Characteristics of the Cloud Liquid Water Content Profiles over Mosul, Iraq

Mosul is one of Iraq's provinces (see Figure 1), located in the semi-arid zone of the world’s environmental categorization in northwest Iraq. In the wintertime, the climate is characterized by chilly conditions, with many days having low air temperature below 0 °C and snowfall, while it is a comfortable summer environment. The city was previously referred to as Umm Al-Rabeen. As for now, the summers are hot, dry, and clear, and the winters will be cold and partly cloudy, with temperatures between 3–42 °C throughout the year. Geographical coordinates are between the longitude (41° 25ʺ), latitude (25° 44ʺ) and altitude of 223 m above mean sea level. Over the course of the year, there has been a considerable seasonal change in the average amount of sky covered by clouds, and the year is divided into two parts. The first section is clearer, starting in May and ending in approximately October, and July is the month with the most clarity. The year’s cloudier period starts in October and ends around May. January is the cloudiest month of the year [Ahmad et al. 2013].

Figure 1.

The study area (Mosul)

Hourly vertical air temperature data, relative humidity, atmospheric pressure, and CLWC were analyzed at four-time intervals (00, 06, 12, and 18 UTC) across all pressure levels from 1000 to 1 hPa for selected days in the winter of 2019. These data were obtained from ERA5, the high-resolution fifth-generation atmospheric reanalysis dataset, which is widely used for weather research, climate studies, and environmental monitoring due to its detailed temporal and spatial resolution [Mahmood et al. 2022]. ERA5, produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset, provides high-resolution hourly data of a wide range of climate, atmospheric, oceanic, and land surface data using advanced data assimilation and numerical weather prediction models. Hourly and monthly data cover the entire globe from 1950 to the present. The data are distributed on a 0.25° × 0.25° grid and resolve the atmosphere in 137 vertical levels, from the surface to 80 km. Additional observational data for low, medium, and high clouds and precipitation were obtained from the Iraqi Meteorological Organization and Seismology (IMOS). Several software programs, including Matlab, Origin, and Excel, were used to analyze and plot the data.

To make the data obtained from ECMWF data more uniform and comparable, they were already standardized relative to climatological baselines. Thus, CLWC values were converted from kg/kg to g/m³ for plotting and analysis. The CLWC data was in units of kg/kg and the common units are g\m³ so we converted the units by multiplying by 1000 and using the following equation (1) CLWCgm3=CLWCgkg*PRT \text{CLWC}\left( \frac{\text{g}}{{{\text{m}}^{3}}} \right)=\text{CLWC}\left( \frac{\text{g}}{\text{kg}} \right)*\frac{\text{P}}{\text{RT}} where T is the air temperature (in Kelvin), P is the atmospheric pressure (hPa), and R is the specific gas constant for dry air (=287 J.kg⁻¹.K⁻¹). After converting the units, we also had to convert the data from pressure levels to height units (km) to display the vertical profile of CLWC in the cloud using the equation below [ROGERS, YAU 1989]. (2) Z=T0σ*1−PoP−Rσg \text{Z}=\frac{{{\text{T}}_{0}}}{\sigma }*{{\left[ 1-\frac{{{\text{P}}_{\text{o}}}}{\text{P}} \right]}^{-\frac{\text{R}\sigma }{\text{g}}}} where T_o is the mean temperature (=288.15 K), σ the vertical lapse rate (=6.5 K.km⁻¹), P_o the pressure at sea level =1013.25 hPa, and Z is altitude in (m). After completing the unit conversion, we found 36 cases characterized by rainfall in 2019. We selected only 9 heavy cases for analysis by comparing the prevailing cloud types under specific weather conditions with the corresponding CLWC values. Each cloudy case was categorized, and the highest and lowest CLWC values were identified to establish typical CLWC ranges. Table 2 summarizes the rainy conditions for 9 cases analyzed in this study, with their dates and daily averages for rainfall and CLWC. As shown in this table, the highest rainfalls were on March 16 (50.3 mm), January 28 (40.2 mm), and March 24 (33.5 mm), while the lowest rainfall occurred on March 3 (5.2 mm). Also, CLWC and rainfall do not always correlate; for example, March 16 had the highest rainfall but a relatively low CLWC.

The profile of CLWC is important in the evolution of clouds, and we selected only two cases, which represent winter (28/1/2019) and spring (14/3/2019) with significant amounts of daily rainfall, 40.2 and 26.4 mm, respectively, to examine in detail. Their CLWC profiles are displayed in Figures 2 and 3 at different times of the day (00, 06, 12, and 18 UTC). They provide a good indication, in general, of how the CLWC varies with altitude, reflecting cloud evolution over time.

Figure 2.

CLWC profile within the cloud over Mosul on date 28\1\2019

Figure 3.

CLWC profile in four times over Mosul on date 14\3\2019

Figure 2 shows the vertical variation of the CLWC within the cloud. At 00 UTC, the CLWC values were few, as the CLWC values started from a height of 4 km to a height of 8 km, reaching their highest value of about 2.3 g.m⁻³ at a height of 6 km. The type of cloud was cumulus. But at 06 UTC, the CLWC developed, and its value began to appear at a height of approximately 2 km. Then it decreased to a height of 6 km, then increased again to diminish at an altitude of approximately 10.5 km. Its maximum value reached 4 g.m⁻³ at an altitude of 8 km, suggesting a cloud deepening leading to cloud-like cumulus. At 12 UTC, LWC values were at low altitudes, as the values started at a height of 1 km and reached a maximum value of 8.5 g.m⁻³ at an altitude of 2 km. Most of the values were high, and the type of cloud became cumulonimbus, which reached a mature cloud development with a higher rain potential. At 18 UTC, the total amount of clouds became large, extending from a height of about 1 km to about 8 km. The CLWC values were high, as most were higher than 5 g.m⁻³, and the highest value reached 8.3 g.m⁻³ at a height of about 2 km, in which the cloud type is cumulonimbus. However, the cloud evolution was deep cloud formation (6–12 UTC) and low CLWC values in the evening (18 UTC).

In Figure 3, the vertical profile of the CLWC above Mosul at 00 UTC shows that the values started at an altitude of 3.5 km and ended at an altitude of approximately 8 km. The highest value occurred at an altitude of approximately 6.5 km and amounted to about 2.5 g.m⁻³. At 6 UTC, the CLWC values started at an elevation of 2 km and ended at 11 km. The highest CLWC value is about 4 g/m³, at an elevation of about 8 km. At 12 UTC, the values appeared at an altitude of 1 km and ended at 10 km. The highest value was about 8.5 g\m³ at an altitude of about 2 km. At 18 UTC, CLWC values began at less than 1 km and ended at 10 km. Cloud thickness about 9 km. Most CLWC values were above 3 g.m⁻³. The highest CLWC value was about 8 g.m⁻³ at an altitude of 2 km. In conclusion, the clouds evolved from low-level development (00 UTC) to mature rain-producing clouds (12 UTC), ending with deep convection at 18 UTC, influencing Mosul’s rainfall and weather patterns.

From the previous two cases and all other cases analyzed in this paper, we showed the significant difference in the behavior of CLWC within the cloud. Other study cases were drawn and placed in the appendix to avoid repetition.

In this section, to relate CLWC to RH, we take all the CLWC data for the cases recorded in Table 2 - the total number of hourly data points of CLWC is 36. We plot the dispersion relationship between the hourly rates for the values of CLWC and RH for all the cases studied. The hourly rates are the sum of the values of the variable, such as CLWC or RH, through the height, divided by their number (where there is a value at each pressure level that is different from other levels). The fitted line or best-loo king line through the data points obeys the following non-linear equation [Al-Jiboori et al. 2020]: (3) RH=α*CLWCb \text{RH}=\alpha *{{\left( \text{CLWC} \right)}^{\text{b}}} where the constants α=79.6 %.m³/g and the exponent b=0.07 were derived from the observation data of CLWC and RH.

Figure 4 shows the relationship between CLWC and relative humidity data over Mosul. We notice that the RH is high, as there are no values below 60%. The RH values started from 60% to 100%, and the CLWC values started from less than 0.1 to 6.5 g.m⁻³. Most of the CLWC values between 80–100% of RH are dispersed close to the best line, and between 60–80% values are slightly dispersed. All CLWC values almost increase with increasing RH in nonlinear behaviour.

Figure 4.

Relationship between CLWC with relative humidity

Here the correlation between the hourly rates of CLWC and air temperatures were described the scatter plot presented in Figure 5. The total number of cases is the same as that used with relative humidity 36. The best-fit line going through the data plotted in this figure as a (parabolic) nonlinear behavior was made, in which the values of CLWC increase with decreasing air temperatures. Using Eq. (3) above but for temperature, the line could pass these data points, but with different values of the constants (α=−13.96 and b=−0.17). In this figure, the values of temperature ranged between −22 - 0 °C and the values of CLWC ranged between 0.2–6 g.m⁻³. Most of the values of CLWC occurred between −20 and −10 °C, and from −10 to −5 there were only 5 values for CLWC and two values. At about 0 °C, the highest value of CLWC was about 6 g.m⁻³ and occurred at temperature=−10 °C. In general, the behavior of the values in all forms was an inverse relationship, as the CLWC increased as the temperature decreased.

Figure 5.

Relationship between CLWC and air temperature

Because the data is available for rain only daily, the study will review the experimental relationship between rain and CLWC on a daily scale. Figure 6 represents the variation between the daily averages of the CLWC and rainfall rates with 9 data points. The best-looking line was passed through these points, which follows the equation below: (4) Daily rainfall=β*ln−γ*lnCLWC \text{Daily}\ \text{rainfall}=\beta *\ln \left( -\gamma *\ln \left( \text{CLWC} \right) \right) where β and γ are empirical constants (β=20.64 mm and γ=−1.61). Figure 6 illustrates the relationship over Mosul, described by Eq. 4. Daily precipitation rates ranged from approximately 5 to 50 mm, while daily CLWC values varied between 4.6 and 14 g.m⁻³. These values indicate that both rainfall and CLWC daily averages were relatively high. For instance, a daily CLWC rate of 6.5 g.m⁻³ corresponded to 50 mm of rainfall, while a CLWC rate of 14 g.m⁻³ yielded approximately 40 mm of rainfall.

Figure 6.

Relationship between CLWC and rainfall

Analysis of the relationship between daily precipitation rates and CLWC, along with the best-fit line through the data points, shows a direct correlation: as CLWC increases, precipitation rates also increase. However, there are some outliers, which are typical for precipitation, as precipitation is an unpredictable phenomenon that varies from cloud to cloud and year to year. In general, precipitation is unlikely to occur if the cloud contains less than 4 g.m⁻³ of liquid water.

The CLWC varies greatly from cloud to cloud and from place to place, and the liquid water content and cloud origin significantly affect the cloud’s classification. Low-density clouds have comparatively low liquid water content values because they contain very little water. High-density clouds have much higher liquid water content values. It is noticed that the difference in CLWC values over Mosul in each type of cloud, so we find it in the clouds of type St, it has values between 0.03–10.6 g.m⁻³. Therefore, we have developed a table of values for the typical CLWC of different types of clouds. Table 3 shows the range of CLWC values for different types of clouds.