Convective Clouds. Convective clouds 1

Transcription

1 Convective clouds 1 Convective Clouds Introduction Convective clouds are formed in vertical motions that result from the instability of the atmosphere. This instability can be caused by: a. heating at the bottom of an air layer; b. cooling at the top of an air layer; c. lifting or saturation of a potentially unstable layer; d. a combination of all the above. Over land the diurnal variation of convective clouds is often clearly visible. On clear mornings, when the sun is able to heat the earth s surface the conditions for Cumulus formation are favourable. If the instability and relative humidity is large enough Cumulus clouds begin to form early. There is no Cu-formation is the atmosphere is only slightly unstable or stable and the relative humidity is low. After reaching a maximum in the afternoon the Cu activity decreases and finally the clouds dissolve in the late afternoon or early evening. Over sea the diurnal variation of Cu-formation usually is very small. If there is any activity it is near the end of the night. The ascending motions of convection decrease or stop when these motions encounter stable layers, or especially inversions. The form into which Cu-clouds develop mainly depends on the vertical distance between the condensation level (the level where temperature equals the dew point) and the cloud base of the stable layer, from the stability and the thickness (vertical distance from top to bottom) of this layer: if it is very stable the cumulus tops may be forced to spread horizontally. If the layer is not very thick only some tops will spread horizontally and some will shoot through the layer and may continue to grow. Observations have shown that the initiation of convection and its weather is only seldom a local phenomenon. On satellite photos it is often apparent that convective conditions are occurring simultaneously over extensive areas and are a result of the vertical structure of an air mass and the characteristics of the actual dominant large-scale weather systems. The convection process In Figure 1 the environmental lapse rate (γ) in the early morning (at the time of the minimum temperature) is indicated by the thick in the θ s,p-diagram; the dew point curve is indicated by the thick dashed line. If an air parcel with temperature T and dew point T d was lifted from the surface, then on level A it could reach its lifting condensation level (LCL). On that level the temperature of the air parcel is lower than the temperature of the environment so that it will sink back and condensation will not occur. Absorption of radiation at the surface will increase the surface temperature (T 1, T 3 etc.) creating unstable conditions in the lowest layers of the atmosphere. Therefore air parcels will rise and subsequently cool dry adiabatically; as a result the environmental lapse rate near the surface will change (e.g. the curve at T 3 ). The LCL will increase but the temperature of the environment at this level will be higher than the temperature of the air parcel so that no clouds will form.

2 Convective clouds 2 Figure 1. Changes in the environmental lapse rate during the day. The dew point temperature will change only slightly: usually some water, e.g. dew on the soil or on vegetation will evaporate into the atmosphere and mix with the lowest air layers. It appears that as a rule the dew point will slightly increase except when the near-surface air is too dry. A good estimate of the surface dew point that will be reached can be made by drawing on the θ s,p-diagram the average maximum mixing ratio line (<χ> in Figure 1) of a layer with a thickness of 50 mb from the surface upwards using equal areas ; the intersection of the line with the surface pressure isobar will give the dew point. When T 3 is reached an air parcel with temperature T 3 and dew point T d3 will lift from the surface. However, before it reaches LCL3 the air parcel again has reached a temperature by cooling that is lower than the temperature of the environment, hence LCL3 will not be reached. Ongoing radiation will increase the temperature even further and T 4 will be reached. Air parcels that rise from the surface will reach level C where the line <χ> is intersected (LCL4); at that height condensation will occur and consecutive ascent will be along a saturated-adiabat. In the case of Figure 1 the environmental lapse rate above level C is located to the left of the saturated adiabat the situation is unstable (in this case is het conditionally unstable). Hence as soon as the temperature T 4 has been reached cumuliform clouds will appear. The value of the surface temperature where convective clouds start to develop is called the convection-temperature (T c ), level C is called the convective condensation level (CCL). In Figure 1 the dry adiabat through T c and the average maximum mixing ratio line <χ> intersect in the convective condensation level. As long as the atmosphere is colder than the rising air parcel, i.e. in Figure 1 as long as the saturated adiabat γ s is located to the right of the environmental lapse rate (γ) the parcel will continue to rise until the intersection of γ s with γ (level D). At this height the vertical motion will

3 Convective clouds 3 stop. The area between γ s and γ between levels C and D is a measure for the amount of energy (in J/kg) available for the parcel to rise (increase its potential energy). This area is called CAPE (Convective Available Potential Energy). The formation of convective clouds In the previous text the explanation of rising air parcels changed into an explanation of convective clouds. With the so-called parcel method the assumption was made that the processes within the parcel were adiabatic and that the atmosphere was not perturbed by the motion of the parcel If a whole layer is involved these assumptions may not be true: a. in measurements within Cu clouds the saturated adiabatic lapse (γ s ) rate is never observed; temperatures and amount of water are lower than expected from the parcel method; the lapse rate within the cloud is often larger than γ s, b. rising large air parcels create downdrafts in the surrounding air, c. evaporation takes place at cloud boundaries. Entrainment The effects mentioned under a are a result that the rising air also mixes with environmental air while also some air from the parcel is expelled. The characteristics of the air in the parcel will change continuously. This phenomenon is called "entrainment". The consequences of entrainment are: a. Sometimes the environmental lapse rate suggests strong formation of cumulus or even showers; in practice only small cumulus (Cu humilis) will form and no showers develop. b. The radiosonde data suggest the cloud tops to be at the intersection of γ and γ s (D in Figure 1); in reality cloud tops may be considerably lower. If the radiosonde data indicate cloud tops at a level higher than 500 mb, then almost always showers will form if the average relative humidity between the CCL and the 500 mb level is larger than 75%. If the average relative humidity is less than 30% then showers are very unlikely. These rules only cover a part of the possible cumulus clouds: it is very well possible to have showers with clouds not even reaching the 500 mb level. Nevertheless these rules illustrate the influence of entrainment. Compensating downdrafts A large amount of air that ascends rapidly creates in the immediate environment compensating downward motions. These downdrafts reduce the temperature difference between cloud and environment; hence they also decrease the amount of CAPE (Figure 2). AB is the environmental lapse rate, AC is the saturated-adiabat. On level P the temperature difference between cloud and environment is T C -T D. As a results of the downdraft air surrounding the cloud is brought from level P 1 to level P dry-adiabatically, reaching a temperature T E. From the figure it appears that T C -T E < T C -T D. Therefore the downdraft also lowers the cloud tops. Evaporation Especially at the top of the ascending cumulus clouds turbulent mixing with the environment will occur; there most evaporation will take place. The energy needed for evaporation is supplied by cooling of the cloud top. Suddenly the dissipating top may vanish completely. However, evaporating cumulus tops will moisten the dry layer making it easier for consecutive cloud tops to survive and grow. From observations it appears that shortly after the top of a cumulus congestus dissipates a shower may form. In this case cloud droplets of a critical size for active coalescence were first moved to the cloud top with the rest of the air. The evaporative cooling at the top either creates a

4 Convective clouds 4 downdraft or reduces the upward motions enabling these cloud droplets to descend into the cloud and start the coalescence and precipitation processes. Figure 2. The effect of downdraft around a rising cloud. Forecasting convective clouds Considering the diurnal behaviour of the formation of convective clouds a forecast made at e.g. 08 UTC for the afternoon of the same day must contain a statement of the chance of the formation of cumuliform clouds. Figure 1 serves as a starting point: a. determine the maximum mixing ratio <χ> averaged over the lowest 50 mb using the dew point curve of the radiosonde data of 00 UTC; the intersection of this line with the environmental lapse rate will give you the CCL; b. from the CCL follow the dry adiabat to the surface pressure isobar and read T C ; when this temperature is reached convective clouds will appear; c. decide if the air temperature will exceed T C ; and if so at what time this will happen. The diurnal temperature changes in the same air mass of previous days may be an important indication; d. if T C will be exceeded then the height of the cloud base must be determined. This should coincide with the CCL because that is where condensation will start; due to a required supersaturation the cloud base may be somewhat higher. Furthermore, during ascend some entrainment will occur decreasing the humidity of the air parcel and increasing the cloud base slightly above the CCL. The cloud base may also be higher than the CCL because air parcels may not lift directly once T C has been reached: a so-called superadiabatic lapse rate is the result. The air parcel sticks to the surface and only starts ascending when the wind disconnects it from the earth s surface. Therefore a wind speed of a few knots is necessary. If such an air parcel having T > T C leaves the surface, it will follow a different dry adiabat which intersects <χ> at a higher level than the CCL. On average the cloud base is approximately 25 mb higher than the CCL. An empirical formula for the height of the CCL (in m) is h CCL (m) = 125 (T - T d ) where T and T d are the temperature and dew point at the surface, respectively. e. Theoretically the cloud top is located in D (Figure 1). In practice, due to the effects mentioned before, the cloud top is at least 25 mb below level D. f. Forecasting the cloud cover (N) of the convective clouds is not easy. In practice the situation of the previous days, if similar, will give some guidance. An empirical rule, giving

5 Convective clouds 5 a rough indication is: N 4 UCCL = where U CCL is the relative humidity (in %) at the CCL. For forecasting precipitation from convective clouds it is important to realize that a cloud can only produce precipitation if the cloud layer is thick enough and/or the temperature in the upper part of the cloud is low enough. In the Netherlands cumulus clouds must have a considerable vertical extent and the 0 C-level must be exceeded in order to produce precipitation. If the cloud top exceeds the -7 C-level and the relative humidity between the CCL and the cloud top height is large enough, then surely showers are possible. The higher the cloud top and the larger the area of instability on the thermodynamic diagram the more intense the precipitation will be. An important aspect in considering the environmental lapse rate is the fact that there is less development of convective clouds in and area with anticyclonically curved isobars. Of course subsiding motions are important, especially when a subsidence inversion is present which may gradually sink and oppose the vertical development of cumuliform clouds. After reaching the maximum temperature convection will diminish gradually and will stop once a surface inversion develops. Remaining clouds will evaporate creating downdrafts because of evaporative cooling of the air. Cumulus clouds will dissolve and now Sc cumulogenitus may be observed. Cloud dissipation will occur more rapidly if the air is dry. Especially in the winter cumuliform clouds, originating over the North Sea, may remain present during the night. With strong NW-winds these clouds may penetrate a considerable distance inland before dissolving. Forecasting thunderstorms The most important conditions for the formation of thunderstorms are: - A large amount of CAPE that can be released by door convection. - Vertical windshear over the entire convective layer. The intensity of thunderstorms depends on the number of clouds where CAPE is released: many small clouds or a few large ones. Windshear has the following effects: - in the absence of shear, updrafts and downdrafts will coincide. Initially the cloud will grow and the updrafts become stronger; next precipitation will fall from the upper parts of the cloud. The precipitation will fall trough the region with updrafts, weakening the updrafts (evaporative cooling) and the cloud dissipates rapidly. - if the wind speed, but not the wind direction, varies with height, then updrafts are directed at an angle with the vertical and precipitation will fall next to the updraft region. Updrafts and downdrafts exist next to one another and the cloud will not dissipate as quickly as in the situation without windshear. Updrafts may be stopped by lateral spreading of cold air when the downdrafts reach the surface. - if there is shear both in wind direction and wind speed then complex mesoscale systems may develop. These may live for several hours, more or less independent of surface heating. Forecasting thunderstorms For the forecast of thunderstorms a number of rules and indices have been developed. A brief overview is presented here.

6 - Indication for thunderstorms from the cloud top height: - top below 4000 m: thunderstorms not likely - top between 4000 and 5000 m: thunderstorms is likely - top over 5000 m: thunderstorms very likely. Convective clouds 6 - Boyden Index The Boyden Index (I) can be calculated from the radiosonde data: I = (Z-200) - T Here Z is the thickness of the hpa layer (in dam!) and T is the temperature (in C) at 700 hpa. Thunderstorms are likely if I >= This index is only valid in or near Western Europe. - Total Totals Index This index is commonly used as a severe weather indicator and is defined by: TT = T T d,850-2 T 500 TT Index < 45 weak moderate > 55 strong - S Index This index is primarily used to indicate thunderstorm potential from April through September. It is defined by: S{TT} = TT - (T-T d ) K where TT is the Total Totals Index (see above) and K is defined as 0 when T T when T T 500 > 22 and < 25 6 when T T S{TT} < 40 none possible > 46 likely - KO index Defined by: KO = [(Θ e,500 + Θ e,700 )/2] - [(Θ e,850 + Θ e,1000 )/2] where Θ e,ppp is the potential equivalent temperature at level ppp hpa. NB Θ e,ppp can be found in the RAOB Data window as ept. This index was developed to estimate thunderstorm potential in Europe. It is more sensitive to moisture than other more traditional indices and is best used in cooler, moist climates. If the surface pressure is below 1000 hpa then Θ e, surface is used. KO Index > 6 weak 2-6 moderate < 2 strong

7 Convective clouds 7 - Lifted Index A stability index used to determine thunderstorm potential. The LI is calculated by lifting an air parcel adiabatically from a level 50 hpa above the surface to 500 hpa. The difference between the temperature of the air parcel and the environmental temperature at 500 hpa represents the LI. LI > -3 weak -3 to -5 moderate < -5 strong - K Index Defined by: KI = T T T d,850 - (T T d,700 ) KI < 20 none weak: isolated thunderstorms moderate: widely scattered thunderstorms moderate: scattered thunderstorms > 35 strong: numerous thunderstorms The above indices must always be used in conjunction with large-scale indicators of thunderstorms. Extra convective energy may depend on: - Position and movement of upper-air troughs or lows. Thunderstorms are usually found along or just in front of upper-air troughs or lows. - The existence and movement of low-level convergence lines, such as fronts. - Elevated areas may receive extra heating. Other useful synoptic tools are: - Analysis of the dew point and/or Θ w. Tongues with high dew points or Θ w may help to define areas with a high risk of thunderstorms; - Difference in Θ w on 500 and 850 hpa - Cyclonic curvature of surface isobars. Conditions favourable for heavy thunderstorms: Heavy showers are mesoscale systems that might develop into heavy thunderstorms. It is important to note that: - some of the heaviest and most extensive thunderstorm activity did not occur before hours after the temp showed a warm layer over a convective layer. Such a warm layer may prevent the release of convective energy, thereby building up the amount of energy which is subsequently released explosively. - In heavy thunderstorms the dew point is often higher than 13 C and may even reach 18 C. At 850 hpa analogous values for Θ w are found. Winds are from the SE to SSW with speeds between kts in a narrow tongue. - Advection of dry air at mid-atmospheric levels. With potential instability Θ w at 500 hpa may 2-5 C lower than at 850 hpa. Winds at 500 hpa must have veered degrees relative to the winds at 850 hpa, speeds kts. - Further veering above 500 hpa, with 300 hpa winds from SSW-W and speeds of kts are good conditions to have the downdrafts in a favourable position for generating heavy thunderstorms.