These will be discussed later. Clouds occur at various levels in the atmosphere. Ranging from near the surface to 70, ft. Cloud composition varies; whether it is water, ice, or a mixture is a function of the surrounding temperatures. The potential for cloud formation and precipitation depends on the amount of water vapor in the atmosphere. As a parcel of air rises, the moisture it contains cools and condenses out onto small particles of dust called cloud condensation nuclei until a cloud forms.
As a volume of unsaturated air cools, its relative humidity increases. Here's a fun hands-on activity applet to help you explore the relationship between temperature, dew point temperature and relative humidity. Lifting , also referred to as adiabatic cooling , is the most common method of humidification of air to form clouds. As air rises it expands because pressure decreases with altitude. Convergence of air occurs here and the warm air is advected downward toward the land surface.
This downward-moving warm air does not allow thermals generated over the cold patch to reach the ABL top and thereby prevents entrainment over the cold patch.
The suppression of upward-moving thermals is thus enhanced as heterogeneity amplitude increases. We discuss now how the temperature at the ABL top is influenced by the modified entrainment. Figure 3 shows the 1-h-averaged space-dependent values for the potential temperature and the absolute temperature near the top of the ABL.
The potential temperature is nearly uniform in case 1 as expected, although there are some fluctuations that are the result of the domain size.
These values are colder than their surroundings because the thermals have become negatively buoyant in the temperature inversion. As shown in Fig. The entrainment events are very well visible in case 5, with the two temperature peaks of The absolute temperature at the top of the ABL shows more variation than the potential temperature because of the variation in boundary layer height over the patches see Fig.
The homogeneous case 1 has an absolute temperature of For the strongest amplitude case case 5 , we find an absolute temperature of In consequence, under the assumption of equal moisture conditions, the relative humidity will increase for larger heterogeneity amplitudes.
In cases characterized by weaker inversions smaller temperature jump at the ABL top , we found identical variation in the ABL height within the domain not shown and therefore the entrainment zone has a similar absolute temperature variability.
The fact that weaker inversions do not yield a larger variability in ABL height and temperature may be explained by the nature of the motions in the entrainment zone in these conditions. If the inversion is weak, folding of the interface between the ABL and the free atmosphere occurs and these types of motions are associated with larger horizontal spatial scales rather than entrainment events in strong inversion cases Sullivan et al. Therefore, additional ABL height variation in heterogeneous cases with weak inversions may not exist as the effects are spread out in the horizontal.
Here, we further discuss the effects of surface variability on the thermodynamic vertical profiles. To address these effects, the vertical profiles of the homogeneous case 1 are compared with the four heterogeneous cases.
Just as a reminder, notice that all cases have the same area-averaged sensible and latent heat fluxes and initial thermodynamic profiles. Consequently, differences among the simulations must be induced by the heterogeneous forcings and by the subsequent local effects on entrainment.
Figure 4 shows the 1-h area-averaged heat flux profiles for all the simulations. In spite of the large structural changes that heterogeneity induces, there are only small differences between the homogeneous and heterogeneous cases, although as we show later the distribution between mesoscale and turbulent contributions to the heat flux varies considerably.
The curved profiles of the heat flux that AS98 found for heterogeneous cases are not present in our cases. We found linear profiles similar to those in PSM Therefore, we assume that heterogeneous cases should yield linear profiles and that AS98 results are the effect of the low resolution of their model runs. The region with negative heat flux is a measure of the contribution of entrainment vanZanten et al. Figure 5 shows the temporal evolution of the area-averaged ABL height for all five cases computed following the maximum temperature gradient method Sullivan et al.
If all cases have the same entrainment velocity, the entrainment differences found in the previous section cannot exist. Therefore, the suggested enhancement found in Fig. This connects with the findings of Lilly , who suggests that the smooth heat flux profiles in the entrainment zone found in LES are mostly an effect of horizontal averaging and the that link to the entrainment rate should be made carefully.
PSM05 found no significant enhancement of entrainment when they performed a sensitivity analysis of the effect of the heterogeneity length, but they did not vary the heterogeneity amplitude. We showed by varying the amplitude that the results of PSM05 are correct and we thus disagree with previous suggestions of AS98 and Letzel and Raasch that the area-averaged entrainment is enhanced.
Changes in ABL height are the result of the model spinup, and once the model is in quasi-stationary state, the entrainment rate is constant for all heterogeneity amplitudes. Therefore, only the spatial changes will influence the structure of the RH; the spatially averaged ABL growth is not affected.
We discussed previously that Fig. Nevertheless, there is a fundamental difference between the cases. In the homogeneous case all the heat transport is driven by small-scale turbulence, but in the heterogeneous cases the total heat flux is the sum of the mesoscale and turbulent part that add up to the linear profile. The left-hand side of Fig. Thus, only the strongest engulfing motions reach the lower half of the entrainment zone. Because of the induced circulation, these events are always at the same location see the peaks in Fig.
They are associated with large pockets of warm tropospheric air that enter the ABL at the edges of the strong thermals that are pushing the inversion layer. In contrast, the small-scale mixing, quantified by the turbulent part, occurs only at the top of the warm patch, but not at a fixed location.
For increasing heterogeneity amplitude, the mesoscale contribution to entrainment increases see the right-hand side of Fig. We show in Fig. In the previous section we focused on the heterogeneity amplitude to study how the strength of the mesoscale circulation influenced entrainment and ABL growth.
Now, we discuss the effect of these structure changes on the spatial distribution of specific humidity in the ABL top. We find that the largest values of the specific humidity are found over the center of the warm patch 6. Just as for the potential temperature at z i , the specific humidity near the ABL top is only slightly dependent on heterogeneity amplitude because the concentration of specific humidity is nearly the same in the rising thermals 6.
The increasing ABL height see Fig. The homogeneous case exhibits a large fluctuation, which is a result of the domain size see the appendix and of the fact that the scale of moisture fluctuations has a tendency to increase in time Jonker et al.
Therefore, a larger domain in the y axis should yield smaller fluctuations because the mean then consists of multiple cycles of the largest spatial scales. The low specific humidity at the top of the ABL near the center of the cold patch 5. The downward transport dries the cold patch from the top, which explains the pocket of dry air that Avissar and Liu and AS98 found over the cold patch. Thus, the atmosphere over the warm patch is moist, despite the low surface evaporation that characterizes this patch, and the cold patch with the high rate of evaporation is dry.
Therefore, the location of the largest moisture content in the ABL top coincides with the location of the lowest absolute temperatures see Fig.
The beneficial effect of the heterogeneity-induced moisture transport toward the thermals over the warm patch becomes obvious in a scatterplot of vertical wind speed and specific humidity for case 1 large and case 5 large see Fig. We have conditionally sampled the vertical wind and the specific humidity for the thermals by taking the area where the vertical velocity exceeds 1.
The plot shows that the thermals for the homogeneous case 1 large are on average drier because there are significantly more points in the range from 5. If we assume that the largest vertical wind speeds are associated with the core of the thermals, then both cases have the same specific humidity in the core 6. Under the assumption that lower wind speeds occur at the edges of the thermals, we can argue that for this region the heterogeneous case is more efficient in transporting moisture.
Because of the merging of thermals by the circulation, the thermals in the heterogeneous case may have a larger ratio of volume to surface and suffer less from drying by detrainment at the side of the thermals. Therefore, in heterogeneous conditions, more moisture reaches the entrainment zone, and thus the RH zi may increase. Relative humidity is the indicator that links the results of the boundary layer growth and temperature analyses with the findings on the moisture structure.
Here, we include the simulations that are performed for the regimes with weaker temperature inversions and a drier upper atmosphere to investigate the importance of the thermodynamic structure of the entrainment zone.
Figure 9 shows the 1-h-averaged cross section of for case 5 and case 5 dry. The structure of case 5 dry is similar to case 5 over the warm patch. A comparison of case 5 weak and case 5 weak—dry yielded similar structures not shown. In the cases characterized by a drier upper atmosphere, the RH enhancement effect is still important, despite the intense dry air entrainment, because this air is horizontally advected toward the cold patch and does not directly influence the RH over the center of the warm patch see Fig.
Because RH is our chosen indicator for cloud formation Ek and Mahrt , we therefore expect that in free convective conditions cloud formation may occur earlier over heterogeneous land, independent of temperature and moisture inversion strengths. This finding provides a more complete framework than previous observational and modeling studies Avissar and Liu ; Chagnon et al.
Figure 10 shows the 1-h-averaged vertical profiles of the relative humidity. In the left figure, the shaded area is the range of the 1-h-average relative humidities found in case 5, and the hatched area is the range of temporal averaged RHs in case 5 weak.
The large-amplitude cases are characterized by a deeper entrainment zone see Fig. Notice that the area-averaged RH is not enhanced by heterogeneity, which is supported by the fact that we have identical surface fluxes and entrainment velocities for all amplitudes see Fig.
The large RH-variability of case 5 gray shaded area indicates the importance of variability on possible cloud formation. The mean profiles of cases 1 and 5 are very similar, but in case 1 there is hardly any variability within the domain not shown. The cases with a dry upper atmosphere right-hand side of Fig. By comparing the left and right sides of Fig. An important consequence, therefore, is that even with dry upper atmosphere conditions, the RH over the warm patch is enhanced compared to the homogeneous case sharing the same conditions.
Figure 11 shows the maximum time-averaged found in the entrainment zone for all cases. The maximum value of RH increases with heterogeneity amplitude for the four defined regimes of potential temperature and specific humidity inversion strengths. For the numerical experiments with a strong inversion, the values range from Therefore, there might be a difference in cloud onset between the homogeneous and heterogeneous cases on the order of an hour, if we take initial conditions for the LES simulations closer to saturation.
We conclude this study with an analysis of the variances of the potential temperature and specific humidity in the entrainment zone to show that these variables are influenced by the nonuniform forcings at the land surface see Fig.
The first finding we discuss is the creation of mesoscale variance. The figure shows that for potential temperature one-third 0. Specific humidity at this height has a significantly larger part of the variance in the mesoscale see bottom right panel in Fig. An explanation for the shift is that in the heterogeneous case, thermals are organized over the center of the warm patch in a line on the y axis.
Therefore, if we apply our statistical procedure over the warm patch, there are only marginal turbulent fluctuations because most of the y axis is covered by thermals. Also, over the cold patch there is a reduction of the turbulence because here there are no thermals, but only a mean sinking motion that is enclosed in the mesoscale contribution see Fig.
An explanation of why there is more mesoscale contribution in the specific humidity variance than in the temperature variance connects with the findings in the next paragraph. Our second finding is that the total potential temperature variance peaks in the entrainment zone for case 1 large and case 5 large see Fig.
This finding could be explained by the opposing entrainment ratios of potential temperature and specific humidity. The specific humidity has in both cases a positive entrainment ratio approximately 0. We will use the table of saturation mixing ratios from an in-class handout. There is no liquid water at first. Thus, there is The remaining water vapor has condensed into liquid, therefore the parcel contains, Most clouds form well above the ground surface.
Air parcels near the Earth's surface contain water vapor, which evaporated from liquid surfaces, e. When these parcels move upward, they take this water in the form of water vapor with them. As parcels move upward, they expand and cool, i. This increases the relative humidity of the air in the parcel because as air temperature falls, the capacity for water vapor in the air decreases.
Once this point is reached, when air parcels rise higher, their temperature becomes so low that they cannot hold all of the water vapor that they started with.
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