowers to a little more to 320m and there is greater mixing of warmer air to the surface. Figure 13. Rawinsonde profiles for 19, 21, 22 July respectively showing the lowering of subsidence inversion and increased mixing to the surface.

18 IV. CONCLUSIONS AND RECOMMENDATIONS The correlation between LCL calculated and observed cloud base height is good for the first stage of the cruise when there was a thick layer of stratus present. The LCL heights correspond to visually observed base heights during the first part of the cruise. Due to differing climatic effects over land this trend does not correspond well during the second part of the cruise when a period of clear conditions was present. This is not to say the calculated LCL heights during this period were less accurate, more that ground truth comparisons with Watsonville experienced different land based cooling overnight giving altered conditions. It is recommended that future research incorporates a ceilometer or similar method to measure cloud base at the time of balloon launch to eliminate this disparity and more definitively assess the accuracy of LCL calculated heights. Trends in the meteorological variables indicate there is a correlation with the stratus extent and pressure, wind direction and speed and relative humidity variables. This could enable forecasters, based on model predictions of these parameters, to estimate cloud base height and extent. However, a more systematic comparison of ceilometer readings taken over the water against these variables is needed to validate the accuracy of these relationships. From such a statistically valid empirical dataset rules of thumb for the stratus layer extent could be developed. The effects of entrainment can be seen from the rawinsonde data to a certain extent. A greater understanding of the characteristic ascent profiles that are associated with high mixing ratios and therefore clearance of the cloud layer would further assist forecasters in assessing the timings and extent to which the cloud would lift and clear.

19 REFERENCES Horner., M. S., 2005: Masters Thesis: Determining inversion structure at the top of the planetary boundary layer. Stull., R. B., 1988: An introduction to boundary layer meteorology.