Science

☁️ Word on the Streets: Soil moisture impacts on cloud streets over the Sahel

Previous Research • August 2026

Click me to see the poster ٩(^ᗜ^ )و ´-

Introduction

This project investigates the development of cloud streets (long, parallel rows of clouds) over the Sahel. Specifically, we want to understand how pre-existing soil moisture (SM) influences cloud formation over this region! This soil moisture can be affected by things like irrigation and vegetation which are important for the people living there, especially since many of them rely on rain-fed agriculture.

Figure 1: EUMETSAT screenshot over the Sahel by me!

Surface Energy Budget

Five simulations with varying soil moistures (0%, 25%, 50%, 75%, 100%) were run using the Regional Atmospheric Modeling System (RAMS) to model the surface energy budget and cloud development. The surface energy budget is the total balance of all heat and radiation energy moving into and out of the Earth's surface. Shortwave radiation is energy coming directly from the sun and includes visible light and ultraviolet rays. Longwave radiation is thermal infrared energy emitted by the Earth's surface, atmosphere, and clouds. Sensible Heat Flux (SHF) is heat that moves from the ground into the air via conduction and convection. Latent Heat Flux (LHF) is energy used when water changes state, like when liquid water evaporates into water vapor.

Figure 2: surface energy budget schematic by me!

How does the Bowen Ratio change with soil moisture?

The two specific variables in the surface energy budget that were looked at are sensible heat flux, which provides temperature change, and latent heat flux, which provides evaporation and adds moisture to the air. These two fluxes provide us with the Bowen Ratio (BR; β = SHF/LHF). As seen in the plot provided, the Bowen Ratio decreases as the soil moisture increases. This is because higher soil moistures have a higher LHF since there is more water to evaporate.

Figure 3: Bowen Ratio Plot

The Bowen Ratio critically affects two key heights: the lifting condensation level (LCL) and the planetary boundary layer (PBL). The PBL rises with increasing SHF, promoting vertical lifting, while the LCL lowers with increasing LHF, enhancing moisture availability. For cloud formation to occur, the LCL must be at or below the PBL height.

How does this affect cloud formation?

Now that we know how the Bowen Ratio changes with pre-existing soil moisture, what does this have to do with the formation of clouds let alone cloud streets???

As SHF increases, the PBL increases as well, giving the air parcels more lift. As the LHF increases, the LCL decreases, which is the altitude where a rising air parcel cools enough through expansion to reach 100% relative humidity. In order to get clouds, we want the LCL to be close to or below the PBL because if the PBL is too low, the air parcels will return to Earth’s surface before they have the chance to become clouds.

The plot below shows that the lower soil moisture run has a higher PBL and LCL on average compared to the higher soil moisture run. We also see that the LCL and PBL meet a little after 10:00 in the higher SM run whereas the lower SM run shows the LCL and PBL meeting closer to 11:00. Basically, clouds form later in the day the lower the soil moisture is.

Figure 4: PBL vs. LCL plot

Show us the clouds!

To illustrate the actual formation of clouds, the cloud mixing ratio was used! This is where the air in the atmosphere is well-mixed enough to form clouds. As seen in the plot below, the higher SM level starts to form clouds earlier in the day and they’re also in the street formation! As the day progresses, the clouds in the higher SM separate into a more cellular formation while the lower SM run is just starting to form clouds. However, compared to the higher SM run, the lower SM clouds do not form in a street formation.

Figure 5: Cloud Mixing Ratio plot

Source?

I promise I’m not pulling your leg! The presence of cloud streets (or lack thereof) can be proven mathematically! By taking the negative value of the PBL (-Z_i) and dividing it by the Monin-Obukhov length (L), we get the roll threshold parameter. The PBL favors convective rolls (gives us cloud streets) if the values are between 5 and 25 (Weckwerth et al. 1999).

The plot below shows us that the higher SM run spends more time in the region where cloud streets are favored than the lower SM run!

Figure 6: -Z_i/L vs. Time

Future Questions

How does pre-existing soil moisture affect precipitation?
Do these cloud streets tell us anything about when and where rain will fall?

Acknowledgements

Huge thanks to the NSF for funding the STORM REU, all the awesome friends I made at UW, my super cool mentor Bee, and BIG Z for making this possible.

Keywords: Cloud streets • Surface Energy Budget • Modeling • Sahel • Boundary layer

⚛️ Synthesis and Characterization of Lead-Free Halide Perovskites

Previous Research • April 2026

Introduction

Halide perovskites are a family of materials making big waves in the science world for next-generation technology, including medical biosensing, light-emitting diodes (LEDs), and solar cells. Usually, these materials are lead-based. However, traditional lead-based perovskites face two major roadblocks: lead is super toxic, and the materials tend to degrade quickly when exposed to normal air and moisture.

To solve this, scientists are working to develop lead-free alternatives that are both environmentally friendly and stable. One method to supercharge these new materials is a process called "doping". Which is introducing tiny amounts of rare-earth elements, like Erbium (Er³⁺). These rare-earth elements act like internal light bulbs, producing incredibly sharp, predictable colors of light when energized.

This study investigates how Erbium doping changes the optical behavior of two lead-free materials: CAIC (Cs₂AgInCl₆) and CMBC (Cs₄MnBi₂Cl₁₂). To see how well they fluoresce, we compared them to a classic lead-based benchmark material: KPC (KPb₂Cl₅).

Er-doped KPC, CMBC, and CAIC in ambient lighting vs UV

Figure 1: Er-doped KPC, CMBC, and CAIC in ambient light vs UV.

Goals and Methods:

The primary goals of this study were:

Synthesize the Erbium-doped lead-free materials using a high pressure, hydrothermal method.
Characterize exactly how these materials absorb and emit light using a spectrofluorometer.
Compare our lead-free creations to the lead-based benchmark (KPC).

Hydrothermal Synthesis

To create these crystals, we used a hydrothermal method, which acts like an ultra-high-temperature pressure cooker to force high-quality crystal growth. This method involved:

Carefully weighing out precise precursor chemicals.
Placing the mixture into a tightly sealed, acid-resistant teflon vessel with 10 mL of hydrochloric acid (HCl).
Locking the vessel inside a stainless steel jacket and heating it in a furnace at 180°C (356°F) for 48 hours.
Safely cooling the mixture to room temperature, washing the brand-new crystal powders with ethanol, and drying them.

Figure 2: Teflon vessel and stainless steel jacket used for material synthesis.

Characterization Techniques

To analyze how our new crystals handle light, we placed them into a spectrofluorometer to perform:

Emission scans - Shining a fixed laser (like a UV or blue laser) at the sample to see what color it glows.
Excitation scans - Shifting through different wavelengths of light to find out exactly which energy level triggers the absolute brightest glow.

Results

Visible Fluorescence

Sample Name	Base Perovskite Type	Glow Color Under UV
Er: KPC	Lead-Based Benchmark	Vibrant Green
Er: CMBC	Lead-Free Alternative	Bright Orange
Er: CAIC	Lead-Free Alternative	None (Completely Dark)

Table 1: Fluorescence colors observed under UV excitation for each sample.

Both Er: CMBC and Er: KPC exhibited strong visible fluorescence under UV light, whereas Er: CAIC did not show significant emission. This suggests that the host material plays a critical role in the optical activity of Er³⁺ ions.

Excitation and Emission Spectroscopy

Er: KPC showed incredibly sharp, easily-identifiable peaks that align with the characteristics of Er³⁺ transitions.
Er: CMBC, however, presented broad energy bands. This tells us that the Manganese (Mn²⁺) is absorbing broad spectrum light and efficiently passing that energy off to our Erbium ions.

Sample Calculation: Er³⁺ Transition Energy

To verify the origin of the observed emission peaks, transition energies were calculated using the known electronic energy levels of Er³⁺ from a Dieke diagram.

Using the equation λ = 10⁷ / ΔE allows for the wavelengths emitted/absorbed by different electron transitions. For example:

For the transition ⁴I_15/2 → ⁴S_3/2:

ΔE = E_final - E_initial → 18500cm^-1 - 0

So... ΔE = 18500cm^-1

λ = 10⁷ / 18500cm^-1 = 540.5 nm

Emission and excitation spectrum graphs alongside energy transitions

Figure 3: Emission and excitation graphs alongside energy transitions. The blue arrows represent transitions shown.

Conclusions

Success without Lead: The high-pressure hydrothermal method effectively cooks up brilliant, optically active crystals without needing toxic lead ingredients.
The Host with the Most... Fluorescence: The choice of host material plays a huge role in the final performance. Multiple different colors can be achieved (or the light can be snuffed entirely) by swapping the structural host lattice.
Direct vs. Hand-off Energy: Er:KPC displays direct excitation paths, whereas Er:CMBC relies on broad Mn²⁺ interactions to absorb light and pass it off to the Er³⁺.
Customizable Tech: Because we can alter emission pathways and colors effortlessly, lead-free perovskites prove to be highly tunable and useful for future green tech, biophotonics, and future commercial lighting.

Keywords: Perovskites • Lead-free • Solar cells • Synthesis • Materials science